INTRODUCTION.
When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America, and
in the geological relations of the present to the past inhabitants of that
continent. These facts seemed to me to throw some light on the origin of
species—that mystery of mysteries, as it has been called by one of
our greatest philosophers. On my return home, it occurred to me, in 1837,
that something might perhaps be made out on this question by patiently
accumulating and reflecting on all sorts of facts which could possibly
have any bearing
on it. After five years' work I allowed myself to
speculate on the subject, and drew up some short notes; these I enlarged
in 1844 into a sketch of the conclusions, which then seemed to me
probable: from that period to the present day I have steadily pursued the
same object. I hope that I may be excused for entering on these personal
details, as I give them to show that I have not been hasty in coming to a
decision.
This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements; and
I must trust to the reader reposing some confidence in my accuracy. No
doubt errors will have crept in, though I hope I have always been cautious
in trusting to good authorities alone. I can here give only the general
conclusions at which I have arrived, with a few facts in illustration, but
which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the
facts, with references, on which my conclusions have been grounded; and I
hope in a future work to do this. For I am well aware that scarcely a
single point is discussed in this volume on which facts cannot be adduced,
often apparently leading to conclusions directly opposite to those at
which I have arrived. A fair result can be obtained only by fully stating
and balancing the facts and arguments on both sides of each question; and
this cannot possibly be here done.
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many
naturalists, some of them personally unknown to me. I cannot, however, let
this opportunity pass without expressing my deep obligations to Dr.
Hooker, who for the last fifteen years has aided me in every possible way
by his large stores of knowledge and his excellent judgment.
In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution, geological
succession, and other such facts, might come to the conclusion that each
species had not been independently created, but had descended, like
varieties, from other species. Nevertheless, such a conclusion, even if
well founded, would be unsatisfactory, until it could be shown how the
innumerable species inhabiting this world have been modified, so as to
acquire that perfection of structure and coadaptation which most justly
excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, etc., as the only possible cause of
variation. In one very limited sense, as we shall hereafter see, this may
be true; but it is preposterous to attribute to mere external conditions,
the structure, for instance, of the woodpecker, with its feet, tail, beak,
and tongue, so admirably adapted to catch insects under the bark of trees.
In the case of the misseltoe, which draws its nourishment from certain
trees, which has seeds that must be transported by certain birds, and
which has flowers with separate sexes absolutely requiring the agency of
certain insects to bring pollen from one flower to the other, it is
equally preposterous to account for the structure of this parasite, with
its relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant itself.
The author of the 'Vestiges of Creation' would, I presume, say that, after
a certain unknown number of generations, some bird had given birth to a
woodpecker, and some plant to the misseltoe, and that these had been
produced perfect as we now see them; but this assumption seems to me to be
no explanation, for it leaves the case of the coadaptations of organic
beings to each other and to their physical conditions of life, untouched
and unexplained.
It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making out
this obscure problem. Nor have I been disappointed; in this and in all
other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the
best and safest clue. I may venture to express my conviction of the high
value of such studies, although they have been very commonly neglected by
naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a large
amount of hereditary modification is at least possible, and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then
pass on to the variability of species in a state of nature; but I shall,
unfortunately, be compelled to treat this subject far too briefly, as it
can be treated properly only by giving long catalogues of facts. We shall,
however, be enabled to discuss what circumstances are most favourable to
variation. In the next chapter the Struggle for Existence amongst all
organic beings throughout the world, which inevitably follows from their
high geometrical powers of increase, will be treated of. This is the
doctrine of Malthus, applied to the whole animal and vegetable kingdoms.
As many more individuals of each species are born than can possibly
survive; and as, consequently, there is a frequently recurring struggle
for existence, it follows that any being, if it vary however slightly in
any manner profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus be
NATURALLY SELECTED. From the strong principle of inheritance, any selected
variety will tend to propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural Selection
almost inevitably causes much Extinction of the less improved forms of
life and induces what I have called Divergence of Character. In the next
chapter I shall discuss the complex and little known laws of variation and
of correlation of growth. In the four succeeding chapters, the most
apparent and gravest difficulties on the theory will be given: namely,
first, the difficulties of transitions, or in understanding how a simple
being or a simple organ can be changed and perfected into a highly
developed being or elaborately constructed organ; secondly the subject of
Instinct, or the mental powers of animals, thirdly, Hybridism, or the
infertility of species and the fertility of varieties when intercrossed;
and fourthly, the imperfection of the Geological Record. In the next
chapter I shall consider the geological succession of organic beings
throughout time; in the eleventh and twelfth, their geographical
distribution throughout space; in the thirteenth, their classification or
mutual affinities, both when mature and in an embryonic condition. In the
last chapter I shall give a brief recapitulation of the whole work, and a
few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due allowance
for our profound ignorance in regard to the mutual relations of all the
beings which live around us. Who can explain why one species ranges widely
and is very numerous, and why another allied species has a narrow range
and is rare? Yet these relations are of the highest importance, for they
determine the present welfare, and, as I believe, the future success and
modification of every inhabitant of this world. Still less do we know of
the mutual relations of the innumerable inhabitants of the world during
the many past geological epochs in its history. Although much remains
obscure, and will long remain obscure, I can entertain no doubt, after the
most deliberate study and dispassionate judgment of which I am capable,
that the view which most naturalists entertain, and which I formerly
entertained—namely, that each species has been independently created—is
erroneous. I am fully convinced that species are not immutable; but that
those belonging to what are called the same genera are lineal descendants
of some other and generally extinct species, in the same manner as the
acknowledged varieties of any one species are the descendants of that
species. Furthermore, I am convinced that Natural Selection has been the
main but not exclusive means of modification.
1. VARIATION UNDER DOMESTICATION.
Causes of Variability. Effects of Habit. Correlation of Growth.
Inheritance. Character of Domestic Varieties. Difficulty of distinguishing
between Varieties and Species. Origin of Domestic Varieties from one or
more Species. Domestic Pigeons, their Differences and Origin. Principle of
Selection anciently followed, its Effects. Methodical and Unconscious
Selection. Unknown Origin of our Domestic Productions. Circumstances
favourable to Man's power of Selection.
When we look to the individuals of the same variety or sub-variety of our
older cultivated plants and animals, one of the first points which strikes
us, is, that they generally differ much more from each other, than do the
individuals of any one species or variety in a state of nature. When we
reflect on the vast diversity of the plants and animals which have been
cultivated, and which have varied during all ages under the most different
climates and treatment, I think we are driven to conclude that this
greater variability is simply due to our domestic productions having been
raised under conditions of life not so uniform as, and somewhat different
from, those to which the parent-species have been exposed under nature.
There is, also, I think, some probability in the view propounded by Andrew
Knight, that this variability may be partly connected with excess of food.
It seems pretty clear that organic beings must be exposed during several
generations to the new conditions of life to cause any appreciable amount
of variation; and that when the organisation has once begun to vary, it
generally continues to vary for many generations. No case is on record of
a variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties: our
oldest domesticated animals are still capable of rapid improvement or
modification.
It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by any
clear line of distinction from mere variations. But I am strongly inclined
to suspect that the most frequent cause of variability may be attributed
to the male and female reproductive elements having been affected prior to
the act of conception. Several reasons make me believe in this; but the
chief one is the remarkable effect which confinement or cultivation has on
the functions of the reproductive system; this system appearing to be far
more susceptible than any other part of the organisation, to the action of
any change in the conditions of life. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely under
confinement, even in the many cases when the male and female unite. How
many animals there are which will not breed, though living long under not
very close confinement in their native country! This is generally
attributed to vitiated instincts; but how many cultivated plants display
the utmost vigour, and yet rarely or never seed! In some few such cases it
has been found out that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether or
not the plant sets a seed. I cannot here enter on the copious details
which I have collected on this curious subject; but to show how singular
the laws are which determine the reproduction of animals under
confinement, I may just mention that carnivorous animals, even from the
tropics, breed in this country pretty freely under confinement, with the
exception of the plantigrades or bear family; whereas, carnivorous birds,
with the rarest exceptions, hardly ever lay fertile eggs. Many exotic
plants have pollen utterly worthless, in the same exact condition as in
the most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see individuals,
though taken young from a state of nature, perfectly tamed, long-lived,
and healthy (of which I could give numerous instances), yet having their
reproductive system so seriously affected by unperceived causes as to fail
in acting, we need not be surprised at this system, when it does act under
confinement, acting not quite regularly, and producing offspring not
perfectly like their parents or variable.
Sterility has been said to be the bane of horticulture; but on this view
we owe variability to the same cause which produces sterility; and
variability is the source of all the choicest productions of the garden. I
may add, that as some organisms will breed most freely under the most
unnatural conditions (for instance, the rabbit and ferret kept in
hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly—perhaps hardly more than in a
state of nature.
A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
Such buds can be propagated by grafting, etc., and sometimes by seed.
These "sports" are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent has
affected a bud or offset, and not the ovules or pollen. But it is the
opinion of most physiologists that there is no essential difference
between a bud and an ovule in their earliest stages of formation; so that,
in fact, "sports" support my view, that variability may be largely
attributed to the ovules or pollen, or to both, having been affected by
the treatment of the parent prior to the act of conception. These cases
anyhow show that variation is not necessarily connected, as some authors
have supposed, with the act of generation.
Seedlings from the same fruit, and the young of the same litter, sometimes
differ considerably from each other, though both the young and the
parents, as Muller has remarked, have apparently been exposed to exactly
the same conditions of life; and this shows how unimportant the direct
effects of the conditions of life are in comparison with the laws of
reproduction, and of growth, and of inheritance; for had the action of the
conditions been direct, if any of the young had varied, all would probably
have varied in the same manner. To judge how much, in the case of any
variation, we should attribute to the direct action of heat, moisture,
light, food, etc., is most difficult: my impression is, that with animals
such agencies have produced very little direct effect, though apparently
more in the case of plants. Under this point of view, Mr. Buckman's recent
experiments on plants seem extremely valuable. When all or nearly all the
individuals exposed to certain conditions are affected in the same way,
the change at first appears to be directly due to such conditions; but in
some cases it can be shown that quite opposite conditions produce similar
changes of structure. Nevertheless some slight amount of change may, I
think, be attributed to the direct action of the conditions of life—as,
in some cases, increased size from amount of food, colour from particular
kinds of food and from light, and perhaps the thickness of fur from
climate.
Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion
to the whole skeleton, than do the same bones in the wild-duck; and I
presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and
inherited development of the udders in cows and goats in countries where
they are habitually milked, in comparison with the state of these organs
in other countries, is another instance of the effect of use. Not a single
domestic animal can be named which has not in some country drooping ears;
and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed
by danger, seems probable.
There are many laws regulating variation, some few of which can be dimly
seen, and will be hereafter briefly mentioned. I will here only allude to
what may be called correlation of growth. Any change in the embryo or
larva will almost certainly entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire's
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation are
quite whimsical; thus cats with blue eyes are invariably deaf; colour and
constitutional peculiarities go together, of which many remarkable cases
could be given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently affected
from coloured individuals by certain vegetable poisons. Hairless dogs have
imperfect teeth; long-haired and coarse-haired animals are apt to have, as
is asserted, long or many horns; pigeons with feathered feet have skin
between their outer toes; pigeons with short beaks have small feet, and
those with long beaks large feet. Hence, if man goes on selecting, and
thus augmenting, any peculiarity, he will almost certainly unconsciously
modify other parts of the structure, owing to the mysterious laws of the
correlation of growth.
The result of the various, quite unknown, or dimly seen laws of variation
is infinitely complex and diversified. It is well worth while carefully to
study the several treatises published on some of our old cultivated
plants, as on the hyacinth, potato, even the dahlia, etc.; and it is
really surprising to note the endless points in structure and constitution
in which the varieties and sub-varieties differ slightly from each other.
The whole organisation seems to have become plastic, and tends to depart
in some small degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But the number
and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, is endless. Dr.
Prosper Lucas's treatise, in two large volumes, is the fullest and the
best on this subject. No breeder doubts how strong is the tendency to
inheritance: like produces like is his fundamental belief: doubts have
been thrown on this principle by theoretical writers alone. When a
deviation appears not unfrequently, and we see it in the father and child,
we cannot tell whether it may not be due to the same original cause acting
on both; but when amongst individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination
of circumstances, appears in the parent—say, once amongst several
million individuals—and it reappears in the child, the mere doctrine
of chances almost compels us to attribute its reappearance to inheritance.
Every one must have heard of cases of albinism, prickly skin, hairy
bodies, etc., appearing in several members of the same family. If strange
and rare deviations of structure are truly inherited, less strange and
commoner deviations may be freely admitted to be inheritable. Perhaps the
correct way of viewing the whole subject, would be, to look at the
inheritance of every character whatever as the rule, and non-inheritance
as the anomaly.
The laws governing inheritance are quite unknown; no one can say why the
same peculiarity in different individuals of the same species, and in
individuals of different species, is sometimes inherited and sometimes not
so; why the child often reverts in certain characters to its grandfather
or grandmother or other much more remote ancestor; why a peculiarity is
often transmitted from one sex to both sexes or to one sex alone, more
commonly but not exclusively to the like sex. It is a fact of some little
importance to us, that peculiarities appearing in the males of our
domestic breeds are often transmitted either exclusively, or in a much
greater degree, to males alone. A much more important rule, which I think
may be trusted, is that, at whatever period of life a peculiarity first
appears, it tends to appear in the offspring at a corresponding age,
though sometimes earlier. In many cases this could not be otherwise: thus
the inherited peculiarities in the horns of cattle could appear only in
the offspring when nearly mature; peculiarities in the silkworm are known
to appear at the corresponding caterpillar or cocoon stage. But hereditary
diseases and some other facts make me believe that the rule has a wider
extension, and that when there is no apparent reason why a peculiarity
should appear at any particular age, yet that it does tend to appear in
the offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the laws
of embryology. These remarks are of course confined to the first
APPEARANCE of the peculiarity, and not to its primary cause, which may
have acted on the ovules or male element; in nearly the same manner as in
the crossed offspring from a short-horned cow by a long-horned bull, the
greater length of horn, though appearing late in life, is clearly due to
the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic
varieties, when run wild, gradually but certainly revert in character to
their aboriginal stocks. Hence it has been argued that no deductions can
be drawn from domestic races to species in a state of nature. I have in
vain endeavoured to discover on what decisive facts the above statement
has so often and so boldly been made. There would be great difficulty in
proving its truth: we may safely conclude that very many of the most
strongly-marked domestic varieties could not possibly live in a wild
state. In many cases we do not know what the aboriginal stock was, and so
could not tell whether or not nearly perfect reversion had ensued. It
would be quite necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in its
new home. Nevertheless, as our varieties certainly do occasionally revert
in some of their characters to ancestral forms, it seems to me not
improbable, that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance, of
the cabbage, in very poor soil (in which case, however, some effect would
have to be attributed to the direct action of the poor soil), that they
would to a large extent, or even wholly, revert to the wild aboriginal
stock. Whether or not the experiment would succeed, is not of great
importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion,—that is, to
lose their acquired characters, whilst kept under unchanged conditions,
and whilst kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations of structure, in such
case, I grant that we could deduce nothing from domestic varieties in
regard to species. But there is not a shadow of evidence in favour of this
view: to assert that we could not breed our cart and race-horses, long and
short-horned cattle, and poultry of various breeds, and esculent
vegetables, for an almost infinite number of generations, would be opposed
to all experience. I may add, that when under nature the conditions of
life do change, variations and reversions of character probably do occur;
but natural selection, as will hereafter be explained, will determine how
far the new characters thus arising shall be preserved.
When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with species closely allied together, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same
species, also, often have a somewhat monstrous character; by which I mean,
that, although differing from each other, and from the other species of
the same genus, in several trifling respects, they often differ in an
extreme degree in some one part, both when compared one with another, and
more especially when compared with all the species in nature to which they
are nearest allied. With these exceptions (and with that of the perfect
fertility of varieties when crossed,—a subject hereafter to be
discussed), domestic races of the same species differ from each other in
the same manner as, only in most cases in a lesser degree than, do
closely-allied species of the same genus in a state of nature. I think
this must be admitted, when we find that there are hardly any domestic
races, either amongst animals or plants, which have not been ranked by
some competent judges as mere varieties, and by other competent judges as
the descendants of aboriginally distinct species. If any marked
distinction existed between domestic races and species, this source of
doubt could not so perpetually recur. It has often been stated that
domestic races do not differ from each other in characters of generic
value. I think it could be shown that this statement is hardly correct;
but naturalists differ most widely in determining what characters are of
generic value; all such valuations being at present empirical. Moreover,
on the view of the origin of genera which I shall presently give, we have
no right to expect often to meet with generic differences in our
domesticated productions.
When we attempt to estimate the amount of structural difference between
the domestic races of the same species, we are soon involved in doubt,
from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the offspring of any single species, then such
facts would have great weight in making us doubt about the immutability of
the many very closely allied and natural species—for instance, of
the many foxes—inhabiting different quarters of the world. I do not
believe, as we shall presently see, that all our dogs have descended from
any one wild species; but, in the case of some other domestic races, there
is presumptive, or even strong, evidence in favour of this view.
It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and likewise
to withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions; but
how could a savage possibly know, when he first tamed an animal, whether
it would vary in succeeding generations, and whether it would endure other
climates? Has the little variability of the ass or guinea-fowl, or the
small power of endurance of warmth by the rein-deer, or of cold by the
common camel, prevented their domestication? I cannot doubt that if other
animals and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would vary on an average as largely
as the parent species of our existing domesticated productions have
varied.
In the case of most of our anciently domesticated animals and plants, I do
not think it is possible to come to any definite conclusion, whether they
have descended from one or several species. The argument mainly relied on
by those who believe in the multiple origin of our domestic animals is,
that we find in the most ancient records, more especially on the monuments
of Egypt, much diversity in the breeds; and that some of the breeds
closely resemble, perhaps are identical with, those still existing. Even
if this latter fact were found more strictly and generally true than seems
to me to be the case, what does it show, but that some of our breeds
originated there, four or five thousand years ago? But Mr. Horner's
researches have rendered it in some degree probable that man sufficiently
civilized to have manufactured pottery existed in the valley of the Nile
thirteen or fourteen thousand years ago; and who will pretend to say how
long before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in
Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs
have descended from several wild species. In regard to sheep and goats I
can form no opinion. I should think, from facts communicated to me by Mr.
Blyth, on the habits, voice, and constitution, etc., of the humped Indian
cattle, that these had descended from a different aboriginal stock from
our European cattle; and several competent judges believe that these
latter have had more than one wild parent. With respect to horses, from
reasons which I cannot give here, I am doubtfully inclined to believe, in
opposition to several authors, that all the races have descended from one
wild stock. Mr. Blyth, whose opinion, from his large and varied stores of
knowledge, I should value more than that of almost any one, thinks that
all the breeds of poultry have proceeded from the common wild Indian fowl
(Gallus bankiva). In regard to ducks and rabbits, the breeds of which
differ considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive
characters be ever so slight, has had its wild prototype. At this rate
there must have existed at least a score of species of wild cattle, as
many sheep, and several goats in Europe alone, and several even within
Great Britain. One author believes that there formerly existed in Great
Britain eleven wild species of sheep peculiar to it! When we bear in mind
that Britain has now hardly one peculiar mammal, and France but few
distinct from those of Germany and conversely, and so with Hungary, Spain,
etc., but that each of these kingdoms possesses several peculiar breeds of
cattle, sheep, etc., we must admit that many domestic breeds have
originated in Europe; for whence could they have been derived, as these
several countries do not possess a number of peculiar species as distinct
parent-stocks? So it is in India. Even in the case of the domestic dogs of
the whole world, which I fully admit have probably descended from several
wild species, I cannot doubt that there has been an immense amount of
inherited variation. Who can believe that animals closely resembling the
Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so
unlike all wild Canidae—ever existed freely in a state of nature? It
has often been loosely said that all our races of dogs have been produced
by the crossing of a few aboriginal species; but by crossing we can get
only forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must admit the
former existence of the most extreme forms, as the Italian greyhound,
bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility
of making distinct races by crossing has been greatly exaggerated. There
can be no doubt that a race may be modified by occasional crosses, if
aided by the careful selection of those individual mongrels, which present
any desired character; but that a race could be obtained nearly
intermediate between two extremely different races or species, I can
hardly believe. Sir J. Sebright expressly experimentised for this object,
and failed. The offspring from the first cross between two pure breeds is
tolerably and sometimes (as I have found with pigeons) extremely uniform,
and everything seems simple enough; but when these mongrels are crossed
one with another for several generations, hardly two of them will be
alike, and then the extreme difficulty, or rather utter hopelessness, of
the task becomes apparent. Certainly, a breed intermediate between TWO
VERY DISTINCT breeds could not be got without extreme care and
long-continued selection; nor can I find a single case on record of a
permanent race having been thus formed.
ON THE BREEDS OF THE DOMESTIC PIGEON.
Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured with
skins from several quarters of the world, more especially by the
Honourable W. Elliot from India, and by the Honourable C. Murray from
Persia. Many treatises in different languages have been published on
pigeons, and some of them are very important, as being of considerable
antiquity. I have associated with several eminent fanciers, and have been
permitted to join two of the London Pigeon Clubs. The diversity of the
breeds is something astonishing. Compare the English carrier and the
short-faced tumbler, and see the wonderful difference in their beaks,
entailing corresponding differences in their skulls. The carrier, more
especially the male bird, is also remarkable from the wonderful
development of the carunculated skin about the head, and this is
accompanied by greatly elongated eyelids, very large external orifices to
the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak
in outline almost like that of a finch; and the common tumbler has the
singular and strictly inherited habit of flying at a great height in a
compact flock, and tumbling in the air head over heels. The runt is a bird
of great size, with long, massive beak and large feet; some of the
sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the carrier,
but, instead of a very long beak, has a very short and very broad one. The
pouter has a much elongated body, wings, and legs; and its enormously
developed crop, which it glories in inflating, may well excite
astonishment and even laughter. The turbit has a very short and conical
beak, with a line of reversed feathers down the breast; and it has the
habit of continually expanding slightly the upper part of the oesophagus.
The Jacobin has the feathers so much reversed along the back of the neck
that they form a hood, and it has, proportionally to its size, much
elongated wing and tail feathers. The trumpeter and laugher, as their
names express, utter a very different coo from the other breeds. The
fantail has thirty or even forty tail-feathers, instead of twelve or
fourteen, the normal number in all members of the great pigeon family; and
these feathers are kept expanded, and are carried so erect that in good
birds the head and tail touch; the oil-gland is quite aborted. Several
other less distinct breeds might have been specified.
In the skeletons of the several breeds, the development of the bones of
the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The number of the caudal and sacral
vertebrae vary; as does the number of the ribs, together with their
relative breadth and the presence of processes. The size and shape of the
apertures in the sternum are highly variable; so is the degree of
divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and of
the upper part of the oesophagus; the development and abortion of the
oil-gland; the number of the primary wing and caudal feathers; the
relative length of wing and tail to each other and to the body; the
relative length of leg and of the feet; the number of scutellae on the
toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight differs remarkably; as does in some breeds the voice and
disposition. Lastly, in certain breeds, the males and females have come to
differ to a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which if shown to
an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species. Moreover, I
do not believe that any ornithologist would place the English carrier, the
short-faced tumbler, the runt, the barb, pouter, and fantail in the same
genus; more especially as in each of these breeds several truly-inherited
sub-breeds, or species as he might have called them, could be shown him.
Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely, that
all have descended from the rock-pigeon (Columba livia), including under
this term several geographical races or sub-species, which differ from
each other in the most trifling respects. As several of the reasons which
have led me to this belief are in some degree applicable in other cases, I
will here briefly give them. If the several breeds are not varieties, and
have not proceeded from the rock-pigeon, they must have descended from at
least seven or eight aboriginal stocks; for it is impossible to make the
present domestic breeds by the crossing of any lesser number: how, for
instance, could a pouter be produced by crossing two breeds unless one of
the parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons, that is, not breeding
or willingly perching on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of rock-pigeons
are known; and these have not any of the characters of the domestic
breeds. Hence the supposed aboriginal stocks must either still exist in
the countries where they were originally domesticated, and yet be unknown
to ornithologists; and this, considering their size, habits, and
remarkable characters, seems very improbable; or they must have become
extinct in the wild state. But birds breeding on precipices, and good
fliers, are unlikely to be exterminated; and the common rock-pigeon, which
has the same habits with the domestic breeds, has not been exterminated
even on several of the smaller British islets, or on the shores of the
Mediterranean. Hence the supposed extermination of so many species having
similar habits with the rock-pigeon seems to me a very rash assumption.
Moreover, the several above-named domesticated breeds have been
transported to all parts of the world, and, therefore, some of them must
have been carried back again into their native country; but not one has
ever become wild or feral, though the dovecot-pigeon, which is the
rock-pigeon in a very slightly altered state, has become feral in several
places. Again, all recent experience shows that it is most difficult to
get any wild animal to breed freely under domestication; yet on the
hypothesis of the multiple origin of our pigeons, it must be assumed that
at least seven or eight species were so thoroughly domesticated in ancient
times by half-civilized man, as to be quite prolific under confinement.
An argument, as it seems to me, of great weight, and applicable in several
other cases, is, that the above-specified breeds, though agreeing
generally in constitution, habits, voice, colouring, and in most parts of
their structure, with the wild rock-pigeon, yet are certainly highly
abnormal in other parts of their structure: we may look in vain throughout
the whole great family of Columbidae for a beak like that of the English
carrier, or that of the short-faced tumbler, or barb; for reversed
feathers like those of the jacobin; for a crop like that of the pouter;
for tail-feathers like those of the fantail. Hence it must be assumed not
only that half-civilized man succeeded in thoroughly domesticating several
species, but that he intentionally or by chance picked out extraordinarily
abnormal species; and further, that these very species have since all
become extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white rump
(the Indian sub-species, C. intermedia of Strickland, having it bluish);
the tail has a terminal dark bar, with the bases of the outer feathers
externally edged with white; the wings have two black bars; some
semi-domestic breeds and some apparently truly wild breeds have, besides
the two black bars, the wings chequered with black. These several marks do
not occur together in any other species of the whole family. Now, in every
one of the domestic breeds, taking thoroughly well-bred birds, all the
above marks, even to the white edging of the outer tail-feathers,
sometimes concur perfectly developed. Moreover, when two birds belonging
to two distinct breeds are crossed, neither of which is blue or has any of
the above-specified marks, the mongrel offspring are very apt suddenly to
acquire these characters; for instance, I crossed some uniformly white
fantails with some uniformly black barbs, and they produced mottled brown
and black birds; these I again crossed together, and one grandchild of the
pure white fantail and pure black barb was of as beautiful a blue colour,
with the white rump, double black wing-bar, and barred and white-edged
tail-feathers, as any wild rock-pigeon! We can understand these facts, on
the well-known principle of reversion to ancestral characters, if all the
domestic breeds have descended from the rock-pigeon. But if we deny this,
we must make one of the two following highly improbable suppositions.
Either, firstly, that all the several imagined aboriginal stocks were
coloured and marked like the rock-pigeon, although no other existing
species is thus coloured and marked, so that in each separate breed there
might be a tendency to revert to the very same colours and markings. Or,
secondly, that each breed, even the purest, has within a dozen or, at
most, within a score of generations, been crossed by the rock-pigeon: I
say within a dozen or twenty generations, for we know of no fact
countenancing the belief that the child ever reverts to some one ancestor,
removed by a greater number of generations. In a breed which has been
crossed only once with some distinct breed, the tendency to reversion to
any character derived from such cross will naturally become less and less,
as in each succeeding generation there will be less of the foreign blood;
but when there has been no cross with a distinct breed, and there is a
tendency in both parents to revert to a character, which has been lost
during some former generation, this tendency, for all that we can see to
the contrary, may be transmitted undiminished for an indefinite number of
generations. These two distinct cases are often confounded in treatises on
inheritance.
Lastly, the hybrids or mongrels from between all the domestic breeds of
pigeons are perfectly fertile. I can state this from my own observations,
purposely made on the most distinct breeds. Now, it is difficult, perhaps
impossible, to bring forward one case of the hybrid offspring of two
animals CLEARLY DISTINCT being themselves perfectly fertile. Some authors
believe that long-continued domestication eliminates this strong tendency
to sterility: from the history of the dog I think there is some
probability in this hypothesis, if applied to species closely related
together, though it is unsupported by a single experiment. But to extend
the hypothesis so far as to suppose that species, aboriginally as distinct
as carriers, tumblers, pouters, and fantails now are, should yield
offspring perfectly fertile, inter se, seems to me rash in the extreme.
From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed species of pigeons to breed freely
under domestication; these supposed species being quite unknown in a wild
state, and their becoming nowhere feral; these species having very
abnormal characters in certain respects, as compared with all other
Columbidae, though so like in most other respects to the rock-pigeon; the
blue colour and various marks occasionally appearing in all the breeds,
both when kept pure and when crossed; the mongrel offspring being
perfectly fertile;—from these several reasons, taken together, I can
feel no doubt that all our domestic breeds have descended from the Columba
livia with its geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we can make
an almost perfect series between the extremes of structure. Thirdly, those
characters which are mainly distinctive of each breed, for instance the
wattle and length of beak of the carrier, the shortness of that of the
tumbler, and the number of tail-feathers in the fantail, are in each breed
eminently variable; and the explanation of this fact will be obvious when
we come to treat of selection. Fourthly, pigeons have been watched, and
tended with the utmost care, and loved by many people. They have been
domesticated for thousands of years in several quarters of the world; the
earliest known record of pigeons is in the fifth Aegyptian dynasty, about
3000 B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch
informs me that pigeons are given in a bill of fare in the previous
dynasty. In the time of the Romans, as we hear from Pliny, immense prices
were given for pigeons; "nay, they are come to this pass, that they can
reckon up their pedigree and race." Pigeons were much valued by Akber Khan
in India, about the year 1600; never less than 20,000 pigeons were taken
with the court. "The monarchs of Iran and Turan sent him some very rare
birds;" and, continues the courtly historian, "His Majesty by crossing the
breeds, which method was never practised before, has improved them
astonishingly." About this same period the Dutch were as eager about
pigeons as were the old Romans. The paramount importance of these
considerations in explaining the immense amount of variation which pigeons
have undergone, will be obvious when we treat of Selection. We shall then,
also, see how it is that the breeds so often have a somewhat monstrous
character. It is also a most favourable circumstance for the production of
distinct breeds, that male and female pigeons can be easily mated for
life; and thus different breeds can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and watched
the several kinds, knowing well how true they bred, I felt fully as much
difficulty in believing that they could ever have descended from a common
parent, as any naturalist could in coming to a similar conclusion in
regard to the many species of finches, or other large groups of birds, in
nature. One circumstance has struck me much; namely, that all the breeders
of the various domestic animals and the cultivators of plants, with whom I
have ever conversed, or whose treatises I have read, are firmly convinced
that the several breeds to which each has attended, are descended from so
many aboriginally distinct species. Ask, as I have asked, a celebrated
raiser of Hereford cattle, whether his cattle might not have descended
from long horns, and he will laugh you to scorn. I have never met a
pigeon, or poultry, or duck, or rabbit fancier, who was not fully
convinced that each main breed was descended from a distinct species. Van
Mons, in his treatise on pears and apples, shows how utterly he
disbelieves that the several sorts, for instance a Ribston-pippin or
Codlin-apple, could ever have proceeded from the seeds of the same tree.
Innumerable other examples could be given. The explanation, I think, is
simple: from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that each
race varies slightly, for they win their prizes by selecting such slight
differences, yet they ignore all general arguments, and refuse to sum up
in their minds slight differences accumulated during many successive
generations. May not those naturalists who, knowing far less of the laws
of inheritance than does the breeder, and knowing no more than he does of
the intermediate links in the long lines of descent, yet admit that many
of our domestic races have descended from the same parents—may they
not learn a lesson of caution, when they deride the idea of species in a
state of nature being lineal descendants of other species?
SELECTION.
Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some little
effect may, perhaps, be attributed to the direct action of the external
conditions of life, and some little to habit; but he would be a bold man
who would account by such agencies for the differences of a dray and race
horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of
the most remarkable features in our domesticated races is that we see in
them adaptation, not indeed to the animal's or plant's own good, but to
man's use or fancy. Some variations useful to him have probably arisen
suddenly, or by one step; many botanists, for instance, believe that the
fuller's teazle, with its hooks, which cannot be rivalled by any
mechanical contrivance, is only a variety of the wild Dipsacus; and this
amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with the wool of one breed
good for one purpose, and that of another breed for another purpose; when
we compare the many breeds of dogs, each good for man in very different
ways; when we compare the game-cock, so pertinacious in battle, with other
breeds so little quarrelsome, with "everlasting layers" which never desire
to sit, and with the bantam so small and elegant; when we compare the host
of agricultural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly produced
as perfect and as useful as we now see them; indeed, in several cases, we
know that this has not been their history. The key is man's power of
accumulative selection: nature gives successive variations; man adds them
up in certain directions useful to him. In this sense he may be said to
make for himself useful breeds.
The great power of this principle of selection is not hypothetical. It is
certain that several of our eminent breeders have, even within a single
lifetime, modified to a large extent some breeds of cattle and sheep. In
order fully to realise what they have done, it is almost necessary to read
several of the many treatises devoted to this subject, and to inspect the
animals. Breeders habitually speak of an animal's organisation as
something quite plastic, which they can model almost as they please. If I
had space I could quote numerous passages to this effect from highly
competent authorities. Youatt, who was probably better acquainted with the
works of agriculturalists than almost any other individual, and who was
himself a very good judge of an animal, speaks of the principle of
selection as "that which enables the agriculturist, not only to modify the
character of his flock, but to change it altogether. It is the magician's
wand, by means of which he may summon into life whatever form and mould he
pleases." Lord Somerville, speaking of what breeders have done for sheep,
says:—"It would seem as if they had chalked out upon a wall a form
perfect in itself, and then had given it existence." That most skilful
breeder, Sir John Sebright, used to say, with respect to pigeons, that "he
would produce any given feather in three years, but it would take him six
years to obtain head and beak." In Saxony the importance of the principle
of selection in regard to merino sheep is so fully recognised, that men
follow it as a trade: the sheep are placed on a table and are studied,
like a picture by a connoisseur; this is done three times at intervals of
months, and the sheep are each time marked and classed, so that the very
best may ultimately be selected for breeding.
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders
are strongly opposed to this practice, except sometimes amongst closely
allied sub-breeds. And when a cross has been made, the closest selection
is far more indispensable even than in ordinary cases. If selection
consisted merely in separating some very distinct variety, and breeding
from it, the principle would be so obvious as hardly to be worth notice;
but its importance consists in the great effect produced by the
accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye—differences
which I for one have vainly attempted to appreciate. Not one man in a
thousand has accuracy of eye and judgment sufficient to become an eminent
breeder. If gifted with these qualities, and he studies his subject for
years, and devotes his lifetime to it with indomitable perseverance, he
will succeed, and may make great improvements; if he wants any of these
qualities, he will assuredly fail. Few would readily believe in the
natural capacity and years of practice requisite to become even a skilful
pigeon-fancier.
The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest productions
have been produced by a single variation from the aboriginal stock. We
have proofs that this is not so in some cases, in which exact records have
been kept; thus, to give a very trifling instance, the steadily-increasing
size of the common gooseberry may be quoted. We see an astonishing
improvement in many florists' flowers, when the flowers of the present day
are compared with drawings made only twenty or thirty years ago. When a
race of plants is once pretty well established, the seed-raisers do not
pick out the best plants, but merely go over their seed-beds, and pull up
the "rogues," as they call the plants that deviate from the proper
standard. With animals this kind of selection is, in fact, also followed;
for hardly any one is so careless as to allow his worst animals to breed.
In regard to plants, there is another means of observing the accumulated
effects of selection—namely, by comparing the diversity of flowers
in the different varieties of the same species in the flower-garden; the
diversity of leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the same varieties; and
the diversity of fruit of the same species in the orchard, in comparison
with the leaves and flowers of the same set of varieties. See how
different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the
leaves; how much the fruit of the different kinds of gooseberries differ
in size, colour, shape, and hairiness, and yet the flowers present very
slight differences. It is not that the varieties which differ largely in
some one point do not differ at all in other points; this is hardly ever,
perhaps never, the case. The laws of correlation of growth, the importance
of which should never be overlooked, will ensure some differences; but, as
a general rule, I cannot doubt that the continued selection of slight
variations, either in the leaves, the flowers, or the fruit, will produce
races differing from each other chiefly in these characters.
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century; it
has certainly been more attended to of late years, and many treatises have
been published on the subject; and the result, I may add, has been, in a
corresponding degree, rapid and important. But it is very far from true
that the principle is a modern discovery. I could give several references
to the full acknowledgment of the importance of the principle in works of
high antiquity. In rude and barbarous periods of English history choice
animals were often imported, and laws were passed to prevent their
exportation: the destruction of horses under a certain size was ordered,
and this may be compared to the "roguing" of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman classical
writers. From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes cross
their dogs with wild canine animals, to improve the breed, and they
formerly did so, as is attested by passages in Pliny. The savages in South
Africa match their draught cattle by colour, as do some of the Esquimaux
their teams of dogs. Livingstone shows how much good domestic breeds are
valued by the negroes of the interior of Africa who have not associated
with Europeans. Some of these facts do not show actual selection, but they
show that the breeding of domestic animals was carefully attended to in
ancient times, and is now attended to by the lowest savages. It would,
indeed, have been a strange fact, had attention not been paid to breeding,
for the inheritance of good and bad qualities is so obvious.
At the present time, eminent breeders try by methodical selection, with a
distinct object in view, to make a new strain or sub-breed, superior to
anything existing in the country. But, for our purpose, a kind of
Selection, which may be called Unconscious, and which results from every
one trying to possess and breed from the best individual animals, is more
important. Thus, a man who intends keeping pointers naturally tries to get
as good dogs as he can, and afterwards breeds from his own best dogs, but
he has no wish or expectation of permanently altering the breed.
Nevertheless I cannot doubt that this process, continued during centuries,
would improve and modify any breed, in the same way as Bakewell, Collins,
etc., by this very same process, only carried on more methodically, did
greatly modify, even during their own lifetimes, the forms and qualities
of their cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the breeds in
question had been made long ago, which might serve for comparison. In some
cases, however, unchanged or but little changed individuals of the same
breed may be found in less civilised districts, where the breed has been
less improved. There is reason to believe that King Charles's spaniel has
been unconsciously modified to a large extent since the time of that
monarch. Some highly competent authorities are convinced that the setter
is directly derived from the spaniel, and has probably been slowly altered
from it. It is known that the English pointer has been greatly changed
within the last century, and in this case the change has, it is believed,
been chiefly effected by crosses with the fox-hound; but what concerns us
is, that the change has been effected unconsciously and gradually, and yet
so effectually, that, though the old Spanish pointer certainly came from
Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in
Spain like our pointer.
By a similar process of selection, and by careful training, the whole body
of English racehorses have come to surpass in fleetness and size the
parent Arab stock, so that the latter, by the regulations for the Goodwood
Races, are favoured in the weights they carry. Lord Spencer and others
have shown how the cattle of England have increased in weight and in early
maturity, compared with the stock formerly kept in this country. By
comparing the accounts given in old pigeon treatises of carriers and
tumblers with these breeds as now existing in Britain, India, and Persia,
we can, I think, clearly trace the stages through which they have
insensibly passed, and come to differ so greatly from the rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far
that the breeders could never have expected or even have wished to have
produced the result which ensued—namely, the production of two
distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley
and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from the
original stock of Mr. Bakewell for upwards of fifty years. There is not a
suspicion existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any one instance
from the pure blood of Mr. Bakewell's flock, and yet the difference
between the sheep possessed by these two gentlemen is so great that they
have the appearance of being quite different varieties."
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are so
liable, and such choice animals would thus generally leave more offspring
than the inferior ones; so that in this case there would be a kind of
unconscious selection going on. We see the value set on animals even by
the barbarians of Tierra del Fuego, by their killing and devouring their
old women, in times of dearth, as of less value than their dogs.
In plants the same gradual process of improvement, through the occasional
preservation of the best individuals, whether or not sufficiently distinct
to be ranked at their first appearance as distinct varieties, and whether
or not two or more species or races have become blended together by
crossing, may plainly be recognised in the increased size and beauty which
we now see in the varieties of the heartsease, rose, pelargonium, dahlia,
and other plants, when compared with the older varieties or with their
parent-stocks. No one would ever expect to get a first-rate heartsease or
dahlia from the seed of a wild plant. No one would expect to raise a
first-rate melting pear from the seed of a wild pear, though he might
succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears,
from Pliny's description, to have been a fruit of very inferior quality. I
have seen great surprise expressed in horticultural works at the wonderful
skill of gardeners, in having produced such splendid results from such
poor materials; but the art, I cannot doubt, has been simple, and, as far
as the final result is concerned, has been followed almost unconsciously.
It has consisted in always cultivating the best known variety, sowing its
seeds, and, when a slightly better variety has chanced to appear,
selecting it, and so onwards. But the gardeners of the classical period,
who cultivated the best pear they could procure, never thought what
splendid fruit we should eat; though we owe our excellent fruit, in some
small degree, to their having naturally chosen and preserved the best
varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact,
that in a vast number of cases we cannot recognise, and therefore do not
know, the wild parent-stocks of the plants which have been longest
cultivated in our flower and kitchen gardens. If it has taken centuries or
thousands of years to improve or modify most of our plants up to their
present standard of usefulness to man, we can understand how it is that
neither Australia, the Cape of Good Hope, nor any other region inhabited
by quite uncivilised man, has afforded us a single plant worth culture. It
is not that these countries, so rich in species, do not by a strange
chance possess the aboriginal stocks of any useful plants, but that the
native plants have not been improved by continued selection up to a
standard of perfection comparable with that given to the plants in
countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it should not
be overlooked that they almost always have to struggle for their own food,
at least during certain seasons. And in two countries very differently
circumstanced, individuals of the same species, having slightly different
constitutions or structure, would often succeed better in the one country
than in the other, and thus by a process of "natural selection," as will
hereafter be more fully explained, two sub-breeds might be formed. This,
perhaps, partly explains what has been remarked by some authors, namely,
that the varieties kept by savages have more of the character of species
than the varieties kept in civilised countries.
On the view here given of the all-important part which selection by man
has played, it becomes at once obvious, how it is that our domestic races
show adaptation in their structure or in their habits to man's wants or
fancies. We can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their differences being so
great in external characters and relatively so slight in internal parts or
organs. Man can hardly select, or only with much difficulty, any deviation
of structure excepting such as is externally visible; and indeed he rarely
cares for what is internal. He can never act by selection, excepting on
variations which are first given to him in some slight degree by nature.
No man would ever try to make a fantail, till he saw a pigeon with a tail
developed in some slight degree in an unusual manner, or a pouter till he
saw a pigeon with a crop of somewhat unusual size; and the more abnormal
or unusual any character was when it first appeared, the more likely it
would be to catch his attention. But to use such an expression as trying
to make a fantail, is, I have no doubt, in most cases, utterly incorrect.
The man who first selected a pigeon with a slightly larger tail, never
dreamed what the descendants of that pigeon would become through
long-continued, partly unconscious and partly methodical selection.
Perhaps the parent bird of all fantails had only fourteen tail-feathers
somewhat expanded, like the present Java fantail, or like individuals of
other and distinct breeds, in which as many as seventeen tail-feathers
have been counted. Perhaps the first pouter-pigeon did not inflate its
crop much more than the turbit now does the upper part of its oesophagus,—a
habit which is disregarded by all fanciers, as it is not one of the points
of the breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
be set on any slight differences in the individuals of the same species,
be judged of by the value which would now be set on them, after several
breeds have once fairly been established. Many slight differences might,
and indeed do now, arise amongst pigeons, which are rejected as faults or
deviations from the standard of perfection of each breed. The common goose
has not given rise to any marked varieties; hence the Thoulouse and the
common breed, which differ only in colour, that most fleeting of
characters, have lately been exhibited as distinct at our poultry-shows.
I think these views further explain what has sometimes been noticed—namely
that we know nothing about the origin or history of any of our domestic
breeds. But, in fact, a breed, like a dialect of a language, can hardly be
said to have had a definite origin. A man preserves and breeds from an
individual with some slight deviation of structure, or takes more care
than usual in matching his best animals and thus improves them, and the
improved individuals slowly spread in the immediate neighbourhood. But as
yet they will hardly have a distinct name, and from being only slightly
valued, their history will be disregarded. When further improved by the
same slow and gradual process, they will spread more widely, and will get
recognised as something distinct and valuable, and will then probably
first receive a provincial name. In semi-civilised countries, with little
free communication, the spreading and knowledge of any new sub-breed will
be a slow process. As soon as the points of value of the new sub-breed are
once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend,—perhaps more at one period
than at another, as the breed rises or falls in fashion,—perhaps
more in one district than in another, according to the state of
civilisation of the inhabitants—slowly to add to the characteristic
features of the breed, whatever they may be. But the chance will be
infinitely small of any record having been preserved of such slow,
varying, and insensible changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to work
on; not that mere individual differences are not amply sufficient, with
extreme care, to allow of the accumulation of a large amount of
modification in almost any desired direction. But as variations manifestly
useful or pleasing to man appear only occasionally, the chance of their
appearance will be much increased by a large number of individuals being
kept; and hence this comes to be of the highest importance to success. On
this principle Marshall has remarked, with respect to the sheep of parts
of Yorkshire, that "as they generally belong to poor people, and are
mostly IN SMALL LOTS, they never can be improved." On the other hand,
nurserymen, from raising large stocks of the same plants, are generally
far more successful than amateurs in getting new and valuable varieties.
The keeping of a large number of individuals of a species in any country
requires that the species should be placed under favourable conditions of
life, so as to breed freely in that country. When the individuals of any
species are scanty, all the individuals, whatever their quality may be,
will generally be allowed to breed, and this will effectually prevent
selection. But probably the most important point of all, is, that the
animal or plant should be so highly useful to man, or so much valued by
him, that the closest attention should be paid to even the slightest
deviation in the qualities or structure of each individual. Unless such
attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary
just when gardeners began to attend closely to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked out
individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then, there appeared (aided by some crossing with distinct
species) those many admirable varieties of the strawberry which have been
raised during the last thirty or forty years.
In the case of animals with separate sexes, facility in preventing crosses
is an important element of success in the formation of new races,—at
least, in a country which is already stocked with other races. In this
respect enclosure of the land plays a part. Wandering savages or the
inhabitants of open plains rarely possess more than one breed of the same
species. Pigeons can be mated for life, and this is a great convenience to
the fancier, for thus many races may be kept true, though mingled in the
same aviary; and this circumstance must have largely favoured the
improvement and formation of new breeds. Pigeons, I may add, can be
propagated in great numbers and at a very quick rate, and inferior birds
may be freely rejected, as when killed they serve for food. On the other
hand, cats, from their nocturnal rambling habits, cannot be matched, and,
although so much valued by women and children, we hardly ever see a
distinct breed kept up; such breeds as we do sometimes see are almost
always imported from some other country, often from islands. Although I do
not doubt that some domestic animals vary less than others, yet the rarity
or absence of distinct breeds of the cat, the donkey, peacock, goose,
etc., may be attributed in main part to selection not having been brought
into play: in cats, from the difficulty in pairing them; in donkeys, from
only a few being kept by poor people, and little attention paid to their
breeding; in peacocks, from not being very easily reared and a large stock
not kept; in geese, from being valuable only for two purposes, food and
feathers, and more especially from no pleasure having been felt in the
display of distinct breeds.
To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the reproductive
system, are so far of the highest importance as causing variability. I do
not believe that variability is an inherent and necessary contingency,
under all circumstances, with all organic beings, as some authors have
thought. The effects of variability are modified by various degrees of
inheritance and of reversion. Variability is governed by many unknown
laws, more especially by that of correlation of growth. Something may be
attributed to the direct action of the conditions of life. Something must
be attributed to use and disuse. The final result is thus rendered
infinitely complex. In some cases, I do not doubt that the intercrossing
of species, aboriginally distinct, has played an important part in the
origin of our domestic productions. When in any country several domestic
breeds have once been established, their occasional intercrossing, with
the aid of selection, has, no doubt, largely aided in the formation of new
sub-breeds; but the importance of the crossing of varieties has, I
believe, been greatly exaggerated, both in regard to animals and to those
plants which are propagated by seed. In plants which are temporarily
propagated by cuttings, buds, etc., the importance of the crossing both of
distinct species and of varieties is immense; for the cultivator here
quite disregards the extreme variability both of hybrids and mongrels, and
the frequent sterility of hybrids; but the cases of plants not propagated
by seed are of little importance to us, for their endurance is only
temporary. Over all these causes of Change I am convinced that the
accumulative action of Selection, whether applied methodically and more
quickly, or unconsciously and more slowly, but more efficiently, is by far
the predominant Power.
2. VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much
diffused, and common species vary most. Species of the larger genera in
any country vary more than the species of the smaller genera. Many of the
species of the larger genera resemble varieties in being very closely, but
unequally, related to each other, and in having restricted ranges.
Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these latter
are subject to any variation. To treat this subject at all properly, a
long catalogue of dry facts should be given; but these I shall reserve for
my future work. Nor shall I here discuss the various definitions which
have been given of the term species. No one definition has as yet
satisfied all naturalists; yet every naturalist knows vaguely what he
means when he speaks of a species. Generally the term includes the unknown
element of a distinct act of creation. The term "variety" is almost
equally difficult to define; but here community of descent is almost
universally implied, though it can rarely be proved. We have also what are
called monstrosities; but they graduate into varieties. By a monstrosity I
presume is meant some considerable deviation of structure in one part,
either injurious to or not useful to the species, and not generally
propagated. Some authors use the term "variation" in a technical sense, as
implying a modification directly due to the physical conditions of life;
and "variations" in this sense are supposed not to be inherited: but who
can say that the dwarfed condition of shells in the brackish waters of the
Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an
animal from far northwards, would not in some cases be inherited for at
least some few generations? and in this case I presume that the form would
be called a variety.
Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring from
the same parents, or which may be presumed to have thus arisen, from being
frequently observed in the individuals of the same species inhabiting the
same confined locality. No one supposes that all the individuals of the
same species are cast in the very same mould. These individual differences
are highly important for us, as they afford materials for natural
selection to accumulate, in the same manner as man can accumulate in any
given direction individual differences in his domesticated productions.
These individual differences generally affect what naturalists consider
unimportant parts; but I could show by a long catalogue of facts, that
parts which must be called important, whether viewed under a physiological
or classificatory point of view, sometimes vary in the individuals of the
same species. I am convinced that the most experienced naturalist would be
surprised at the number of the cases of variability, even in important
parts of structure, which he could collect on good authority, as I have
collected, during a course of years. It should be remembered that
systematists are far from pleased at finding variability in important
characters, and that there are not many men who will laboriously examine
internal and important organs, and compare them in many specimens of the
same species. I should never have expected that the branching of the main
nerves close to the great central ganglion of an insect would have been
variable in the same species; I should have expected that changes of this
nature could have been effected only by slow degrees: yet quite recently
Mr. Lubbock has shown a degree of variability in these main nerves in
Coccus, which may almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add, has also quite
recently shown that the muscles in the larvae of certain insects are very
far from uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank that
character as important (as some few naturalists have honestly confessed)
which does not vary; and, under this point of view, no instance of an
important part varying will ever be found: but under any other point of
view many instances assuredly can be given.
There is one point connected with individual differences, which seems to
me extremely perplexing: I refer to those genera which have sometimes been
called "protean" or "polymorphic," in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which
forms to rank as species and which as varieties. We may instance Rubus,
Rosa, and Hieracium amongst plants, several genera of insects, and several
genera of Brachiopod shells. In most polymorphic genera some of the
species have fixed and definite characters. Genera which are polymorphic
in one country seem to be, with some few exceptions, polymorphic in other
countries, and likewise, judging from Brachiopod shells, at former periods
of time. These facts seem to be very perplexing, for they seem to show
that this kind of variability is independent of the conditions of life. I
am inclined to suspect that we see in these polymorphic genera variations
in points of structure which are of no service or disservice to the
species, and which consequently have not been seized on and rendered
definite by natural selection, as hereafter will be explained.
Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do not
like to rank them as distinct species, are in several respects the most
important for us. We have every reason to believe that many of these
doubtful and closely-allied forms have permanently retained their
characters in their own country for a long time; for as long, as far as we
know, as have good and true species. Practically, when a naturalist can
unite two forms together by others having intermediate characters, he
treats the one as a variety of the other, ranking the most common, but
sometimes the one first described, as the species, and the other as the
variety. But cases of great difficulty, which I will not here enumerate,
sometimes occur in deciding whether or not to rank one form as a variety
of another, even when they are closely connected by intermediate links;
nor will the commonly-assumed hybrid nature of the intermediate links
always remove the difficulty. In very many cases, however, one form is
ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed; and
here a wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species by
at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of the
United States, drawn up by different botanists, and see what a surprising
number of forms have been ranked by one botanist as good species, and by
another as mere varieties. Mr. H. C. Watson, to whom I lie under deep
obligation for assistance of all kinds, has marked for me 182 British
plants, which are generally considered as varieties, but which have all
been ranked by botanists as species; and in making this list he has
omitted many trifling varieties, but which nevertheless have been ranked
by some botanists as species, and he has entirely omitted several highly
polymorphic genera. Under genera, including the most polymorphic forms,
Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112,—a
difference of 139 doubtful forms! Amongst animals which unite for each
birth, and which are highly locomotive, doubtful forms, ranked by one
zoologist as a species and by another as a variety, can rarely be found
within the same country, but are common in separated areas. How many of
those birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist as
undoubted species, and by another as varieties, or, as they are often
called, as geographical races! Many years ago, when comparing, and seeing
others compare, the birds from the separate islands of the Galapagos
Archipelago, both one with another, and with those from the American
mainland, I was much struck how entirely vague and arbitrary is the
distinction between species and varieties. On the islets of the little
Madeira group there are many insects which are characterized as varieties
in Mr. Wollaston's admirable work, but which it cannot be doubted would be
ranked as distinct species by many entomologists. Even Ireland has a few
animals, now generally regarded as varieties, but which have been ranked
as species by some zoologists. Several most experienced ornithologists
consider our British red grouse as only a strongly-marked race of a
Norwegian species, whereas the greater number rank it as an undoubted
species peculiar to Great Britain. A wide distance between the homes of
two doubtful forms leads many naturalists to rank both as distinct
species; but what distance, it has been well asked, will suffice? if that
between America and Europe is ample, will that between the Continent and
the Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It
must be admitted that many forms, considered by highly-competent judges as
varieties, have so perfectly the character of species that they are ranked
by other highly-competent judges as good and true species. But to discuss
whether they are rightly called species or varieties, before any
definition of these terms has been generally accepted, is vainly to beat
the air.
Many of the cases of strongly-marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear on the attempt to determine their rank. I will here
give only a single instance,—the well-known one of the primrose and
cowslip, or Primula veris and elatior. These plants differ considerably in
appearance; they have a different flavour and emit a different odour; they
flower at slightly different periods; they grow in somewhat different
stations; they ascend mountains to different heights; they have different
geographical ranges; and lastly, according to very numerous experiments
made during several years by that most careful observer Gartner, they can
be crossed only with much difficulty. We could hardly wish for better
evidence of the two forms being specifically distinct. On the other hand,
they are united by many intermediate links, and it is very doubtful
whether these links are hybrids; and there is, as it seems to me, an
overwhelming amount of experimental evidence, showing that they descend
from common parents, and consequently must be ranked as varieties.
Close investigation, in most cases, will bring naturalists to an agreement
how to rank doubtful forms. Yet it must be confessed, that it is in the
best-known countries that we find the greatest number of forms of doubtful
value. I have been struck with the fact, that if any animal or plant in a
state of nature be highly useful to man, or from any cause closely attract
his attention, varieties of it will almost universally be found recorded.
These varieties, moreover, will be often ranked by some authors as
species. Look at the common oak, how closely it has been studied; yet a
German author makes more than a dozen species out of forms, which are very
generally considered as varieties; and in this country the highest
botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or mere
varieties.
When a young naturalist commences the study of a group of organisms quite
unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he knows
nothing of the amount and kind of variation to which the group is subject;
and this shows, at least, how very generally there is some variation. But
if he confine his attention to one class within one country, he will soon
make up his mind how to rank most of the doubtful forms. His general
tendency will be to make many species, for he will become impressed, just
like the pigeon or poultry-fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he has
little general knowledge of analogical variation in other groups and in
other countries, by which to correct his first impressions. As he extends
the range of his observations, he will meet with more cases of difficulty;
for he will encounter a greater number of closely-allied forms. But if his
observations be widely extended, he will in the end generally be enabled
to make up his own mind which to call varieties and which species; but he
will succeed in this at the expense of admitting much variation,—and
the truth of this admission will often be disputed by other naturalists.
When, moreover, he comes to study allied forms brought from countries not
now continuous, in which case he can hardly hope to find the intermediate
links between his doubtful forms, he will have to trust almost entirely to
analogy, and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species—that is, the forms which in the opinion of
some naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other in an insensible series; and a series impresses the
mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the
systematist, as of high importance for us, as being the first step towards
such slight varieties as are barely thought worth recording in works on
natural history. And I look at varieties which are in any degree more
distinct and permanent, as steps leading to more strongly marked and more
permanent varieties; and at these latter, as leading to sub-species, and
to species. The passage from one stage of difference to another and higher
stage may be, in some cases, due merely to the long-continued action of
different physical conditions in two different regions; but I have not
much faith in this view; and I attribute the passage of a variety, from a
state in which it differs very slightly from its parent to one in which it
differs more, to the action of natural selection in accumulating (as will
hereafter be more fully explained) differences of structure in certain
definite directions. Hence I believe a well-marked variety may be justly
called an incipient species; but whether this belief be justifiable must
be judged of by the general weight of the several facts and views given
throughout this work.
It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this incipient
state become extinct, or they may endure as varieties for very long
periods, as has been shown to be the case by Mr. Wollaston with the
varieties of certain fossil land-shells in Madeira. If a variety were to
flourish so as to exceed in numbers the parent species, it would then rank
as the species, and the species as the variety; or it might come to
supplant and exterminate the parent species; or both might co-exist, and
both rank as independent species. But we shall hereafter have to return to
this subject.
From these remarks it will be seen that I look at the term species, as one
arbitrarily given for the sake of convenience to a set of individuals
closely resembling each other, and that it does not essentially differ
from the term variety, which is given to less distinct and more
fluctuating forms. The term variety, again, in comparison with mere
individual differences, is also applied arbitrarily, and for mere
convenience sake.
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance on
this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for my
future work the discussion of these difficulties, and the tables
themselves of the proportional numbers of the varying species. Dr. Hooker
permits me to add, that after having carefully read my manuscript, and
examined the tables, he thinks that the following statements are fairly
well established. The whole subject, however, treated as it necessarily
here is with much brevity, is rather perplexing, and allusions cannot be
avoided to the "struggle for existence," "divergence of character," and
other questions, hereafter to be discussed.
Alph. De Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected, as
they become exposed to diverse physical conditions, and as they come into
competition (which, as we shall hereafter see, is a far more important
circumstance) with different sets of organic beings. But my tables further
show that, in any limited country, the species which are most common, that
is abound most in individuals, and the species which are most widely
diffused within their own country (and this is a different consideration
from wide range, and to a certain extent from commonness), often give rise
to varieties sufficiently well-marked to have been recorded in botanical
works. Hence it is the most flourishing, or, as they may be called, the
dominant species,—those which range widely over the world, are the
most diffused in their own country, and are the most numerous in
individuals,—which oftenest produce well-marked varieties, or, as I
consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree
permanent, necessarily have to struggle with the other inhabitants of the
country, the species which are already dominant will be the most likely to
yield offspring which, though in some slight degree modified, will still
inherit those advantages that enabled their parents to become dominant
over their compatriots.
If the plants inhabiting a country and described in any Flora be divided
into two equal masses, all those in the larger genera being placed on one
side, and all those in the smaller genera on the other side, a somewhat
larger number of the very common and much diffused or dominant species
will be found on the side of the larger genera. This, again, might have
been anticipated; for the mere fact of many species of the same genus
inhabiting any country, shows that there is something in the organic or
inorganic conditions of that country favourable to the genus; and,
consequently, we might have expected to have found in the larger genera,
or those including many species, a large proportional number of dominant
species. But so many causes tend to obscure this result, that I am
surprised that my tables show even a small majority on the side of the
larger genera. I will here allude to only two causes of obscurity.
Fresh-water and salt-loving plants have generally very wide ranges and are
much diffused, but this seems to be connected with the nature of the
stations inhabited by them, and has little or no relation to the size of
the genera to which the species belong. Again, plants low in the scale of
organisation are generally much more widely diffused than plants higher in
the scale; and here again there is no close relation to the size of the
genera. The cause of lowly-organised plants ranging widely will be
discussed in our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of the
smaller genera; for wherever many closely related species (i.e. species of
the same genus) have been formed, many varieties or incipient species
ought, as a general rule, to be now forming. Where many large trees grow,
we expect to find saplings. Where many species of a genus have been formed
through variation, circumstances have been favourable for variation; and
hence we might expect that the circumstances would generally be still
favourable to variation. On the other hand, if we look at each species as
a special act of creation, there is no apparent reason why more varieties
should occur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into two
nearly equal masses, the species of the larger genera on one side, and
those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the side
of the larger genera present varieties, than on the side of the smaller
genera. Moreover, the species of the large genera which present any
varieties, invariably present a larger average number of varieties than do
the species of the small genera. Both these results follow when another
division is made, and when all the smallest genera, with from only one to
four species, are absolutely excluded from the tables. These facts are of
plain signification on the view that species are only strongly marked and
permanent varieties; for wherever many species of the same genus have been
formed, or where, if we may use the expression, the manufactory of species
has been active, we ought generally to find the manufactory still in
action, more especially as we have every reason to believe the process of
manufacturing new species to be a slow one. And this certainly is the
case, if varieties be looked at as incipient species; for my tables
clearly show as a general rule that, wherever many species of a genus have
been formed, the species of that genus present a number of varieties, that
is of incipient species, beyond the average. It is not that all large
genera are now varying much, and are thus increasing in the number of
their species, or that no small genera are now varying and increasing; for
if this had been so, it would have been fatal to my theory; inasmuch as
geology plainly tells us that small genera have in the lapse of time often
increased greatly in size; and that large genera have often come to their
maxima, declined, and disappeared. All that we want to show is, that where
many species of a genus have been formed, on an average many are still
forming; and this holds good.
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been
found between doubtful forms, naturalists are compelled to come to a
determination by the amount of difference between them, judging by analogy
whether or not the amount suffices to raise one or both to the rank of
species. Hence the amount of difference is one very important criterion in
settling whether two forms should be ranked as species or varieties. Now
Fries has remarked in regard to plants, and Westwood in regard to insects,
that in large genera the amount of difference between the species is often
exceedingly small. I have endeavoured to test this numerically by
averages, and, as far as my imperfect results go, they always confirm the
view. I have also consulted some sagacious and most experienced observers,
and, after deliberation, they concur in this view. In this respect,
therefore, the species of the larger genera resemble varieties, more than
do the species of the smaller genera. Or the case may be put in another
way, and it may be said, that in the larger genera, in which a number of
varieties or incipient species greater than the average are now
manufacturing, many of the species already manufactured still to a certain
extent resemble varieties, for they differ from each other by a less than
usual amount of difference.
Moreover, the species of the large genera are related to each other, in
the same manner as the varieties of any one species are related to each
other. No naturalist pretends that all the species of a genus are equally
distinct from each other; they may generally be divided into sub-genera,
or sections, or lesser groups. As Fries has well remarked, little groups
of species are generally clustered like satellites around certain other
species. And what are varieties but groups of forms, unequally related to
each other, and clustered round certain forms—that is, round their
parent-species? Undoubtedly there is one most important point of
difference between varieties and species; namely, that the amount of
difference between varieties, when compared with each other or with their
parent-species, is much less than that between the species of the same
genus. But when we come to discuss the principle, as I call it, of
Divergence of Character, we shall see how this may be explained, and how
the lesser differences between varieties will tend to increase into the
greater differences between species.
There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed scarcely
more than a truism, for if a variety were found to have a wider range than
that of its supposed parent-species, their denominations ought to be
reversed. But there is also reason to believe, that those species which
are very closely allied to other species, and in so far resemble
varieties, often have much restricted ranges. For instance, Mr. H. C.
Watson has marked for me in the well-sifted London Catalogue of plants
(4th edition) 63 plants which are therein ranked as species, but which he
considers as so closely allied to other species as to be of doubtful
value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this
same catalogue, 53 acknowledged varieties are recorded, and these range
over 7.7 provinces; whereas, the species to which these varieties belong
range over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but which
are almost universally ranked by British botanists as good and true
species.
Finally, then, varieties have the same general characters as species, for
they cannot be distinguished from species,—except, firstly, by the
discovery of intermediate linking forms, and the occurrence of such links
cannot affect the actual characters of the forms which they connect; and
except, secondly, by a certain amount of difference, for two forms, if
differing very little, are generally ranked as varieties, notwithstanding
that intermediate linking forms have not been discovered; but the amount
of difference considered necessary to give to two forms the rank of
species is quite indefinite. In genera having more than the average number
of species in any country, the species of these genera have more than the
average number of varieties. In large genera the species are apt to be
closely, but unequally, allied together, forming little clusters round
certain species. Species very closely allied to other species apparently
have restricted ranges. In all these several respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species have once existed as varieties, and
have thus originated: whereas, these analogies are utterly inexplicable if
each species has been independently created.
We have, also, seen that it is the most flourishing and dominant species
of the larger genera which on an average vary most; and varieties, as we
shall hereafter see, tend to become converted into new and distinct
species. The larger genera thus tend to become larger; and throughout
nature the forms of life which are now dominant tend to become still more
dominant by leaving many modified and dominant descendants. But by steps
hereafter to be explained, the larger genera also tend to break up into
smaller genera. And thus, the forms of life throughout the universe become
divided into groups subordinate to groups.
3. STRUGGLE FOR EXISTENCE.
Bears on natural selection. The term used in a wide sense. Geometrical
powers of increase. Rapid increase of naturalised animals and plants.
Nature of the checks to increase. Competition universal. Effects of
climate. Protection from the number of individuals. Complex relations of
all animals and plants throughout nature. Struggle for life most severe
between individuals and varieties of the same species; often severe
between species of the same genus. The relation of organism to organism
the most important of all relations.
Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual variability;
indeed I am not aware that this has ever been disputed. It is immaterial
for us whether a multitude of doubtful forms be called species or
sub-species or varieties; what rank, for instance, the two or three
hundred doubtful forms of British plants are entitled to hold, if the
existence of any well-marked varieties be admitted. But the mere existence
of individual variability and of some few well-marked varieties, though
necessary as the foundation for the work, helps us but little in
understanding how species arise in nature. How have all those exquisite
adaptations of one part of the organisation to another part, and to the
conditions of life, and of one distinct organic being to another being,
been perfected? We see these beautiful co-adaptations most plainly in the
woodpecker and missletoe; and only a little less plainly in the humblest
parasite which clings to the hairs of a quadruped or feathers of a bird;
in the structure of the beetle which dives through the water; in the
plumed seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of species,
which constitute what are called distinct genera, and which differ from
each other more than do the species of the same genus, arise? All these
results, as we shall more fully see in the next chapter, follow inevitably
from the struggle for life. Owing to this struggle for life, any
variation, however slight and from whatever cause proceeding, if it be in
any degree profitable to an individual of any species, in its infinitely
complex relations to other organic beings and to external nature, will
tend to the preservation of that individual, and will generally be
inherited by its offspring. The offspring, also, will thus have a better
chance of surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is preserved, by the
term of Natural Selection, in order to mark its relation to man's power of
selection. We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand of
Nature. But Natural Selection, as we shall hereafter see, is a power
incessantly ready for action, and is as immeasurably superior to man's
feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence. In
my future work this subject shall be treated, as it well deserves, at much
greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently the
result of his great horticultural knowledge. Nothing is easier than to
admit in words the truth of the universal struggle for life, or more
difficult—at least I have found it so—than constantly to bear
this conclusion in mind. Yet unless it be thoroughly engrained in the
mind, I am convinced that the whole economy of nature, with every fact on
distribution, rarity, abundance, extinction, and variation, will be dimly
seen or quite misunderstood. We behold the face of nature bright with
gladness, we often see superabundance of food; we do not see, or we
forget, that the birds which are idly singing round us mostly live on
insects or seeds, and are thus constantly destroying life; or we forget
how largely these songsters, or their eggs, or their nestlings, are
destroyed by birds and beasts of prey; we do not always bear in mind, that
though food may be now superabundant, it is not so at all seasons of each
recurring year.
I should premise that I use the term Struggle for Existence in a large and
metaphorical sense, including dependence of one being on another, and
including (which is more important) not only the life of the individual,
but success in leaving progeny. Two canine animals in a time of dearth,
may be truly said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a thousand
seeds, of which on an average only one comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds which
already clothe the ground. The missletoe is dependent on the apple and a
few other trees, but can only in a far-fetched sense be said to struggle
with these trees, for if too many of these parasites grow on the same
tree, it will languish and die. But several seedling missletoes, growing
close together on the same branch, may more truly be said to struggle with
each other. As the missletoe is disseminated by birds, its existence
depends on birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these several
senses, which pass into each other, I use for convenience sake the general
term of struggle for existence.
A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its natural
lifetime produces several eggs or seeds, must suffer destruction during
some period of its life, and during some season or occasional year,
otherwise, on the principle of geometrical increase, its numbers would
quickly become so inordinately great that no country could support the
product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either one
individual with another of the same species, or with the individuals of
distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase
of food, and no prudential restraint from marriage. Although some species
may be now increasing, more or less rapidly, in numbers, all cannot do so,
for the world would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man has
doubled in twenty-five years, and at this rate, in a few thousand years,
there would literally not be standing room for his progeny. Linnaeus has
calculated that if an annual plant produced only two seeds—and there
is no plant so unproductive as this—and their seedlings next year
produced two, and so on, then in twenty years there would be a million
plants. The elephant is reckoned to be the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum rate
of natural increase: it will be under the mark to assume that it breeds
when thirty years old, and goes on breeding till ninety years old,
bringing forth three pair of young in this interval; if this be so, at the
end of the fifth century there would be alive fifteen million elephants,
descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when circumstances
have been favourable to them during two or three following seasons. Still
more striking is the evidence from our domestic animals of many kinds
which have run wild in several parts of the world: if the statements of
the rate of increase of slow-breeding cattle and horses in South America,
and latterly in Australia, had not been well authenticated, they would
have been quite incredible. So it is with plants: cases could be given of
introduced plants which have become common throughout whole islands in a
period of less than ten years. Several of the plants now most numerous
over the wide plains of La Plata, clothing square leagues of surface
almost to the exclusion of all other plants, have been introduced from
Europe; and there are plants which now range in India, as I hear from Dr.
Falconer, from Cape Comorin to the Himalaya, which have been imported from
America since its discovery. In such cases, and endless instances could be
given, no one supposes that the fertility of these animals or plants has
been suddenly and temporarily increased in any sensible degree. The
obvious explanation is that the conditions of life have been very
favourable, and that there has consequently been less destruction of the
old and young, and that nearly all the young have been enabled to breed.
In such cases the geometrical ratio of increase, the result of which never
fails to be surprising, simply explains the extraordinarily rapid increase
and wide diffusion of naturalised productions in their new homes.
In a state of nature almost every plant produces seed, and amongst animals
there are very few which do not annually pair. Hence we may confidently
assert, that all plants and animals are tending to increase at a
geometrical ratio, that all would most rapidly stock every station in
which they could any how exist, and that the geometrical tendency to
increase must be checked by destruction at some period of life. Our
familiarity with the larger domestic animals tends, I think, to mislead
us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of nature
an equal number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or seeds
by the thousand, and those which produce extremely few, is, that the
slow-breeders would require a few more years to people, under favourable
conditions, a whole district, let it be ever so large. The condor lays a
couple of eggs and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two: the Fulmar petrel lays but one
egg, yet it is believed to be the most numerous bird in the world. One fly
deposits hundreds of eggs, and another, like the hippobosca, a single one;
but this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species, which depend on a rapidly fluctuating amount
of food, for it allows them rapidly to increase in number. But the real
importance of a large number of eggs or seeds is to make up for much
destruction at some period of life; and this period in the great majority
of cases is an early one. If an animal can in any way protect its own eggs
or young, a small number may be produced, and yet the average stock be
fully kept up; but if many eggs or young are destroyed, many must be
produced, or the species will become extinct. It would suffice to keep up
the full number of a tree, which lived on an average for a thousand years,
if a single seed were produced once in a thousand years, supposing that
this seed were never destroyed, and could be ensured to germinate in a
fitting place. So that in all cases, the average number of any animal or
plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single
organic being around us may be said to be striving to the utmost to
increase in numbers; that each lives by a struggle at some period of its
life; that heavy destruction inevitably falls either on the young or old,
during each generation or at recurrent intervals. Lighten any check,
mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount. The face of Nature may
be compared to a yielding surface, with ten thousand sharp wedges packed
close together and driven inwards by incessant blows, sometimes one wedge
being struck, and then another with greater force.
What checks the natural tendency of each species to increase in number is
most obscure. Look at the most vigorous species; by as much as it swarms
in numbers, by so much will its tendency to increase be still further
increased. We know not exactly what the checks are in even one single
instance. Nor will this surprise any one who reflects how ignorant we are
on this head, even in regard to mankind, so incomparably better known than
any other animal. This subject has been ably treated by several authors,
and I shall, in my future work, discuss some of the checks at considerable
length, more especially in regard to the feral animals of South America.
Here I will make only a few remarks, just to recall to the reader's mind
some of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is a
vast destruction of seeds, but, from some observations which I have made,
I believe that it is the seedlings which suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece of
ground three feet long and two wide, dug and cleared, and where there
could be no choking from other plants, I marked all the seedlings of our
native weeds as they came up, and out of the 357 no less than 295 were
destroyed, chiefly by slugs and insects. If turf which has long been mown,
and the case would be the same with turf closely browsed by quadrupeds, be
let to grow, the more vigorous plants gradually kill the less vigorous,
though fully grown, plants: thus out of twenty species growing on a little
plot of turf (three feet by four) nine species perished from the other
species being allowed to grow up freely.
The amount of food for each species of course gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends chiefly on
the destruction of vermin. If not one head of game were shot during the
next twenty years in England, and, at the same time, if no vermin were
destroyed, there would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually killed. On
the other hand, in some cases, as with the elephant and rhinoceros, none
are destroyed by beasts of prey: even the tiger in India most rarely dares
to attack a young elephant protected by its dam.
Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought, I believe to
be the most effective of all checks. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my own grounds; and this is
a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it
brings on the most severe struggle between the individuals, whether of the
same or of distinct species, which subsist on the same kind of food. Even
when climate, for instance extreme cold, acts directly, it will be the
least vigorous, or those which have got least food through the advancing
winter, which will suffer most. When we travel from south to north, or
from a damp region to a dry, we invariably see some species gradually
getting rarer and rarer, and finally disappearing; and the change of
climate being conspicuous, we are tempted to attribute the whole effect to
its direct action. But this is a very false view: we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competitors
for the same place and food; and if these enemies or competitors be in the
least degree favoured by any slight change of climate, they will increase
in numbers, and, as each area is already fully stocked with inhabitants,
the other species will decrease. When we travel southward and see a
species decreasing in numbers, we may feel sure that the cause lies quite
as much in other species being favoured, as in this one being hurt. So it
is when we travel northward, but in a somewhat lesser degree, for the
number of species of all kinds, and therefore of competitors, decreases
northwards; hence in going northward, or in ascending a mountain, we far
oftener meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southwards or in descending a mountain.
When we reach the Arctic regions, or snow-capped summits, or absolute
deserts, the struggle for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species, we
may clearly see in the prodigious number of plants in our gardens which
can perfectly well endure our climate, but which never become naturalised,
for they cannot compete with our native plants, nor resist destruction by
our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue: and here
we have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of diffusion
amongst the crowded animals, been disproportionably favoured: and here
comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the same
species, relatively to the numbers of its enemies, is absolutely necessary
for its preservation. Thus we can easily raise plenty of corn and
rape-seed, etc., in our fields, because the seeds are in great excess
compared with the number of birds which feed on them; nor can the birds,
though having a superabundance of food at this one season, increase in
number proportionally to the supply of seed, as their numbers are checked
during winter: but any one who has tried, knows how troublesome it is to
get seed from a few wheat or other such plants in a garden; I have in this
case lost every single seed. This view of the necessity of a large stock
of the same species for its preservation, explains, I believe, some
singular facts in nature, such as that of very rare plants being sometimes
extremely abundant in the few spots where they do occur; and that of some
social plants being social, that is, abounding in individuals, even on the
extreme confines of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so favourable
that many could exist together, and thus save each other from utter
destruction. I should add that the good effects of frequent intercrossing,
and the ill effects of close interbreeding, probably come into play in
some of these cases; but on this intricate subject I will not here
enlarge.
Many cases are on record showing how complex and unexpected are the checks
and relations between organic beings, which have to struggle together in
the same country. I will give only a single instance, which, though a
simple one, has interested me. In Staffordshire, on the estate of a
relation where I had ample means of investigation, there was a large and
extremely barren heath, which had never been touched by the hand of man;
but several hundred acres of exactly the same nature had been enclosed
twenty-five years previously and planted with Scotch fir. The change in
the native vegetation of the planted part of the heath was most
remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not
counting grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been still
greater, for six insectivorous birds were very common in the plantations,
which were not to be seen on the heath; and the heath was frequented by
two or three distinct insectivorous birds. Here we see how potent has been
the effect of the introduction of a single tree, nothing whatever else
having been done, with the exception that the land had been enclosed, so
that cattle could not enter. But how important an element enclosure is, I
plainly saw near Farnham, in Surrey. Here there are extensive heaths, with
a few clumps of old Scotch firs on the distant hill-tops: within the last
ten years large spaces have been enclosed, and self-sown firs are now
springing up in multitudes, so close together that all cannot live.
When I ascertained that these young trees had not been sown or planted, I
was so much surprised at their numbers that I went to several points of
view, whence I could examine hundreds of acres of the unenclosed heath,
and literally I could not see a single Scotch fir, except the old planted
clumps. But on looking closely between the stems of the heath, I found a
multitude of seedlings and little trees, which had been perpetually
browsed down by the cattle. In one square yard, at a point some hundred
yards distant from one of the old clumps, I counted thirty-two little
trees; and one of them, judging from the rings of growth, had during
twenty-six years tried to raise its head above the stems of the heath, and
had failed. No wonder that, as soon as the land was enclosed, it became
thickly clothed with vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would ever have imagined
that cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence of
cattle. Perhaps Paraguay offers the most curious instance of this; for
here neither cattle nor horses nor dogs have ever run wild, though they
swarm southward and northward in a feral state; and Azara and Rengger have
shown that this is caused by the greater number in Paraguay of a certain
fly, which lays its eggs in the navels of these animals when first born.
The increase of these flies, numerous as they are, must be habitually
checked by some means, probably by birds. Hence, if certain insectivorous
birds (whose numbers are probably regulated by hawks or beasts of prey)
were to increase in Paraguay, the flies would decrease—then cattle
and horses would become feral, and this would certainly greatly alter (as
indeed I have observed in parts of South America) the vegetation: this
again would largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we
have ended with them. Not that in nature the relations can ever be as
simple as this. Battle within battle must ever be recurring with varying
success; and yet in the long-run the forces are so nicely balanced, that
the face of nature remains uniform for long periods of time, though
assuredly the merest trifle would often give the victory to one organic
being over another. Nevertheless so profound is our ignorance, and so high
our presumption, that we marvel when we hear of the extinction of an
organic being; and as we do not see the cause, we invoke cataclysms to
desolate the world, or invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens, in this part of England, is never visited by insects, and
consequently, from its peculiar structure, never can set a seed. Many of
our orchidaceous plants absolutely require the visits of moths to remove
their pollen-masses and thus to fertilise them. I have, also, reason to
believe that humble-bees are indispensable to the fertilisation of the
heartsease (Viola tricolor), for other bees do not visit this flower. From
experiments which I have tried, I have found that the visits of bees, if
not indispensable, are at least highly beneficial to the fertilisation of
our clovers; but humble-bees alone visit the common red clover (Trifolium
pratense), as other bees cannot reach the nectar. Hence I have very little
doubt, that if the whole genus of humble-bees became extinct or very rare
in England, the heartsease and red clover would become very rare, or
wholly disappear. The number of humble-bees in any district depends in a
great degree on the number of field-mice, which destroy their combs and
nests; and Mr. H. Newman, who has long attended to the habits of
humble-bees, believes that "more than two thirds of them are thus
destroyed all over England." Now the number of mice is largely dependent,
as every one knows, on the number of cats; and Mr. Newman says, "Near
villages and small towns I have found the nests of humble-bees more
numerous than elsewhere, which I attribute to the number of cats that
destroy the mice." Hence it is quite credible that the presence of a
feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!
In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come into
play; some one check or some few being generally the most potent, but all
concurring in determining the average number or even the existence of the
species. In some cases it can be shown that widely-different checks act on
the same species in different districts. When we look at the plants and
bushes clothing an entangled bank, we are tempted to attribute their
proportional numbers and kinds to what we call chance. But how false a
view is this! Every one has heard that when an American forest is cut
down, a very different vegetation springs up; but it has been observed
that the trees now growing on the ancient Indian mounds, in the Southern
United States, display the same beautiful diversity and proportion of
kinds as in the surrounding virgin forests. What a struggle between the
several kinds of trees must here have gone on during long centuries, each
annually scattering its seeds by the thousand; what war between insect and
insect—between insects, snails, and other animals with birds and
beasts of prey—all striving to increase, and all feeding on each
other or on the trees or their seeds and seedlings, or on the other plants
which first clothed the ground and thus checked the growth of the trees!
Throw up a handful of feathers, and all must fall to the ground according
to definite laws; but how simple is this problem compared to the action
and reaction of the innumerable plants and animals which have determined,
in the course of centuries, the proportional numbers and kinds of trees
now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
often the case with those which may strictly be said to struggle with each
other for existence, as in the case of locusts and grass-feeding
quadrupeds. But the struggle almost invariably will be most severe between
the individuals of the same species, for they frequent the same districts,
require the same food, and are exposed to the same dangers. In the case of
varieties of the same species, the struggle will generally be almost
equally severe, and we sometimes see the contest soon decided: for
instance, if several varieties of wheat be sown together, and the mixed
seed be resown, some of the varieties which best suit the soil or climate,
or are naturally the most fertile, will beat the others and so yield more
seed, and will consequently in a few years quite supplant the other
varieties. To keep up a mixed stock of even such extremely close varieties
as the variously coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in numbers and disappear. So again
with the varieties of sheep: it has been asserted that certain
mountain-varieties will starve out other mountain-varieties, so that they
cannot be kept together. The same result has followed from keeping
together different varieties of the medicinal leech. It may even be
doubted whether the varieties of any one of our domestic plants or animals
have so exactly the same strength, habits, and constitution, that the
original proportions of a mixed stock could be kept up for half a dozen
generations, if they were allowed to struggle together, like beings in a
state of nature, and if the seed or young were not annually sorted.
As species of the same genus have usually, though by no means invariably,
some similarity in habits and constitution, and always in structure, the
struggle will generally be more severe between species of the same genus,
when they come into competition with each other, than between species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under the
most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. One species of charlock
will supplant another, and so in other cases. We can dimly see why the
competition should be most severe between allied forms, which fill nearly
the same place in the economy of nature; but probably in no one case could
we precisely say why one species has been victorious over another in the
great battle of life.
A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related, in
the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or
from which it has to escape, or on which it preys. This is obvious in the
structure of the teeth and talons of the tiger; and in that of the legs
and claws of the parasite which clings to the hair on the tiger's body.
But in the beautifully plumed seed of the dandelion, and in the flattened
and fringed legs of the water-beetle, the relation seems at first confined
to the elements of air and water. Yet the advantage of plumed seeds no
doubt stands in the closest relation to the land being already thickly
clothed by other plants; so that the seeds may be widely distributed and
fall on unoccupied ground. In the water-beetle, the structure of its legs,
so well adapted for diving, allows it to compete with other aquatic
insects, to hunt for its own prey, and to escape serving as prey to other
animals.
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and
beans), when sown in the midst of long grass, I suspect that the chief use
of the nutriment in the seed is to favour the growth of the young
seedling, whilst struggling with other plants growing vigorously all
around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case we
can clearly see that if we wished in imagination to give the plant the
power of increasing in number, we should have to give it some advantage
over its competitors, or over the animals which preyed on it. On the
confines of its geographical range, a change of constitution with respect
to climate would clearly be an advantage to our plant; but we have reason
to believe that only a few plants or animals range so far, that they are
destroyed by the rigour of the climate alone. Not until we reach the
extreme confines of life, in the arctic regions or on the borders of an
utter desert, will competition cease. The land may be extremely cold or
dry, yet there will be competition between some few species, or between
the individuals of the same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the
same as in its former home, yet the conditions of its life will generally
be changed in an essential manner. If we wished to increase its average
numbers in its new home, we should have to modify it in a different way to
what we should have done in its native country; for we should have to give
it some advantage over a different set of competitors or enemies.
It is good thus to try in our imagination to give any form some advantage
over another. Probably in no single instance should we know what to do, so
as to succeed. It will convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it seems to
be difficult to acquire. All that we can do, is to keep steadily in mind
that each organic being is striving to increase at a geometrical ratio;
that each at some period of its life, during some season of the year,
during each generation or at intervals, has to struggle for life, and to
suffer great destruction. When we reflect on this struggle, we may console
ourselves with the full belief, that the war of nature is not incessant,
that no fear is felt, that death is generally prompt, and that the
vigorous, the healthy, and the happy survive and multiply.
4. NATURAL SELECTION.
Natural Selection: its power compared with man's selection, its power on
characters of trifling importance, its power at all ages and on both
sexes. Sexual Selection. On the generality of intercrosses between
individuals of the same species. Circumstances favourable and unfavourable
to Natural Selection, namely, intercrossing, isolation, number of
individuals. Slow action. Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of any
small area, and to naturalisation. Action of Natural Selection, through
Divergence of Character and Extinction, on the descendants from a common
parent. Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection, which
we have seen is so potent in the hands of man, apply in nature? I think we
shall see that it can act most effectually. Let it be borne in mind in
what an endless number of strange peculiarities our domestic productions,
and, in a lesser degree, those under nature, vary; and how strong the
hereditary tendency is. Under domestication, it may be truly said that the
whole organisation becomes in some degree plastic. Let it be borne in mind
how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life. Can
it, then, be thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should sometimes occur in
the course of thousands of generations? If such do occur, can we doubt
(remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight, over
others, would have the best chance of surviving and of procreating their
kind? On the other hand, we may feel sure that any variation in the least
degree injurious would be rigidly destroyed. This preservation of
favourable variations and the rejection of injurious variations, I call
Natural Selection. Variations neither useful nor injurious would not be
affected by natural selection, and would be left a fluctuating element, as
perhaps we see in the species called polymorphic.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants would
almost immediately undergo a change, and some species might become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of some of the
inhabitants, independently of the change of climate itself, would most
seriously affect many of the others. If the country were open on its
borders, new forms would certainly immigrate, and this also would
seriously disturb the relations of some of the former inhabitants. Let it
be remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a country
partly surrounded by barriers, into which new and better adapted forms
could not freely enter, we should then have places in the economy of
nature which would assuredly be better filled up, if some of the original
inhabitants were in some manner modified; for, had the area been open to
immigration, these same places would have been seized on by intruders. In
such case, every slight modification, which in the course of ages chanced
to arise, and which in any way favoured the individuals of any of the
species, by better adapting them to their altered conditions, would tend
to be preserved; and natural selection would thus have free scope for the
work of improvement.
We have reason to believe, as stated in the first chapter, that a change
in the conditions of life, by specially acting on the reproductive system,
causes or increases variability; and in the foregoing case the conditions
of life are supposed to have undergone a change, and this would manifestly
be favourable to natural selection, by giving a better chance of
profitable variations occurring; and unless profitable variations do
occur, natural selection can do nothing. Not that, as I believe, any
extreme amount of variability is necessary; as man can certainly produce
great results by adding up in any given direction mere individual
differences, so could Nature, but far more easily, from having
incomparably longer time at her disposal. Nor do I believe that any great
physical change, as of climate, or any unusual degree of isolation to
check immigration, is actually necessary to produce new and unoccupied
places for natural selection to fill up by modifying and improving some of
the varying inhabitants. For as all the inhabitants of each country are
struggling together with nicely balanced forces, extremely slight
modifications in the structure or habits of one inhabitant would often
give it an advantage over others; and still further modifications of the
same kind would often still further increase the advantage. No country can
be named in which all the native inhabitants are now so perfectly adapted
to each other and to the physical conditions under which they live, that
none of them could anyhow be improved; for in all countries, the natives
have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as foreigners
have thus everywhere beaten some of the natives, we may safely conclude
that the natives might have been modified with advantage, so as to have
better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature effect?
Man can act only on external and visible characters: nature cares nothing
for appearances, except in so far as they may be useful to any being. She
can act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects only for his own
good; Nature only for that of the being which she tends. Every selected
character is fully exercised by her; and the being is placed under
well-suited conditions of life. Man keeps the natives of many climates in
the same country; he seldom exercises each selected character in some
peculiar and fitting manner; he feeds a long and a short beaked pigeon on
the same food; he does not exercise a long-backed or long-legged quadruped
in any peculiar manner; he exposes sheep with long and short wool to the
same climate. He does not allow the most vigorous males to struggle for
the females. He does not rigidly destroy all inferior animals, but
protects during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half-monstrous form; or
at least by some modification prominent enough to catch his eye, or to be
plainly useful to him. Under nature, the slightest difference of structure
or constitution may well turn the nicely-balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts of
man! how short his time! and consequently how poor will his products be,
compared with those accumulated by nature during whole geological periods.
Can we wonder, then, that nature's productions should be far "truer" in
character than man's productions; that they should be infinitely better
adapted to the most complex conditions of life, and should plainly bear
the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting that
which is bad, preserving and adding up all that is good; silently and
insensibly working, whenever and wherever opportunity offers, at the
improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress,
until the hand of time has marked the long lapse of ages, and then so
imperfect is our view into long past geological ages, that we only see
that the forms of life are now different from what they formerly were.
Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, and the black-grouse that
of peaty earth, we must believe that these tints are of service to these
birds and insects in preserving them from danger. Grouse, if not destroyed
at some period of their lives, would increase in countless numbers; they
are known to suffer largely from birds of prey; and hawks are guided by
eyesight to their prey,—so much so, that on parts of the Continent
persons are warned not to keep white pigeons, as being the most liable to
destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of
grouse, and in keeping that colour, when once acquired, true and constant.
Nor ought we to think that the occasional destruction of an animal of any
particular colour would produce little effect: we should remember how
essential it is in a flock of white sheep to destroy every lamb with the
faintest trace of black. In plants the down on the fruit and the colour of
the flesh are considered by botanists as characters of the most trifling
importance: yet we hear from an excellent horticulturist, Downing, that in
the United States smooth-skinned fruits suffer far more from a beetle, a
curculio, than those with down; that purple plums suffer far more from a
certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh. If,
with all the aids of art, these slight differences make a great difference
in cultivating the several varieties, assuredly, in a state of nature,
where the trees would have to struggle with other trees and with a host of
enemies, such differences would effectually settle which variety, whether
a smooth or downy, a yellow or purple fleshed fruit, should succeed.
In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem to be quite unimportant, we
must not forget that climate, food, etc., probably produce some slight and
direct effect. It is, however, far more necessary to bear in mind that
there are many unknown laws of correlation of growth, which, when one part
of the organisation is modified through variation, and the modifications
are accumulated by natural selection for the good of the being, will cause
other modifications, often of the most unexpected nature.
As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the same
period;—for instance, in the seeds of the many varieties of our
culinary and agricultural plants; in the caterpillar and cocoon stages of
the varieties of the silkworm; in the eggs of poultry, and in the colour
of the down of their chickens; in the horns of our sheep and cattle when
nearly adult;—so in a state of nature, natural selection will be
enabled to act on and modify organic beings at any age, by the
accumulation of profitable variations at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its seeds
more and more widely disseminated by the wind, I can see no greater
difficulty in this being effected through natural selection, than in the
cotton-planter increasing and improving by selection the down in the pods
on his cotton-trees. Natural selection may modify and adapt the larva of
an insect to a score of contingencies, wholly different from those which
concern the mature insect. These modifications will no doubt affect,
through the laws of correlation, the structure of the adult; and probably
in the case of those insects which live only for a few hours, and which
never feed, a large part of their structure is merely the correlated
result of successive changes in the structure of their larvae. So,
conversely, modifications in the adult will probably often affect the
structure of the larva; but in all cases natural selection will ensure
that modifications consequent on other modifications at a different period
of life, shall not be in the least degree injurious: for if they became
so, they would cause the extinction of the species.
Natural selection will modify the structure of the young in relation to
the parent, and of the parent in relation to the young. In social animals
it will adapt the structure of each individual for the benefit of the
community; if each in consequence profits by the selected change. What
natural selection cannot do, is to modify the structure of one species,
without giving it any advantage, for the good of another species; and
though statements to this effect may be found in works of natural history,
I cannot find one case which will bear investigation. A structure used
only once in an animal's whole life, if of high importance to it, might be
modified to any extent by natural selection; for instance, the great jaws
possessed by certain insects, and used exclusively for opening the cocoon—or
the hard tip to the beak of nestling birds, used for breaking the egg. It
has been asserted, that of the best short-beaked tumbler-pigeons more
perish in the egg than are able to get out of it; so that fanciers assist
in the act of hatching. Now, if nature had to make the beak of a
full-grown pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of the young birds within the egg, which had the
most powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be selected,
the thickness of the shell being known to vary like every other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, the same fact probably occurs
under nature, and if so, natural selection will be able to modify one sex
in its functional relations to the other sex, or in relation to wholly
different habits of life in the two sexes, as is sometimes the case with
insects. And this leads me to say a few words on what I call Sexual
Selection. This depends, not on a struggle for existence, but on a
struggle between the males for possession of the females; the result is
not death to the unsuccessful competitor, but few or no offspring. Sexual
selection is, therefore, less rigorous than natural selection. Generally,
the most vigorous males, those which are best fitted for their places in
nature, will leave most progeny. But in many cases, victory will depend
not on general vigour, but on having special weapons, confined to the male
sex. A hornless stag or spurless cock would have a poor chance of leaving
offspring. Sexual selection by always allowing the victor to breed might
surely give indomitable courage, length to the spur, and strength to the
wing to strike in the spurred leg, as well as the brutal cock-fighter, who
knows well that he can improve his breed by careful selection of the best
cocks. How low in the scale of nature this law of battle descends, I know
not; male alligators have been described as fighting, bellowing, and
whirling round, like Indians in a war-dance, for the possession of the
females; male salmons have been seen fighting all day long; male
stag-beetles often bear wounds from the huge mandibles of other males. The
war is, perhaps, severest between the males of polygamous animals, and
these seem oftenest provided with special weapons. The males of
carnivorous animals are already well armed; though to them and to others,
special means of defence may be given through means of sexual selection,
as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw
to the male salmon; for the shield may be as important for victory, as the
sword or spear.
Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the severest
rivalry between the males of many species to attract by singing the
females. The rock-thrush of Guiana, birds of Paradise, and some others,
congregate; and successive males display their gorgeous plumage and
perform strange antics before the females, which standing by as
spectators, at last choose the most attractive partner. Those who have
closely attended to birds in confinement well know that they often take
individual preferences and dislikes: thus Sir R. Heron has described how
one pied peacock was eminently attractive to all his hen birds. It may
appear childish to attribute any effect to such apparently weak means: I
cannot here enter on the details necessary to support this view; but if
man can in a short time give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no good reason to doubt
that female birds, by selecting, during thousands of generations, the most
melodious or beautiful males, according to their standard of beauty, might
produce a marked effect. I strongly suspect that some well-known laws with
respect to the plumage of male and female birds, in comparison with the
plumage of the young, can be explained on the view of plumage having been
chiefly modified by sexual selection, acting when the birds have come to
the breeding age or during the breeding season; the modifications thus
produced being inherited at corresponding ages or seasons, either by the
males alone, or by the males and females; but I have not space here to
enter on this subject.
Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour, or
ornament, such differences have been mainly caused by sexual selection;
that is, individual males have had, in successive generations, some slight
advantage over other males, in their weapons, means of defence, or charms;
and have transmitted these advantages to their male offspring. Yet, I
would not wish to attribute all such sexual differences to this agency:
for we see peculiarities arising and becoming attached to the male sex in
our domestic animals (as the wattle in male carriers, horn-like
protuberances in the cocks of certain fowls, etc.), which we cannot
believe to be either useful to the males in battle, or attractive to the
females. We see analogous cases under nature, for instance, the tuft of
hair on the breast of the turkey-cock, which can hardly be either useful
or ornamental to this bird;—indeed, had the tuft appeared under
domestication, it would have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some by
craft, some by strength, and some by fleetness; and let us suppose that
the fleetest prey, a deer for instance, had from any change in the country
increased in numbers, or that other prey had decreased in numbers, during
that season of the year when the wolf is hardest pressed for food. I can
under such circumstances see no reason to doubt that the swiftest and
slimmest wolves would have the best chance of surviving, and so be
preserved or selected,—provided always that they retained strength
to master their prey at this or at some other period of the year, when
they might be compelled to prey on other animals. I can see no more reason
to doubt this, than that man can improve the fleetness of his greyhounds
by careful and methodical selection, or by that unconscious selection
which results from each man trying to keep the best dogs without any
thought of modifying the breed.
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable; for
we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and almost
nightly catching woodcocks or snipes. The tendency to catch rats rather
than mice is known to be inherited. Now, if any slight innate change of
habit or of structure benefited an individual wolf, it would have the best
chance of surviving and of leaving offspring. Some of its young would
probably inherit the same habits or structure, and by the repetition of
this process, a new variety might be formed which would either supplant or
coexist with the parent-form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands, would naturally
be forced to hunt different prey; and from the continued preservation of
the individuals best fitted for the two sites, two varieties might slowly
be formed. These varieties would cross and blend where they met; but to
this subject of intercrossing we shall soon have to return. I may add,
that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky, with
shorter legs, which more frequently attacks the shepherd's flocks.
Let us now take a more complex case. Certain plants excrete a sweet juice,
apparently for the sake of eliminating something injurious from their sap:
this is effected by glands at the base of the stipules in some
Leguminosae, and at the back of the leaf of the common laurel. This juice,
though small in quantity, is greedily sought by insects. Let us now
suppose a little sweet juice or nectar to be excreted by the inner bases
of the petals of a flower. In this case insects in seeking the nectar
would get dusted with pollen, and would certainly often transport the
pollen from one flower to the stigma of another flower. The flowers of two
distinct individuals of the same species would thus get crossed; and the
act of crossing, we have good reason to believe (as will hereafter be more
fully alluded to), would produce very vigorous seedlings, which
consequently would have the best chance of flourishing and surviving. Some
of these seedlings would probably inherit the nectar-excreting power.
Those individual flowers which had the largest glands or nectaries, and
which excreted most nectar, would be oftenest visited by insects, and
would be oftenest crossed; and so in the long-run would gain the upper
hand. Those flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insects which visited
them, so as to favour in any degree the transportal of their pollen from
flower to flower, would likewise be favoured or selected. We might have
taken the case of insects visiting flowers for the sake of collecting
pollen instead of nectar; and as pollen is formed for the sole object of
fertilisation, its destruction appears a simple loss to the plant; yet if
a little pollen were carried, at first occasionally and then habitually,
by the pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed, it might
still be a great gain to the plant; and those individuals which produced
more and more pollen, and had larger and larger anthers, would be
selected.
When our plant, by this process of the continued preservation or natural
selection of more and more attractive flowers, had been rendered highly
attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they can most
effectually do this, I could easily show by many striking instances. I
will give only one—not as a very striking case, but as likewise
illustrating one step in the separation of the sexes of plants, presently
to be alluded to. Some holly-trees bear only male flowers, which have four
stamens producing rather a small quantity of pollen, and a rudimentary
pistil; other holly-trees bear only female flowers; these have a
full-sized pistil, and four stamens with shrivelled anthers, in which not
a grain of pollen can be detected. Having found a female tree exactly
sixty yards from a male tree, I put the stigmas of twenty flowers, taken
from different branches, under the microscope, and on all, without
exception, there were pollen-grains, and on some a profusion of pollen. As
the wind had set for several days from the female to the male tree, the
pollen could not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every
female flower which I examined had been effectually fertilised by the
bees, accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in another flower or on another plant. In
plants under culture and placed under new conditions of life, sometimes
the male organs and sometimes the female organs become more or less
impotent; now if we suppose this to occur in ever so slight a degree under
nature, then as pollen is already carried regularly from flower to flower,
and as a more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals with
this tendency more and more increased, would be continually favoured or
selected, until at last a complete separation of the sexes would be
effected.
Let us now turn to the nectar-feeding insects in our imaginary case: we
may suppose the plant of which we have been slowly increasing the nectar
by continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many facts,
showing how anxious bees are to save time; for instance, their habit of
cutting holes and sucking the nectar at the bases of certain flowers,
which they can, with a very little more trouble, enter by the mouth.
Bearing such facts in mind, I can see no reason to doubt that an
accidental deviation in the size and form of the body, or in the curvature
and length of the proboscis, etc., far too slight to be appreciated by us,
might profit a bee or other insect, so that an individual so characterised
would be able to obtain its food more quickly, and so have a better chance
of living and leaving descendants. Its descendants would probably inherit
a tendency to a similar slight deviation of structure. The tubes of the
corollas of the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length; yet the
hive-bee can easily suck the nectar out of the incarnate clover, but not
out of the common red clover, which is visited by humble-bees alone; so
that whole fields of the red clover offer in vain an abundant supply of
precious nectar to the hive-bee. Thus it might be a great advantage to the
hive-bee to have a slightly longer or differently constructed proboscis.
On the other hand, I have found by experiment that the fertility of clover
greatly depends on bees visiting and moving parts of the corolla, so as to
push the pollen on to the stigmatic surface. Hence, again, if humble-bees
were to become rare in any country, it might be a great advantage to the
red clover to have a shorter or more deeply divided tube to its corolla,
so that the hive-bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one after
the other, modified and adapted in the most perfect manner to each other,
by the continued preservation of individuals presenting mutual and
slightly favourable deviations of structure.
I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which were
at first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now very seldom
hear the action, for instance, of the coast-waves, called a trifling and
insignificant cause, when applied to the excavation of gigantic valleys or
to the formation of the longest lines of inland cliffs. Natural selection
can act only by the preservation and accumulation of infinitesimally small
inherited modifications, each profitable to the preserved being; and as
modern geology has almost banished such views as the excavation of a great
valley by a single diluvial wave, so will natural selection, if it be a
true principle, banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always unite for each birth; but in the case of hermaphrodites this
is far from obvious. Nevertheless I am strongly inclined to believe that
with all hermaphrodites two individuals, either occasionally or
habitually, concur for the reproduction of their kind. This view, I may
add, was first suggested by Andrew Knight. We shall presently see its
importance; but I must here treat the subject with extreme brevity, though
I have the materials prepared for an ample discussion. All vertebrate
animals, all insects, and some other large groups of animals, pair for
each birth. Modern research has much diminished the number of supposed
hermaphrodites, and of real hermaphrodites a large number pair; that is,
two individuals regularly unite for reproduction, which is all that
concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are
hermaphrodites. What reason, it may be asked, is there for supposing in
these cases that two individuals ever concur in reproduction? As it is
impossible here to enter on details, I must trust to some general
considerations alone.
In the first place, I have collected so large a body of facts, showing, in
accordance with the almost universal belief of breeders, that with animals
and plants a cross between different varieties, or between individuals of
the same variety but of another strain, gives vigour and fertility to the
offspring; and on the other hand, that CLOSE interbreeding diminishes
vigour and fertility; that these facts alone incline me to believe that it
is a general law of nature (utterly ignorant though we be of the meaning
of the law) that no organic being self-fertilises itself for an eternity
of generations; but that a cross with another individual is occasionally—perhaps
at very long intervals—indispensable.
On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers
have their anthers and stigmas fully exposed to the weather! but if an
occasional cross be indispensable, the fullest freedom for the entrance of
pollen from another individual will explain this state of exposure, more
especially as the plant's own anthers and pistil generally stand so close
together that self-fertilisation seems almost inevitable. Many flowers, on
the other hand, have their organs of fructification closely enclosed, as
in the great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure of
the flower and the manner in which bees suck the nectar; for, in doing
this, they either push the flower's own pollen on the stigma, or bring
pollen from another flower. So necessary are the visits of bees to
papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits be
prevented. Now, it is scarcely possible that bees should fly from flower
to flower, and not carry pollen from one to the other, to the great good,
as I believe, of the plant. Bees will act like a camel-hair pencil, and it
is quite sufficient just to touch the anthers of one flower and then the
stigma of another with the same brush to ensure fertilisation; but it must
not be supposed that bees would thus produce a multitude of hybrids
between distinct species; for if you bring on the same brush a plant's own
pollen and pollen from another species, the former will have such a
prepotent effect, that it will invariably and completely destroy, as has
been shown by Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely
to ensure self-fertilisation; and no doubt it is useful for this end: but,
the agency of insects is often required to cause the stamens to spring
forward, as Kolreuter has shown to be the case with the barberry; and
curiously in this very genus, which seems to have a special contrivance
for self-fertilisation, it is well known that if very closely-allied forms
or varieties are planted near each other, it is hardly possible to raise
pure seedlings, so largely do they naturally cross. In many other cases,
far from there being any aids for self-fertilisation, there are special
contrivances, as I could show from the writings of C. C. Sprengel and from
my own observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which every one of the infinitely
numerous pollen-granules are swept out of the conjoined anthers of each
flower, before the stigma of that individual flower is ready to receive
them; and as this flower is never visited, at least in my garden, by
insects, it never sets a seed, though by placing pollen from one flower on
the stigma of another, I raised plenty of seedlings; and whilst another
species of Lobelia growing close by, which is visited by bees, seeds
freely. In very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving its own pollen,
yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers
burst before the stigma is ready for fertilisation, or the stigma is ready
before the pollen of that flower is ready, so that these plants have in
fact separated sexes, and must habitually be crossed. How strange are
these facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for instance,
I raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind, and
some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens, but by those
of the many other flowers on the same plant. How, then, comes it that such
a vast number of the seedlings are mongrelized? I suspect that it must
arise from the pollen of a distinct VARIETY having a prepotent effect over
a flower's own pollen; and that this is part of the general law of good
being derived from the intercrossing of distinct individuals of the same
species. When distinct SPECIES are crossed the case is directly the
reverse, for a plant's own pollen is always prepotent over foreign pollen;
but to this subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at
most only from flower to flower on the same tree, and that flowers on the
same tree can be considered as distinct individuals only in a limited
sense. I believe this objection to be valid, but that nature has largely
provided against it by giving to trees a strong tendency to bear flowers
with separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, we can see that pollen
must be regularly carried from flower to flower; and this will give a
better chance of pollen being occasionally carried from tree to tree. That
trees belonging to all Orders have their sexes more often separated than
other plants, I find to be the case in this country; and at my request Dr.
Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the
United States, and the result was as I anticipated. On the other hand, Dr.
Hooker has recently informed me that he finds that the rule does not hold
in Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair. As
yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which offers so
strong a contrast with terrestrial plants, on the view of an occasional
cross being indispensable, by considering the medium in which terrestrial
animals live, and the nature of the fertilising element; for we know of no
means, analogous to the action of insects and of the wind in the case of
plants, by which an occasional cross could be effected with terrestrial
animals without the concurrence of two individuals. Of aquatic animals,
there are many self-fertilising hermaphrodites; but here currents in the
water offer an obvious means for an occasional cross. And, as in the case
of flowers, I have as yet failed, after consultation with one of the
highest authorities, namely, Professor Huxley, to discover a single case
of an hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very great
difficulty under this point of view; but I have been enabled, by a
fortunate chance, elsewhere to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even of
the same genus, though agreeing closely with each other in almost their
whole organisation, yet are not rarely, some of them hermaphrodites, and
some of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference between
hermaphrodites and unisexual species, as far as function is concerned,
becomes very small.
From these several considerations and from the many special facts which I
have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I am
well aware that there are, on this view, many cases of difficulty, some of
which I am trying to investigate. Finally then, we may conclude that in
many organic beings, a cross between two individuals is an obvious
necessity for each birth; in many others it occurs perhaps only at long
intervals; but in none, as I suspect, can self-fertilisation go on for
perpetuity.
CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.
This is an extremely intricate subject. A large amount of inheritable and
diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by giving
a better chance for the appearance within any given period of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, an extremely important element of success.
Though nature grants vast periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving, it may be said, to seize on each place in the economy
of nature, if any one species does not become modified and improved in a
corresponding degree with its competitors, it will soon be exterminated.
In man's methodical selection, a breeder selects for some definite object,
and free intercrossing will wholly stop his work. But when many men,
without intending to alter the breed, have a nearly common standard of
perfection, and all try to get and breed from the best animals, much
improvement and modification surely but slowly follow from this
unconscious process of selection, notwithstanding a large amount of
crossing with inferior animals. Thus it will be in nature; for within a
confined area, with some place in its polity not so perfectly occupied as
might be, natural selection will always tend to preserve all the
individuals varying in the right direction, though in different degrees,
so as better to fill up the unoccupied place. But if the area be large,
its several districts will almost certainly present different conditions
of life; and then if natural selection be modifying and improving a
species in the several districts, there will be intercrossing with the
other individuals of the same species on the confines of each. And in this
case the effects of intercrossing can hardly be counterbalanced by natural
selection always tending to modify all the individuals in each district in
exactly the same manner to the conditions of each; for in a continuous
area, the conditions will generally graduate away insensibly from one
district to another. The intercrossing will most affect those animals
which unite for each birth, which wander much, and which do not breed at a
very quick rate. Hence in animals of this nature, for instance in birds,
varieties will generally be confined to separated countries; and this I
believe to be the case. In hermaphrodite organisms which cross only
occasionally, and likewise in animals which unite for each birth, but
which wander little and which can increase at a very rapid rate, a new and
improved variety might be quickly formed on any one spot, and might there
maintain itself in a body, so that whatever intercrossing took place would
be chiefly between the individuals of the same new variety. A local
variety when once thus formed might subsequently slowly spread to other
districts. On the above principle, nurserymen always prefer getting seed
from a large body of plants of the same variety, as the chance of
intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth, we
must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing that
within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to pair
together.
Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and uniform
in character. It will obviously thus act far more efficiently with those
animals which unite for each birth; but I have already attempted to show
that we have reason to believe that occasional intercrosses take place
with all animals and with all plants. Even if these take place only at
long intervals, I am convinced that the young thus produced will gain so
much in vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving and
propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist
organic beings which never intercross, uniformity of character can be
retained amongst them, as long as their conditions of life remain the
same, only through the principle of inheritance, and through natural
selection destroying any which depart from the proper type; but if their
conditions of life change and they undergo modification, uniformity of
character can be given to their modified offspring, solely by natural
selection preserving the same favourable variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic
and inorganic conditions of life will generally be in a great degree
uniform; so that natural selection will tend to modify all the individuals
of a varying species throughout the area in the same manner in relation to
the same conditions. Intercrosses, also, with the individuals of the same
species, which otherwise would have inhabited the surrounding and
differently circumstanced districts, will be prevented. But isolation
probably acts more efficiently in checking the immigration of better
adapted organisms, after any physical change, such as of climate or
elevation of the land, etc.; and thus new places in the natural economy of
the country are left open for the old inhabitants to struggle for, and
become adapted to, through modifications in their structure and
constitution. Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly improved; and
this may sometimes be of importance in the production of new species. If,
however, an isolated area be very small, either from being surrounded by
barriers, or from having very peculiar physical conditions, the total
number of the individuals supported on it will necessarily be very small;
and fewness of individuals will greatly retard the production of new
species through natural selection, by decreasing the chance of the
appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the total number
of the species inhabiting it, will be found to be small, as we shall see
in our chapter on geographical distribution; yet of these species a very
large proportion are endemic,—that is, have been produced there, and
nowhere else. Hence an oceanic island at first sight seems to have been
highly favourable for the production of new species. But we may thus
greatly deceive ourselves, for to ascertain whether a small isolated area,
or a large open area like a continent, has been most favourable for the
production of new organic forms, we ought to make the comparison within
equal times; and this we are incapable of doing.
Although I do not doubt that isolation is of considerable importance in
the production of new species, on the whole I am inclined to believe that
largeness of area is of more importance, more especially in the production
of species, which will prove capable of enduring for a long period, and of
spreading widely. Throughout a great and open area, not only will there be
a better chance of favourable variations arising from the large number of
individuals of the same species there supported, but the conditions of
life are infinitely complex from the large number of already existing
species; and if some of these many species become modified and improved,
others will have to be improved in a corresponding degree or they will be
exterminated. Each new form, also, as soon as it has been much improved,
will be able to spread over the open and continuous area, and will thus
come into competition with many others. Hence more new places will be
formed, and the competition to fill them will be more severe, on a large
than on a small and isolated area. Moreover, great areas, though now
continuous, owing to oscillations of level, will often have recently
existed in a broken condition, so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude that,
although small isolated areas probably have been in some respects highly
favourable for the production of new species, yet that the course of
modification will generally have been more rapid on large areas; and what
is more important, that the new forms produced on large areas, which
already have been victorious over many competitors, will be those that
will spread most widely, will give rise to most new varieties and species,
and will thus play an important part in the changing history of the
organic world.
We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that
the productions of the smaller continent of Australia have formerly
yielded, and apparently are now yielding, before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental productions have
everywhere become so largely naturalised on islands. On a small island,
the race for life will have been less severe, and there will have been
less modification and less extermination. Hence, perhaps, it comes that
the flora of Madeira, according to Oswald Heer, resembles the extinct
tertiary flora of Europe. All fresh-water basins, taken together, make a
small area compared with that of the sea or of the land; and,
consequently, the competition between fresh-water productions will have
been less severe than elsewhere; new forms will have been more slowly
formed, and old forms more slowly exterminated. And it is in fresh water
that we find seven genera of Ganoid fishes, remnants of a once
preponderant order: and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders now widely
separated in the natural scale. These anomalous forms may almost be called
living fossils; they have endured to the present day, from having
inhabited a confined area, and from having thus been exposed to less
severe competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a broken condition,
will be the most favourable for the production of many new forms of life,
likely to endure long and to spread widely. For the area will first have
existed as a continent, and the inhabitants, at this period numerous in
individuals and kinds, will have been subjected to very severe
competition. When converted by subsidence into large separate islands,
there will still exist many individuals of the same species on each
island: intercrossing on the confines of the range of each species will
thus be checked: after physical changes of any kind, immigration will be
prevented, so that new places in the polity of each island will have to be
filled up by modifications of the old inhabitants; and time will be
allowed for the varieties in each to become well modified and perfected.
When, by renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will be
much extinction of the less improved forms, and the relative proportional
numbers of the various inhabitants of the renewed continent will again be
changed; and again there will be a fair field for natural selection to
improve still further the inhabitants, and thus produce new species.
That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of nature,
which can be better occupied by some of the inhabitants of the country
undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and on
the immigration of better adapted forms having been checked. But the
action of natural selection will probably still oftener depend on some of
the inhabitants becoming slowly modified; the mutual relations of many of
the other inhabitants being thus disturbed. Nothing can be effected,
unless favourable variations occur, and variation itself is apparently
always a very slow process. The process will often be greatly retarded by
free intercrossing. Many will exclaim that these several causes are amply
sufficient wholly to stop the action of natural selection. I do not
believe so. On the other hand, I do believe that natural selection will
always act very slowly, often only at long intervals of time, and
generally on only a very few of the inhabitants of the same region at the
same time. I further believe, that this very slow, intermittent action of
natural selection accords perfectly well with what geology tells us of the
rate and manner at which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble man can do much by
his powers of artificial selection, I can see no limit to the amount of
change, to the beauty and infinite complexity of the coadaptations between
all organic beings, one with another and with their physical conditions of
life, which may be effected in the long course of time by nature's power
of selection.
EXTINCTION.
This subject will be more fully discussed in our chapter on Geology; but
it must be here alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. But as
from the high geometrical powers of increase of all organic beings, each
area is already fully stocked with inhabitants, it follows that as each
selected and favoured form increases in number, so will the less favoured
forms decrease and become rare. Rarity, as geology tells us, is the
precursor to extinction. We can, also, see that any form represented by
few individuals will, during fluctuations in the seasons or in the number
of its enemies, run a good chance of utter extinction. But we may go
further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not indefinitely
increased, geology shows us plainly; and indeed we can see reason why they
should not have thus increased, for the number of places in the polity of
nature is not indefinitely great,—not that we have any means of
knowing that any one region has as yet got its maximum of species.
Probably no region is as yet fully stocked, for at the Cape of Good Hope,
where more species of plants are crowded together than in any other
quarter of the world, some foreign plants have become naturalised, without
causing, as far as we know, the extinction of any natives.
Furthermore, the species which are most numerous in individuals will have
the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the greatest
number of recorded varieties, or incipient species. Hence, rare species
will be less quickly modified or improved within any given period, and
they will consequently be beaten in the race for life by the modified
descendants of the commoner species.
From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the chapter
on the Struggle for Existence that it is the most closely-allied forms,—varieties
of the same species, and species of the same genus or of related genera,—which,
from having nearly the same structure, constitution, and habits, generally
come into the severest competition with each other. Consequently, each new
variety or species, during the progress of its formation, will generally
press hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination amongst our domesticated productions,
through the selection of improved forms by man. Many curious instances
could be given showing how quickly new breeds of cattle, sheep, and other
animals, and varieties of flowers, take the place of older and inferior
kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these "were swept away
by the short-horns" (I quote the words of an agricultural writer) "as if
by some murderous pestilence."
DIVERGENCE OF CHARACTER./
The principle, which I have designated by this term, is of high importance
on my theory, and explains, as I believe, several important facts. In the
first place, varieties, even strongly-marked ones, though having somewhat
of the character of species—as is shown by the hopeless doubts in
many cases how to rank them—yet certainly differ from each other far
less than do good and distinct species. Nevertheless, according to my
view, varieties are species in the process of formation, or are, as I have
called them, incipient species. How, then, does the lesser difference
between varieties become augmented into the greater difference between
species? That this does habitually happen, we must infer from most of the
innumerable species throughout nature presenting well-marked differences;
whereas varieties, the supposed prototypes and parents of future
well-marked species, present slight and ill-defined differences. Mere
chance, as we may call it, might cause one variety to differ in some
character from its parents, and the offspring of this variety again to
differ from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large an amount of
difference as that between varieties of the same species and species of
the same genus.
As has always been my practice, let us seek light on this head from our
domestic productions. We shall here find something analogous. A fancier is
struck by a pigeon having a slightly shorter beak; another fancier is
struck by a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium standard, but
like extremes," they both go on (as has actually occurred with
tumbler-pigeons) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at an
early period one man preferred swifter horses; another stronger and more
bulky horses. The early differences would be very slight; in the course of
time, from the continued selection of swifter horses by some breeders, and
of stronger ones by others, the differences would become greater, and
would be noted as forming two sub-breeds; finally, after the lapse of
centuries, the sub-breeds would become converted into two well-established
and distinct breeds. As the differences slowly become greater, the
inferior animals with intermediate characters, being neither very swift
nor very strong, will have been neglected, and will have tended to
disappear. Here, then, we see in man's productions the action of what may
be called the principle of divergence, causing differences, at first
barely appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places in
the polity of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural powers of increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in its conditions) only by its
varying descendants seizing on places at present occupied by other
animals: some of them, for instance, being enabled to feed on new kinds of
prey, either dead or alive; some inhabiting new stations, climbing trees,
frequenting water, and some perhaps becoming less carnivorous. The more
diversified in habits and structure the descendants of our carnivorous
animal became, the more places they would be enabled to occupy. What
applies to one animal will apply throughout all time to all animals—that
is, if they vary—for otherwise natural selection can do nothing. So
it will be with plants. It has been experimentally proved, that if a plot
of ground be sown with one species of grass, and a similar plot be sown
with several distinct genera of grasses, a greater number of plants and a
greater weight of dry herbage can thus be raised. The same has been found
to hold good when first one variety and then several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one species
of grass were to go on varying, and those varieties were continually
selected which differed from each other in at all the same manner as
distinct species and genera of grasses differ from each other, a greater
number of individual plants of this species of grass, including its
modified descendants, would succeed in living on the same piece of ground.
And we well know that each species and each variety of grass is annually
sowing almost countless seeds; and thus, as it may be said, is striving
its utmost to increase its numbers. Consequently, I cannot doubt that in
the course of many thousands of generations, the most distinct varieties
of any one species of grass would always have the best chance of
succeeding and of increasing in numbers, and thus of supplanting the less
distinct varieties; and varieties, when rendered very distinct from each
other, take the rank of species.
The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any small
piece of ground, could live on it (supposing it not to be in any way
peculiar in its nature), and may be said to be striving to the utmost to
live there; but, it is seen, that where they come into the closest
competition with each other, the advantages of diversification of
structure, with the accompanying differences of habit and constitution,
determine that the inhabitants, which thus jostle each other most closely,
shall, as a general rule, belong to what we call different genera and
orders.
The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants which
have succeeded in becoming naturalised in any land would generally have
been closely allied to the indigenes; for these are commonly looked at as
specially created and adapted for their own country. It might, also,
perhaps have been expected that naturalised plants would have belonged to
a few groups more especially adapted to certain stations in their new
homes. But the case is very different; and Alph. De Candolle has well
remarked in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's 'Manual of the Flora of
the Northern United States,' 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants are
of a highly diversified nature. They differ, moreover, to a large extent
from the indigenes, for out of the 162 genera, no less than 100 genera are
not there indigenous, and thus a large proportional addition is made to
the genera of these States.
By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the
natives would have had to be modified, in order to have gained an
advantage over the other natives; and we may, I think, at least safely
infer that diversification of structure, amounting to new generic
differences, would have been profitable to them.
The advantage of diversification in the inhabitants of the same region is,
in fact, the same as that of the physiological division of labour in the
organs of the same individual body—a subject so well elucidated by
Milne Edwards. No physiologist doubts that a stomach by being adapted to
digest vegetable matter alone, or flesh alone, draws most nutriment from
these substances. So in the general economy of any land, the more widely
and perfectly the animals and plants are diversified for different habits
of life, so will a greater number of individuals be capable of there
supporting themselves. A set of animals, with their organisation but
little diversified, could hardly compete with a set more perfectly
diversified in structure. It may be doubted, for instance, whether the
Australian marsupials, which are divided into groups differing but little
from each other, and feebly representing, as Mr. Waterhouse and others
have remarked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-pronounced orders. In the Australian
mammals, we see the process of diversification in an early and incomplete
stage of development. After the foregoing discussion, which ought to have
been much amplified, we may, I think, assume that the modified descendants
of any one species will succeed by so much the better as they become more
diversified in structure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how this principle of great
benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large in
its own country; these species are supposed to resemble each other in
unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances. I
have said a large genus, because we have seen in the second chapter, that
on an average more of the species of large genera vary than of small
genera; and the varying species of the large genera present a greater
number of varieties. We have, also, seen that the species, which are the
commonest and the most widely-diffused, vary more than rare species with
restricted ranges. Let (A) be a common, widely-diffused, and varying
species, belonging to a genus large in its own country. The little fan of
diverging dotted lines of unequal lengths proceeding from (A), may
represent its varying offspring. The variations are supposed to be
extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals of
time; nor are they all supposed to endure for equal periods. Only those
variations which are in some way profitable will be preserved or naturally
selected. And here the importance of the principle of benefit being
derived from divergence of character comes in; for this will generally
lead to the most different or divergent variations (represented by the
outer dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation is
supposed to have been accumulated to have formed a fairly well-marked
variety, such as would be thought worthy of record in a systematic work.
The intervals between the horizontal lines in the diagram, may represent
each a thousand generations; but it would have been better if each had
represented ten thousand generations. After a thousand generations,
species (A) is supposed to have produced two fairly well-marked varieties,
namely a1 and m1. These two varieties will generally continue to be
exposed to the same conditions which made their parents variable, and the
tendency to variability is in itself hereditary, consequently they will
tend to vary, and generally to vary in nearly the same manner as their
parents varied. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their
common parent (A) more numerous than most of the other inhabitants of the
same country; they will likewise partake of those more general advantages
which made the genus to which the parent-species belonged, a large genus
in its own country. And these circumstances we know to be favourable to
the production of new varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle of
divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing from
each other, and more considerably from their common parent (A). We may
continue the process by similar steps for any length of time; some of the
varieties, after each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two or
three varieties, and some failing to produce any. Thus the varieties or
modified descendants, proceeding from the common parent (A), will
generally go on increasing in number and diverging in character. In the
diagram the process is represented up to the ten-thousandth generation,
and under a condensed and simplified form up to the fourteen-thousandth
generation.
But I must here remark that I do not suppose that the process ever goes on
so regularly as is represented in the diagram, though in itself made
somewhat irregular. I am far from thinking that the most divergent
varieties will invariably prevail and multiply: a medium form may often
long endure, and may or may not produce more than one modified descendant;
for natural selection will always act according to the nature of the
places which are either unoccupied or not perfectly occupied by other
beings; and this will depend on infinitely complex relations. But as a
general rule, the more diversified in structure the descendants from any
one species can be rendered, the more places they will be enabled to seize
on, and the more their modified progeny will be increased. In our diagram
the line of succession is broken at regular intervals by small numbered
letters marking the successive forms which have become sufficiently
distinct to be recorded as varieties. But these breaks are imaginary, and
might have been inserted anywhere, after intervals long enough to have
allowed the accumulation of a considerable amount of divergent variation.
As all the modified descendants from a common and widely-diffused species,
belonging to a large genus, will tend to partake of the same advantages
which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is
represented in the diagram by the several divergent branches proceeding
from (A). The modified offspring from the later and more highly improved
branches in the lines of descent, will, it is probable, often take the
place of, and so destroy, the earlier and less improved branches: this is
represented in the diagram by some of the lower branches not reaching to
the upper horizontal lines. In some cases I do not doubt that the process
of modification will be confined to a single line of descent, and the
number of the descendants will not be increased; although the amount of
divergent modification may have been increased in the successive
generations. This case would be represented in the diagram, if all the
lines proceeding from (A) were removed, excepting that from a1 to a10. In
the same way, for instance, the English race-horse and English pointer
have apparently both gone on slowly diverging in character from their
original stocks, without either having given off any fresh branches or
races.
After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of
sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates the
steps by which the small differences distinguishing varieties are
increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as shown
in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and m14, all descended from
(A). Thus, as I believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary. In
the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters n14 to z14,
are supposed to have been produced. In each genus, the species, which are
already extremely different in character, will generally tend to produce
the greatest number of modified descendants; for these will have the best
chance of filling new and widely different places in the polity of nature:
hence in the diagram I have chosen the extreme species (A), and the nearly
extreme species (I), as those which have largely varied, and have given
rise to new varieties and species. The other nine species (marked by
capital letters) of our original genus, may for a long period continue
transmitting unaltered descendants; and this is shown in the diagram by
the dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played an
important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate in
each stage of descent their predecessors and their original parent. For it
should be remembered that the competition will generally be most severe
between those forms which are most nearly related to each other in habits,
constitution, and structure. Hence all the intermediate forms between the
earlier and later states, that is between the less and more improved state
of a species, as well as the original parent-species itself, will
generally tend to become extinct. So it probably will be with many whole
collateral lines of descent, which will be conquered by later and improved
lines of descent. If, however, the modified offspring of a species get
into some distinct country, or become quickly adapted to some quite new
station, in which child and parent do not come into competition, both may
continue to exist.
If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a14 to m14); and (I)
will have been replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D, than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A) and (I), were also supposed to be very
common and widely diffused species, so that they must originally have had
some advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation,
will probably have inherited some of the same advantages: they have also
been modified and improved in a diversified manner at each stage of
descent, so as to have become adapted to many related places in the
natural economy of their country. It seems, therefore, to me extremely
probable that they will have taken the places of, and thus exterminated,
not only their parents (A) and (I), but likewise some of the original
species which were most nearly related to their parents. Hence very few of
the original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of the
two species which were least closely related to the other nine original
species, has transmitted descendants to this late stage of descent.
The new species in our diagram descended from the original eleven species,
will now be fifteen in number. Owing to the divergent tendency of natural
selection, the extreme amount of difference in character between species
a14 and z14 will be much greater than that between the most different of
the original eleven species. The new species, moreover, will be allied to
each other in a widely different manner. Of the eight descendants from (A)
the three marked a14, q14, p14, will be nearly related from having
recently branched off from a10; b14 and f14, from having diverged at an
earlier period from a5, will be in some degree distinct from the three
first-named species; and lastly, o14, e14, and m14, will be nearly related
one to the other, but from having diverged at the first commencement of
the process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera. But
as the original species (I) differed largely from (A), standing nearly at
the extreme points of the original genus, the six descendants from (I)
will, owing to inheritance, differ considerably from the eight descendants
from (A); the two groups, moreover, are supposed to have gone on diverging
in different directions. The intermediate species, also (and this is a
very important consideration), which connected the original species (A)
and (I), have all become, excepting (F), extinct, and have left no
descendants. Hence the six new species descended from (I), and the eight
descended from (A), will have to be ranked as very distinct genera, or
even as distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by descent,
with modification, from two or more species of the same genus. And the two
or more parent-species are supposed to have descended from some one
species of an earlier genus. In our diagram, this is indicated by the
broken lines, beneath the capital letters, converging in sub-branches
downwards towards a single point; this point representing a single
species, the supposed single parent of our several new sub-genera and
genera.
It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character, but
to have retained the form of (F), either unaltered or altered only in a
slight degree. In this case, its affinities to the other fourteen new
species will be of a curious and circuitous nature. Having descended from
a form which stood between the two parent-species (A) and (I), now
supposed to be extinct and unknown, it will be in some degree intermediate
in character between the two groups descended from these species. But as
these two groups have gone on diverging in character from the type of
their parents, the new species (F14) will not be directly intermediate
between them, but rather between types of the two groups; and every
naturalist will be able to bring some such case before his mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when we
come to our chapter on Geology, have to refer again to this subject, and I
think we shall then see that the diagram throws light on the affinities of
extinct beings, which, though generally belonging to the same orders, or
families, or genera, with those now living, yet are often, in some degree,
intermediate in character between existing groups; and we can understand
this fact, for the extinct species lived at very ancient epochs when the
branching lines of descent had diverged less.
I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in our diagram, we suppose the amount
of change represented by each successive group of diverging dotted lines
to be very great, the forms marked a14 to p14, those marked b14 and f14,
and those marked o14 to m14, will form three very distinct genera. We
shall also have two very distinct genera descended from (I) and as these
latter two genera, both from continued divergence of character and from
inheritance from a different parent, will differ widely from the three
genera descended from (A), the two little groups of genera will form two
distinct families, or even orders, according to the amount of divergent
modification supposed to be represented in the diagram. And the two new
families, or orders, will have descended from two species of the original
genus; and these two species are supposed to have descended from one
species of a still more ancient and unknown genus.
We have seen that in each country it is the species of the larger genera
which oftenest present varieties or incipient species. This, indeed, might
have been expected; for as natural selection acts through one form having
some advantage over other forms in the struggle for existence, it will
chiefly act on those which already have some advantage; and the largeness
of any group shows that its species have inherited from a common ancestor
some advantage in common. Hence, the struggle for the production of new
and modified descendants, will mainly lie between the larger groups, which
are all trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the later
and more highly perfected sub-groups, from branching out and seizing on
many new places in the polity of Nature, will constantly tend to supplant
and destroy the earlier and less improved sub-groups. Small and broken
groups and sub-groups will finally tend to disappear. Looking to the
future, we can predict that the groups of organic beings which are now
large and triumphant, and which are least broken up, that is, which as yet
have suffered least extinction, will for a long period continue to
increase. But which groups will ultimately prevail, no man can predict;
for we well know that many groups, formerly most extensively developed,
have now become extinct. Looking still more remotely to the future, we may
predict that, owing to the continued and steady increase of the larger
groups, a multitude of smaller groups will become utterly extinct, and
leave no modified descendants; and consequently that of the species living
at any one period, extremely few will transmit descendants to a remote
futurity. I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of the
more ancient species having transmitted descendants, and on the view of
all the descendants of the same species making a class, we can understand
how it is that there exist but very few classes in each main division of
the animal and vegetable kingdoms. Although extremely few of the most
ancient species may now have living and modified descendants, yet at the
most remote geological period, the earth may have been as well peopled
with many species of many genera, families, orders, and classes, as at the
present day.
SUMMARY OF CHAPTER.
If during the long course of ages and under varying conditions of life,
organic beings vary at all in the several parts of their organisation, and
I think this cannot be disputed; if there be, owing to the high
geometrical powers of increase of each species, at some age, season, or
year, a severe struggle for life, and this certainly cannot be disputed;
then, considering the infinite complexity of the relations of all organic
beings to each other and to their conditions of existence, causing an
infinite diversity in structure, constitution, and habits, to be
advantageous to them, I think it would be a most extraordinary fact if no
variation ever had occurred useful to each being's own welfare, in the
same way as so many variations have occurred useful to man. But if
variations useful to any organic being do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance they will
tend to produce offspring similarly characterised. This principle of
preservation, I have called, for the sake of brevity, Natural Selection.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as the
adult. Amongst many animals, sexual selection will give its aid to
ordinary selection, by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other males.
Whether natural selection has really thus acted in nature, in modifying
and adapting the various forms of life to their several conditions and
stations, must be judged of by the general tenour and balance of evidence
given in the following chapters. But we already see how it entails
extinction; and how largely extinction has acted in the world's history,
geology plainly declares. Natural selection, also, leads to divergence of
character; for more living beings can be supported on the same area the
more they diverge in structure, habits, and constitution, of which we see
proof by looking at the inhabitants of any small spot or at naturalised
productions. Therefore during the modification of the descendants of any
one species, and during the incessant struggle of all species to increase
in numbers, the more diversified these descendants become, the better will
be their chance of succeeding in the battle of life. Thus the small
differences distinguishing varieties of the same species, will steadily
tend to increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary most;
and these will tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries. Natural
selection, as has just been remarked, leads to divergence of character and
to much extinction of the less improved and intermediate forms of life. On
these principles, I believe, the nature of the affinities of all organic
beings may be explained. It is a truly wonderful fact—the wonder of
which we are apt to overlook from familiarity—that all animals and
all plants throughout all time and space should be related to each other
in group subordinate to group, in the manner which we everywhere behold—namely,
varieties of the same species most closely related together, species of
the same genus less closely and unequally related together, forming
sections and sub-genera, species of distinct genera much less closely
related, and genera related in different degrees, forming sub-families,
families, orders, sub-classes, and classes. The several subordinate groups
in any class cannot be ranked in a single file, but seem rather to be
clustered round points, and these round other points, and so on in almost
endless cycles. On the view that each species has been independently
created, I can see no explanation of this great fact in the classification
of all organic beings; but, to the best of my judgment, it is explained
through inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in the
diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long succession
of extinct species. At each period of growth all the growing twigs have
tried to branch out on all sides, and to overtop and kill the surrounding
twigs and branches, in the same manner as species and groups of species
have tried to overmaster other species in the great battle for life. The
limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was small, budding twigs;
and this connexion of the former and present buds by ramifying branches
may well represent the classification of all extinct and living species in
groups subordinate to groups. Of the many twigs which flourished when the
tree was a mere bush, only two or three, now grown into great branches,
yet survive and bear all the other branches; so with the species which
lived during long-past geological periods, very few now have living and
modified descendants. From the first growth of the tree, many a limb and
branch has decayed and dropped off; and these lost branches of various
sizes may represent those whole orders, families, and genera which have
now no living representatives, and which are known to us only from having
been found in a fossil state. As we here and there see a thin straggling
branch springing from a fork low down in a tree, and which by some chance
has been favoured and is still alive on its summit, so we occasionally see
an animal like the Ornithorhynchus or Lepidosiren, which in some small
degree connects by its affinities two large branches of life, and which
has apparently been saved from fatal competition by having inhabited a
protected station. As buds give rise by growth to fresh buds, and these,
if vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and covers
the surface with its ever branching and beautiful ramifications.
5. LAWS OF VARIATION.
Effects of external conditions. Use and disuse, combined with natural
selection; organs of flight and of vision. Acclimatisation. Correlation of
growth. Compensation and economy of growth. False correlations. Multiple,
rudimentary, and lowly organised structures variable. Parts developed in
an unusual manner are highly variable: specific characters more variable
than generic: secondary sexual characters variable. Species of the same
genus vary in an analogous manner. Reversions to long lost characters.
Summary.
I have hitherto sometimes spoken as if the variations—so common and
multiform in organic beings under domestication, and in a lesser degree in
those in a state of nature—had been due to chance. This, of course,
is a wholly incorrect expression, but it serves to acknowledge plainly our
ignorance of the cause of each particular variation. Some authors believe
it to be as much the function of the reproductive system to produce
individual differences, or very slight deviations of structure, as to make
the child like its parents. But the much greater variability, as well as
the greater frequency of monstrosities, under domestication or
cultivation, than under nature, leads me to believe that deviations of
structure are in some way due to the nature of the conditions of life, to
which the parents and their more remote ancestors have been exposed during
several generations. I have remarked in the first chapter—but a long
catalogue of facts which cannot be here given would be necessary to show
the truth of the remark—that the reproductive system is eminently
susceptible to changes in the conditions of life; and to this system being
functionally disturbed in the parents, I chiefly attribute the varying or
plastic condition of the offspring. The male and female sexual elements
seem to be affected before that union takes place which is to form a new
being. In the case of "sporting" plants, the bud, which in its earliest
condition does not apparently differ essentially from an ovule, is alone
affected. But why, because the reproductive system is disturbed, this or
that part should vary more or less, we are profoundly ignorant.
Nevertheless, we can here and there dimly catch a faint ray of light, and
we may feel sure that there must be some cause for each deviation of
structure, however slight.
How much direct effect difference of climate, food, etc., produces on any
being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that of
plants. We may, at least, safely conclude that such influences cannot have
produced the many striking and complex co-adaptations of structure between
one organic being and another, which we see everywhere throughout nature.
Some little influence may be attributed to climate, food, etc.: thus, E.
Forbes speaks confidently that shells at their southern limit, and when
living in shallow water, are more brightly coloured than those of the same
species further north or from greater depths. Gould believes that birds of
the same species are more brightly coloured under a clear atmosphere, than
when living on islands or near the coast. So with insects, Wollaston is
convinced that residence near the sea affects their colours. Moquin-Tandon
gives a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several other
such cases could be given.
The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree some
of the characters of such species, accords with our view that species of
all kinds are only well-marked and permanent varieties. Thus the species
of shells which are confined to tropical and shallow seas are generally
brighter-coloured than those confined to cold and deeper seas. The birds
which are confined to continents are, according to Mr. Gould,
brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants
which live exclusively on the sea-side are very apt to have fleshy leaves.
He who believes in the creation of each species, will have to say that
this shell, for instance, was created with bright colours for a warm sea;
but that this other shell became bright-coloured by variation when it
ranged into warmer or shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural selection,
and how much to the conditions of life. Thus, it is well known to furriers
that animals of the same species have thicker and better fur the more
severe the climate is under which they have lived; but who can tell how
much of this difference may be due to the warmest-clad individuals having
been favoured and preserved during many generations, and how much to the
direct action of the severe climate? for it would appear that climate has
some direct action on the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly the conditions
of life must act. Again, innumerable instances are known to every
naturalist of species keeping true, or not varying at all, although living
under the most opposite climates. Such considerations as these incline me
to lay very little weight on the direct action of the conditions of life.
Indirectly, as already remarked, they seem to play an important part in
affecting the reproductive system, and in thus inducing variability; and
natural selection will then accumulate all profitable variations, however
slight, until they become plainly developed and appreciable by us.
EFFECTS OF USE AND DISUSE.
From the facts alluded to in the first chapter, I think there can be
little doubt that use in our domestic animals strengthens and enlarges
certain parts, and disuse diminishes them; and that such modifications are
inherited. Under free nature, we can have no standard of comparison, by
which to judge of the effects of long-continued use or disuse, for we know
not the parent-forms; but many animals have structures which can be
explained by the effects of disuse. As Professor Owen has remarked, there
is no greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck. As the larger ground-feeding
birds seldom take flight except to escape danger, I believe that the
nearly wingless condition of several birds, which now inhabit or have
lately inhabited several oceanic islands, tenanted by no beast of prey,
has been caused by disuse. The ostrich indeed inhabits continents and is
exposed to danger from which it cannot escape by flight, but by kicking it
can defend itself from enemies, as well as any of the smaller quadrupeds.
We may imagine that the early progenitor of the ostrich had habits like
those of a bustard, and that as natural selection increased in successive
generations the size and weight of its body, its legs were used more, and
its wings less, until they became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are very often broken
off; he examined seventeen specimens in his own collection, and not one
had even a relic left. In the Onites apelles the tarsi are so habitually
lost, that the insect has been described as not having them. In some other
genera they are present, but in a rudimentary condition. In the Ateuchus
or sacred beetle of the Egyptians, they are totally deficient. There is
not sufficient evidence to induce us to believe that mutilations are ever
inherited; and I should prefer explaining the entire absence of the
anterior tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors; for
as the tarsi are almost always lost in many dung-feeding beetles, they
must be lost early in life, and therefore cannot be much used by these
insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of the
550 species inhabiting Madeira, are so far deficient in wings that they
cannot fly; and that of the twenty-nine endemic genera, no less than
twenty-three genera have all their species in this condition! Several
facts, namely, that beetles in many parts of the world are very frequently
blown to sea and perish; that the beetles in Madeira, as observed by Mr.
Wollaston, lie much concealed, until the wind lulls and the sun shines;
that the proportion of wingless beetles is larger on the exposed Dezertas
than in Madeira itself; and especially the extraordinary fact, so strongly
insisted on by Mr. Wollaston, of the almost entire absence of certain
large groups of beetles, elsewhere excessively numerous, and which groups
have habits of life almost necessitating frequent flight;—these
several considerations have made me believe that the wingless condition of
so many Madeira beetles is mainly due to the action of natural selection,
but combined probably with disuse. For during thousands of successive
generations each individual beetle which flew least, either from its wings
having been ever so little less perfectly developed or from indolent
habit, will have had the best chance of surviving from not being blown out
to sea; and, on the other hand, those beetles which most readily took to
flight will oftenest have been blown to sea and thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their wings
to gain their subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite compatible with the
action of natural selection. For when a new insect first arrived on the
island, the tendency of natural selection to enlarge or to reduce the
wings, would depend on whether a greater number of individuals were saved
by successfully battling with the winds, or by giving up the attempt and
rarely or never flying. As with mariners shipwrecked near a coast, it
would have been better for the good swimmers if they had been able to swim
still further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered up by skin and fur. This state of the
eyes is probably due to gradual reduction from disuse, but aided perhaps
by natural selection. In South America, a burrowing rodent, the tuco-tuco,
or Ctenomys, is even more subterranean in its habits than the mole; and I
was assured by a Spaniard, who had often caught them, that they were
frequently blind; one which I kept alive was certainly in this condition,
the cause, as appeared on dissection, having been inflammation of the
nictitating membrane. As frequent inflammation of the eyes must be
injurious to any animal, and as eyes are certainly not indispensable to
animals with subterranean habits, a reduction in their size with the
adhesion of the eyelids and growth of fur over them, might in such case be
an advantage; and if so, natural selection would constantly aid the
effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though
useless, could be in any way injurious to animals living in darkness, I
attribute their loss wholly to disuse. In one of the blind animals,
namely, the cave-rat, the eyes are of immense size; and Professor Silliman
thought that it regained, after living some days in the light, some slight
power of vision. In the same manner as in Madeira the wings of some of the
insects have been enlarged, and the wings of others have been reduced by
natural selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and to
have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common
view of the blind animals having been separately created for the American
and European caverns, close similarity in their organisation and
affinities might have been expected; but, as Schiodte and others have
remarked, this is not the case, and the cave-insects of the two continents
are not more closely allied than might have been anticipated from the
general resemblance of the other inhabitants of North America and Europe.
On my view we must suppose that American animals, having ordinary powers
of vision, slowly migrated by successive generations from the outer world
into the deeper and deeper recesses of the Kentucky caves, as did European
animals into the caves of Europe. We have some evidence of this gradation
of habit; for, as Schiodte remarks, "animals not far remote from ordinary
forms, prepare the transition from light to darkness. Next follow those
that are constructed for twilight; and, last of all, those destined for
total darkness." By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more or
less perfectly obliterated its eyes, and natural selection will often have
effected other changes, such as an increase in the length of the antennae
or palpi, as a compensation for blindness. Notwithstanding such
modifications, we might expect still to see in the cave-animals of
America, affinities to the other inhabitants of that continent, and in
those of Europe, to the inhabitants of the European continent. And this is
the case with some of the American cave-animals, as I hear from Professor
Dana; and some of the European cave-insects are very closely allied to
those of the surrounding country. It would be most difficult to give any
rational explanation of the affinities of the blind cave-animals to the
other inhabitants of the two continents on the ordinary view of their
independent creation. That several of the inhabitants of the caves of the
Old and New Worlds should be closely related, we might expect from the
well-known relationship of most of their other productions. Far from
feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference to
the reptiles of Europe, I am only surprised that more wrecks of ancient
life have not been preserved, owing to the less severe competition to
which the inhabitants of these dark abodes will probably have been
exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in the
amount of rain requisite for seeds to germinate, in the time of sleep,
etc., and this leads me to say a few words on acclimatisation. As it is
extremely common for species of the same genus to inhabit very hot and
very cold countries, and as I believe that all the species of the same
genus have descended from a single parent, if this view be correct,
acclimatisation must be readily effected during long-continued descent. It
is notorious that each species is adapted to the climate of its own home:
species from an arctic or even from a temperate region cannot endure a
tropical climate, or conversely. So again, many succulent plants cannot
endure a damp climate. But the degree of adaptation of species to the
climates under which they live is often overrated. We may infer this from
our frequent inability to predict whether or not an imported plant will
endure our climate, and from the number of plants and animals brought from
warmer countries which here enjoy good health. We have reason to believe
that species in a state of nature are limited in their ranges by the
competition of other organic beings quite as much as, or more than, by
adaptation to particular climates. But whether or not the adaptation be
generally very close, we have evidence, in the case of some few plants, of
their becoming, to a certain extent, naturally habituated to different
temperatures, or becoming acclimatised: thus the pines and rhododendrons,
raised from seed collected by Dr. Hooker from trees growing at different
heights on the Himalaya, were found in this country to possess different
constitutional powers of resisting cold. Mr. Thwaites informs me that he
has observed similar facts in Ceylon, and analogous observations have been
made by Mr. H. C. Watson on European species of plants brought from the
Azores to England. In regard to animals, several authentic cases could be
given of species within historical times having largely extended their
range from warmer to cooler latitudes, and conversely; but we do not
positively know that these animals were strictly adapted to their native
climate, but in all ordinary cases we assume such to be the case; nor do
we know that they have subsequently become acclimatised to their new
homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary capacity
in our domestic animals of not only withstanding the most different
climates but of being perfectly fertile (a far severer test) under them,
may be used as an argument that a large proportion of other animals, now
in a state of nature, could easily be brought to bear widely different
climates. We must not, however, push the foregoing argument too far, on
account of the probable origin of some of our domestic animals from
several wild stocks: the blood, for instance, of a tropical and arctic
wolf or wild dog may perhaps be mingled in our domestic breeds. The rat
and mouse cannot be considered as domestic animals, but they have been
transported by man to many parts of the world, and now have a far wider
range than any other rodent, living free under the cold climate of Faroe
in the north and of the Falklands in the south, and on many islands in the
torrid zones. Hence I am inclined to look at adaptation to any special
climate as a quality readily grafted on an innate wide flexibility of
constitution, which is common to most animals. On this view, the capacity
of enduring the most different climates by man himself and by his domestic
animals, and such facts as that former species of the elephant and
rhinoceros were capable of enduring a glacial climate, whereas the living
species are now all tropical or sub-tropical in their habits, ought not to
be looked at as anomalies, but merely as examples of a very common
flexibility of constitution, brought, under peculiar circumstances, into
play.
How much of the acclimatisation of species to any peculiar climate is due
to mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to both means combined, is a
very obscure question. That habit or custom has some influence I must
believe, both from analogy, and from the incessant advice given in
agricultural works, even in the ancient Encyclopaedias of China, to be
very cautious in transposing animals from one district to another; for it
is not likely that man should have succeeded in selecting so many breeds
and sub-breeds with constitutions specially fitted for their own
districts: the result must, I think, be due to habit. On the other hand, I
can see no reason to doubt that natural selection will continually tend to
preserve those individuals which are born with constitutions best adapted
to their native countries. In treatises on many kinds of cultivated
plants, certain varieties are said to withstand certain climates better
than others: this is very strikingly shown in works on fruit trees
published in the United States, in which certain varieties are habitually
recommended for the northern, and others for the southern States; and as
most of these varieties are of recent origin, they cannot owe their
constitutional differences to habit. The case of the Jerusalem artichoke,
which is never propagated by seed, and of which consequently new varieties
have not been produced, has even been advanced—for it is now as
tender as ever it was—as proving that acclimatisation cannot be
effected! The case, also, of the kidney-bean has been often cited for a
similar purpose, and with much greater weight; but until some one will
sow, during a score of generations, his kidney-beans so early that a very
large proportion are destroyed by frost, and then collect seed from the
few survivors, with care to prevent accidental crosses, and then again get
seed from these seedlings, with the same precautions, the experiment
cannot be said to have been even tried. Nor let it be supposed that no
differences in the constitution of seedling kidney-beans ever appear, for
an account has been published how much more hardy some seedlings appeared
to be than others.
On the whole, I think we may conclude that habit, use, and disuse, have,
in some cases, played a considerable part in the modification of the
constitution, and of the structure of various organs; but that the effects
of use and disuse have often been largely combined with, and sometimes
overmastered by, the natural selection of innate differences.
CORRELATION OF GROWTH.
I mean by this expression that the whole organisation is so tied together
during its growth and development, that when slight variations in any one
part occur, and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly
understood. The most obvious case is, that modifications accumulated
solely for the good of the young or larva, will, it may safely be
concluded, affect the structure of the adult; in the same manner as any
malconformation affecting the early embryo, seriously affects the whole
organisation of the adult. The several parts of the body which are
homologous, and which, at an early embryonic period, are alike, seem
liable to vary in an allied manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind legs,
and even in the jaws and limbs, varying together, for the lower jaw is
believed to be homologous with the limbs. These tendencies, I do not
doubt, may be mastered more or less completely by natural selection: thus
a family of stags once existed with an antler only on one side; and if
this had been of any great use to the breed it might probably have been
rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of the
petals of the corolla into a tube. Hard parts seem to affect the form of
adjoining soft parts; it is believed by some authors that the diversity in
the shape of the pelvis in birds causes the remarkable diversity in the
shape of their kidneys. Others believe that the shape of the pelvis in the
human mother influences by pressure the shape of the head of the child. In
snakes, according to Schlegel, the shape of the body and the manner of
swallowing determine the position of several of the most important
viscera.
The nature of the bond of correlation is very frequently quite obscure. M.
Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist, without
our being able to assign any reason. What can be more singular than the
relation between blue eyes and deafness in cats, and the tortoise-shell
colour with the female sex; the feathered feet and skin between the outer
toes in pigeons, and the presence of more or less down on the young birds
when first hatched, with the future colour of their plumage; or, again,
the relation between the hair and teeth in the naked Turkish dog, though
here probably homology comes into play? With respect to this latter case
of correlation, I think it can hardly be accidental, that if we pick out
the two orders of mammalia which are most abnormal in their dermal
coverings, viz. Cetacea (whales) and Edentata (armadilloes, scaly
ant-eaters, etc.), that these are likewise the most abnormal in their
teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility
and, therefore, of natural selection, than that of the difference between
the outer and inner flowers in some Compositous and Umbelliferous plants.
Every one knows the difference in the ray and central florets of, for
instance, the daisy, and this difference is often accompanied with the
abortion of parts of the flower. But, in some Compositous plants, the
seeds also differ in shape and sculpture; and even the ovary itself, with
its accessory parts, differs, as has been described by Cassini. These
differences have been attributed by some authors to pressure, and the
shape of the seeds in the ray-florets in some Compositae countenances this
idea; but, in the case of the corolla of the Umbelliferae, it is by no
means, as Dr. Hooker informs me, in species with the densest heads that
the inner and outer flowers most frequently differ. It might have been
thought that the development of the ray-petals by drawing nourishment from
certain other parts of the flower had caused their abortion; but in some
Compositae there is a difference in the seeds of the outer and inner
florets without any difference in the corolla. Possibly, these several
differences may be connected with some difference in the flow of nutriment
towards the central and external flowers: we know, at least, that in
irregular flowers, those nearest to the axis are oftenest subject to
peloria, and become regular. I may add, as an instance of this, and of a
striking case of correlation, that I have recently observed in some garden
pelargoniums, that the central flower of the truss often loses the patches
of darker colour in the two upper petals; and that when this occurs, the
adherent nectary is quite aborted; when the colour is absent from only one
of the two upper petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and exterior
flowers of a head or umbel, I do not feel at all sure that C. C.
Sprengel's idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of these two
orders, is so far-fetched, as it may at first appear: and if it be
advantageous, natural selection may have come into play. But in regard to
the differences both in the internal and external structure of the seeds,
which are not always correlated with any differences in the flowers, it
seems impossible that they can be in any way advantageous to the plant:
yet in the Umbelliferae these differences are of such apparent importance—the
seeds being in some cases, according to Tausch, orthospermous in the
exterior flowers and coelospermous in the central flowers,—that the
elder De Candolle founded his main divisions of the order on analogous
differences. Hence we see that modifications of structure, viewed by
systematists as of high value, may be wholly due to unknown laws of
correlated growth, and without being, as far as we can see, of the
slightest service to the species.
We may often falsely attribute to correlation of growth, structures which
are common to whole groups of species, and which in truth are simply due
to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after thousands
of generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants
with diverse habits, would naturally be thought to be correlated in some
necessary manner. So, again, I do not doubt that some apparent
correlations, occurring throughout whole orders, are entirely due to the
manner alone in which natural selection can act. For instance, Alph. De
Candolle has remarked that winged seeds are never found in fruits which do
not open: I should explain the rule by the fact that seeds could not
gradually become winged through natural selection, except in fruits which
opened; so that the individual plants producing seeds which were a little
better fitted to be wafted further, might get an advantage over those
producing seed less fitted for dispersal; and this process could not
possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period, their
law of compensation or balancement of growth; or, as Goethe expressed it,
"in order to spend on one side, nature is forced to economise on the other
side." I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it
rarely flows, at least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties of
the cabbage do not yield abundant and nutritious foliage and a copious
supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our
poultry, a large tuft of feathers on the head is generally accompanied by
a diminished comb, and a large beard by diminished wattles. With species
in a state of nature it can hardly be maintained that the law is of
universal application; but many good observers, more especially botanists,
believe in its truth. I will not, however, here give any instances, for I
see hardly any way of distinguishing between the effects, on the one hand,
of a part being largely developed through natural selection and another
and adjoining part being reduced by this same process or by disuse, and,
on the other hand, the actual withdrawal of nutriment from one part owing
to the excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions
of life a structure before useful becomes less useful, any diminution,
however slight, in its development, will be seized on by natural
selection, for it will profit the individual not to have its nutriment
wasted in building up an useless structure. I can thus only understand a
fact with which I was much struck when examining cirripedes, and of which
many other instances could be given: namely, that when a cirripede is
parasitic within another and is thus protected, it loses more or less
completely its own shell or carapace. This is the case with the male Ibla,
and in a truly extraordinary manner with the Proteolepas: for the carapace
in all other cirripedes consists of the three highly-important anterior
segments of the head enormously developed, and furnished with great nerves
and muscles; but in the parasitic and protected Proteolepas, the whole
anterior part of the head is reduced to the merest rudiment attached to
the bases of the prehensile antennae. Now the saving of a large and
complex structure, when rendered superfluous by the parasitic habits of
the Proteolepas, though effected by slow steps, would be a decided
advantage to each successive individual of the species; for in the
struggle for life to which every animal is exposed, each individual
Proteolepas would have a better chance of supporting itself, by less
nutriment being wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long run
in reducing and saving every part of the organisation, as soon as it is
rendered superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And, conversely, that natural
selection may perfectly well succeed in largely developing any organ,
without requiring as a necessary compensation the reduction of some
adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in
varieties and in species, that when any part or organ is repeated many
times in the structure of the same individual (as the vertebrae in snakes,
and the stamens in polyandrous flowers) the number is variable; whereas
the number of the same part or organ, when it occurs in lesser numbers, is
constant. The same author and some botanists have further remarked that
multiple parts are also very liable to variation in structure. Inasmuch as
this "vegetative repetition," to use Professor Owen's expression, seems to
be a sign of low organisation; the foregoing remark seems connected with
the very general opinion of naturalists, that beings low in the scale of
nature are more variable than those which are higher. I presume that
lowness in this case means that the several parts of the organisation have
been but little specialised for particular functions; and as long as the
same part has to perform diversified work, we can perhaps see why it
should remain variable, that is, why natural selection should have
preserved or rejected each little deviation of form less carefully than
when the part has to serve for one special purpose alone. In the same way
that a knife which has to cut all sorts of things may be of almost any
shape; whilst a tool for some particular object had better be of some
particular shape. Natural selection, it should never be forgotten, can act
on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe with
truth, are apt to be highly variable. We shall have to recur to the
general subject of rudimentary and aborted organs; and I will here only
add that their variability seems to be owing to their uselessness, and
therefore to natural selection having no power to check deviations in
their structure. Thus rudimentary parts are left to the free play of the
various laws of growth, to the effects of long-continued disuse, and to
the tendency to reversion.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.
Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr. Waterhouse. I infer also from an observation made
by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of this proposition
without giving the long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my conviction that it
is a rule of high generality. I am aware of several causes of error, but I
hope that I have made due allowance for them. It should be understood that
the rule by no means applies to any part, however unusually developed,
unless it be unusually developed in comparison with the same part in
closely allied species. Thus, the bat's wing is a most abnormal structure
in the class mammalia; but the rule would not here apply, because there is
a whole group of bats having wings; it would apply only if some one
species of bat had its wings developed in some remarkable manner in
comparison with the other species of the same genus. The rule applies very
strongly in the case of secondary sexual characters, when displayed in any
unusual manner. The term, secondary sexual characters, used by Hunter,
applies to characters which are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and
females; but as females more rarely offer remarkable secondary sexual
characters, it applies more rarely to them. The rule being so plainly
applicable in the case of secondary sexual characters, may be due to the
great variability of these characters, whether or not displayed in any
unusual manner—of which fact I think there can be little doubt. But
that our rule is not confined to secondary sexual characters is clearly
shown in the case of hermaphrodite cirripedes; and I may here add, that I
particularly attended to Mr. Waterhouse's remark, whilst investigating
this Order, and I am fully convinced that the rule almost invariably holds
good with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates the
rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in different
genera; but in the several species of one genus, Pyrgoma, these valves
present a marvellous amount of diversification: the homologous valves in
the different species being sometimes wholly unlike in shape; and the
amount of variation in the individuals of several of the species is so
great, that it is no exaggeration to state that the varieties differ more
from each other in the characters of these important valves than do other
species of distinct genera.
As birds within the same country vary in a remarkably small degree, I have
particularly attended to them, and the rule seems to me certainly to hold
good in this class. I cannot make out that it applies to plants, and this
would seriously have shaken my belief in its truth, had not the great
variability in plants made it particularly difficult to compare their
relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner
in any species, the fair presumption is that it is of high importance to
that species; nevertheless the part in this case is eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species have descended from
other species, and have been modified through natural selection, I think
we can obtain some light. In our domestic animals, if any part, or the
whole animal, be neglected and no selection be applied, that part (for
instance, the comb in the Dorking fowl) or the whole breed will cease to
have a nearly uniform character. The breed will then be said to have
degenerated. In rudimentary organs, and in those which have been but
little specialised for any particular purpose, and perhaps in polymorphic
groups, we see a nearly parallel natural case; for in such cases natural
selection either has not or cannot come into full play, and thus the
organisation is left in a fluctuating condition. But what here more
especially concerns us is, that in our domestic animals those points,
which at the present time are undergoing rapid change by continued
selection, are also eminently liable to variation. Look at the breeds of
the pigeon; see what a prodigious amount of difference there is in the
beak of the different tumblers, in the beak and wattle of the different
carriers, in the carriage and tail of our fantails, etc., these being the
points now mainly attended to by English fanciers. Even in the sub-breeds,
as in the short-faced tumbler, it is notoriously difficult to breed them
nearly to perfection, and frequently individuals are born which depart
widely from the standard. There may be truly said to be a constant
struggle going on between, on the one hand, the tendency to reversion to a
less modified state, as well as an innate tendency to further variability
of all kinds, and, on the other hand, the power of steady selection to
keep the breed true. In the long run selection gains the day, and we do
not expect to fail so far as to breed a bird as coarse as a common tumbler
from a good short-faced strain. But as long as selection is rapidly going
on, there may always be expected to be much variability in the structure
undergoing modification. It further deserves notice that these variable
characters, produced by man's selection, sometimes become attached, from
causes quite unknown to us, more to one sex than to the other, generally
to the male sex, as with the wattle of carriers and the enlarged crop of
pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification, since the period when the species
branched off from the common progenitor of the genus. This period will
seldom be remote in any extreme degree, as species very rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability, which
has continually been accumulated by natural selection for the benefit of
the species. But as the variability of the extraordinarily-developed part
or organ has been so great and long-continued within a period not
excessively remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation, which
have remained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection on the
one hand, and the tendency to reversion and variability on the other hand,
will in the course of time cease; and that the most abnormally developed
organs may be made constant, I can see no reason to doubt. Hence when an
organ, however abnormal it may be, has been transmitted in approximately
the same condition to many modified descendants, as in the case of the
wing of the bat, it must have existed, according to my theory, for an
immense period in nearly the same state; and thus it comes to be no more
variable than any other structure. It is only in those cases in which the
modification has been comparatively recent and extraordinarily great that
we ought to find the GENERATIVE VARIABILITY, as it may be called, still
present in a high degree. For in this case the variability will seldom as
yet have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of those
tending to revert to a former and less modified condition.
The principle included in these remarks may be extended. It is notorious
that specific characters are more variable than generic. To explain by a
simple example what is meant. If some species in a large genus of plants
had blue flowers and some had red, the colour would be only a specific
character, and no one would be surprised at one of the blue species
varying into red, or conversely; but if all the species had blue flowers,
the colour would become a generic character, and its variation would be a
more unusual circumstance. I have chosen this example because an
explanation is not in this case applicable, which most naturalists would
advance, namely, that specific characters are more variable than generic,
because they are taken from parts of less physiological importance than
those commonly used for classing genera. I believe this explanation is
partly, yet only indirectly, true; I shall, however, have to return to
this subject in our chapter on Classification. It would be almost
superfluous to adduce evidence in support of the above statement, that
specific characters are more variable than generic; but I have repeatedly
noticed in works on natural history, that when an author has remarked with
surprise that some IMPORTANT organ or part, which is generally very
constant throughout large groups of species, has DIFFERED considerably in
closely-allied species, that it has, also, been VARIABLE in the
individuals of some of the species. And this fact shows that a character,
which is generally of generic value, when it sinks in value and becomes
only of specific value, often becomes variable, though its physiological
importance may remain the same. Something of the same kind applies to
monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no
doubt, that the more an organ normally differs in the different species of
the same group, the more subject it is to individual anomalies.
On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same part in
other independently-created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not
see that any explanation can be given. But on the view of species being
only strongly marked and fixed varieties, we might surely expect to find
them still often continuing to vary in those parts of their structure
which have varied within a moderately recent period, and which have thus
come to differ. Or to state the case in another manner:—the points
in which all the species of a genus resemble each other, and in which they
differ from the species of some other genus, are called generic
characters; and these characters in common I attribute to inheritance from
a common progenitor, for it can rarely have happened that natural
selection will have modified several species, fitted to more or less
widely-different habits, in exactly the same manner: and as these
so-called generic characters have been inherited from a remote period,
since that period when the species first branched off from their common
progenitor, and subsequently have not varied or come to differ in any
degree, or only in a slight degree, it is not probable that they should
vary at the present day. On the other hand, the points in which species
differ from other species of the same genus, are called specific
characters; and as these specific characters have varied and come to
differ within the period of the branching off of the species from a common
progenitor, it is probable that they should still often be in some degree
variable,—at least more variable than those parts of the
organisation which have for a very long period remained constant.
In connexion with the present subject, I will make only two other remarks.
I think it will be admitted, without my entering on details, that
secondary sexual characters are very variable; I think it also will be
admitted that species of the same group differ from each other more widely
in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the
males of gallinaceous birds, in which secondary sexual characters are
strongly displayed, with the amount of difference between their females;
and the truth of this proposition will be granted. The cause of the
original variability of secondary sexual characters is not manifest; but
we can see why these characters should not have been rendered as constant
and uniform as other parts of the organisation; for secondary sexual
characters have been accumulated by sexual selection, which is less rigid
in its action than ordinary selection, as it does not entail death, but
only gives fewer offspring to the less favoured males. Whatever the cause
may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus readily have succeeded in giving to the species of the same
group a greater amount of difference in their sexual characters, than in
other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between the
two sexes of the same species are generally displayed in the very same
parts of the organisation in which the different species of the same genus
differ from each other. Of this fact I will give in illustration two
instances, the first which happen to stand on my list; and as the
differences in these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the tarsi is a
character generally common to very large groups of beetles, but in the
Engidae, as Westwood has remarked, the number varies greatly; and the
number likewise differs in the two sexes of the same species: again in
fossorial hymenoptera, the manner of neuration of the wings is a character
of the highest importance, because common to large groups; but in certain
genera the neuration differs in the different species, and likewise in the
two sexes of the same species. This relation has a clear meaning on my
view of the subject: I look at all the species of the same genus as having
as certainly descended from the same progenitor, as have the two sexes of
any one of the species. Consequently, whatever part of the structure of
the common progenitor, or of its early descendants, became variable;
variations of this part would it is highly probable, be taken advantage of
by natural and sexual selection, in order to fit the several species to
their several places in the economy of nature, and likewise to fit the two
sexes of the same species to each other, or to fit the males and females
to different habits of life, or the males to struggle with other males for
the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;—that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in its
congeners; and the not great degree of variability in a part, however
extraordinarily it may be developed, if it be common to a whole group of
species;—that the great variability of secondary sexual characters,
and the great amount of difference in these same characters between
closely allied species;—that secondary sexual and ordinary specific
differences are generally displayed in the same parts of the organisation,—are
all principles closely connected together. All being mainly due to the
species of the same group having descended from a common progenitor, from
whom they have inherited much in common,—to parts which have
recently and largely varied being more likely still to go on varying than
parts which have long been inherited and have not varied,—to natural
selection having more or less completely, according to the lapse of time,
overmastered the tendency to reversion and to further variability,—to
sexual selection being less rigid than ordinary selection,—and to
variations in the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE
SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head and
feathers on the feet,—characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter, may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or roots as commonly called, of the Swedish turnip and Ruta baga,
plants which several botanists rank as varieties produced by cultivation
from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a
third may be added, namely, the common turnip. According to the ordinary
view of each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants, not
to the vera causa of community of descent, and a consequent tendency to
vary in a like manner, but to three separate yet closely related acts of
creation.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, a white rump, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I think confidently come
to this conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing
on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations. But
when a breed has been crossed only once by some other breed, the offspring
occasionally show a tendency to revert in character to the foreign breed
for many generations—some say, for a dozen or even a score of
generations. After twelve generations, the proportion of blood, to use a
common expression, of any one ancestor, is only 1 in 2048; and yet, as we
see, it is generally believed that a tendency to reversion is retained by
this very small proportion of foreign blood. In a breed which has not been
crossed, but in which BOTH parents have lost some character which their
progenitor possessed, the tendency, whether strong or weak, to reproduce
the lost character might be, as was formerly remarked, for all that we can
see to the contrary, transmitted for almost any number of generations.
When a character which has been lost in a breed, reappears after a great
number of generations, the most probable hypothesis is, not that the
offspring suddenly takes after an ancestor some hundred generations
distant, but that in each successive generation there has been a tendency
to reproduce the character in question, which at last, under unknown
favourable conditions, gains an ascendancy. For instance, it is probable
that in each generation of the barb-pigeon, which produces most rarely a
blue and black-barred bird, there has been a tendency in each generation
in the plumage to assume this colour. This view is hypothetical, but could
be supported by some facts; and I can see no more abstract improbability
in a tendency to produce any character being inherited for an endless
number of generations, than in quite useless or rudimentary organs being,
as we all know them to be, thus inherited. Indeed, we may sometimes
observe a mere tendency to produce a rudiment inherited: for instance, in
the common snapdragon (Antirrhinum) a rudiment of a fifth stamen so often
appears, that this plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to have
descended from a common parent, it might be expected that they would
occasionally vary in an analogous manner; so that a variety of one species
would resemble in some of its characters another species; this other
species being on my view only a well-marked and permanent variety. But
characters thus gained would probably be of an unimportant nature, for the
presence of all important characters will be governed by natural
selection, in accordance with the diverse habits of the species, and will
not be left to the mutual action of the conditions of life and of a
similar inherited constitution. It might further be expected that the
species of the same genus would occasionally exhibit reversions to lost
ancestral characters. As, however, we never know the exact character of
the common ancestor of a group, we could not distinguish these two cases:
if, for instance, we did not know that the rock-pigeon was not
feather-footed or turn-crowned, we could not have told, whether these
characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blueness was a case of
reversion, from the number of the markings, which are correlated with the
blue tint, and which it does not appear probable would all appear together
from simple variation. More especially we might have inferred this, from
the blue colour and marks so often appearing when distinct breeds of
diverse colours are crossed. Hence, though under nature it must generally
be left doubtful, what cases are reversions to an anciently existing
character, and what are new but analogous variations, yet we ought, on my
theory, sometimes to find the varying offspring of a species assuming
characters (either from reversion or from analogous variation) which
already occur in some other members of the same group. And this
undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable species in
our systematic works, is due to its varieties mocking, as it were, some of
the other species of the same genus. A considerable catalogue, also, could
be given of forms intermediate between two other forms, which themselves
must be doubtfully ranked as either varieties or species; and this shows,
unless all these forms be considered as independently created species,
that the one in varying has assumed some of the characters of the other,
so as to produce the intermediate form. But the best evidence is afforded
by parts or organs of an important and uniform nature occasionally varying
so as to acquire, in some degree, the character of the same part or organ
in an allied species. I have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and seem
to me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species
of the same genus, partly under domestication and partly under nature. It
is a case apparently of reversion. The ass not rarely has very distinct
transverse bars on its legs, like those on the legs of a zebra: it has
been asserted that these are plainest in the foal, and from inquiries
which I have made, I believe this to be true. It has also been asserted
that the stripe on each shoulder is sometimes double. The shoulder stripe
is certainly very variable in length and outline. A white ass, but NOT an
albino, has been described without either spinal or shoulder-stripe; and
these stripes are sometimes very obscure, or actually quite lost, in
dark-coloured asses. The koulan of Pallas is said to have been seen with a
double shoulder-stripe. The hemionus has no shoulder-stripe; but traces of
it, as stated by Mr. Blyth and others, occasionally appear: and I have
been informed by Colonel Poole that the foals of this species are
generally striped on the legs, and faintly on the shoulder. The quagga,
though so plainly barred like a zebra over the body, is without bars on
the legs; but Dr. Gray has figured one specimen with very distinct
zebra-like bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut: a faint shoulder-stripe may sometimes be seen in
duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes; and a man, whom I can
implicitly trust, has examined for me a small dun Welch pony with THREE
short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined the
breed for the Indian Government, a horse without stripes is not considered
as purely-bred. The spine is always striped; the legs are generally
barred; and the shoulder-stripe, which is sometimes double and sometimes
treble, is common; the side of the face, moreover, is sometimes striped.
The stripes are plainest in the foal; and sometimes quite disappear in old
horses. Colonel Poole has seen both gray and bay Kattywar horses striped
when first foaled. I have, also, reason to suspect, from information given
me by Mr. W. W. Edwards, that with the English race-horse the spinal
stripe is much commoner in the foal than in the full-grown animal. Without
here entering on further details, I may state that I have collected cases
of leg and shoulder stripes in horses of very different breeds, in various
countries from Britain to Eastern China; and from Norway in the north to
the Malay Archipelago in the south. In all parts of the world these
stripes occur far oftenest in duns and mouse-duns; by the term dun a large
range of colour is included, from one between brown and black to a close
approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse have descended from several
aboriginal species—one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But I am not at all satisfied with this theory, and should be loth
to apply it to breeds so distinct as the heavy Belgian cart-horse, Welch
ponies, cobs, the lanky Kattywar race, etc., inhabiting the most distant
parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and horse
is particularly apt to have bars on its legs. I once saw a mule with its
legs so much striped that any one at first would have thought that it must
have been the product of a zebra; and Mr. W. C. Martin, in his excellent
treatise on the horse, has given a figure of a similar mule. In four
coloured drawings, which I have seen, of hybrids between the ass and
zebra, the legs were much more plainly barred than the rest of the body;
and in one of them there was a double shoulder-stripe. In Lord Moreton's
famous hybrid from a chestnut mare and male quagga, the hybrid, and even
the pure offspring subsequently produced from the mare by a black Arabian
sire, were much more plainly barred across the legs than is even the pure
quagga. Lastly, and this is another most remarkable case, a hybrid has
been figured by Dr. Gray (and he informs me that he knows of a second
case) from the ass and the hemionus; and this hybrid, though the ass
seldom has stripes on its legs and the hemionus has none and has not even
a shoulder-stripe, nevertheless had all four legs barred, and had three
short shoulder-stripes, like those on the dun Welch pony, and even had
some zebra-like stripes on the sides of its face. With respect to this
last fact, I was so convinced that not even a stripe of colour appears
from what would commonly be called an accident, that I was led solely from
the occurrence of the face-stripes on this hybrid from the ass and
hemionus, to ask Colonel Poole whether such face-stripes ever occur in the
eminently striped Kattywar breed of horses, and was, as we have seen,
answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation, striped
on the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears—a tint
which approaches to that of the general colouring of the other species of
the genus. The appearance of the stripes is not accompanied by any change
of form or by any other new character. We see this tendency to become
striped most strongly displayed in hybrids from between several of the
most distinct species. Now observe the case of the several breeds of
pigeons: they are descended from a pigeon (including two or three
sub-species or geographical races) of a bluish colour, with certain bars
and other marks; and when any breed assumes by simple variation a bluish
tint, these bars and other marks invariably reappear; but without any
other change of form or character. When the oldest and truest breeds of
various colours are crossed, we see a strong tendency for the blue tint
and bars and marks to reappear in the mongrels. I have stated that the
most probable hypothesis to account for the reappearance of very ancient
characters, is—that there is a TENDENCY in the young of each
successive generation to produce the long-lost character, and that this
tendency, from unknown causes, sometimes prevails. And we have just seen
that in several species of the horse-genus the stripes are either plainer
or appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the horse-genus!
For myself, I venture confidently to look back thousands on thousands of
generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our domestic
horse, whether or not it be descended from one or more wild stocks, of the
ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will,
I presume, assert that each species has been created with a tendency to
vary, both under nature and under domestication, in this particular
manner, so as often to become striped like other species of the genus; and
that each has been created with a strong tendency, when crossed with
species inhabiting distant quarters of the world, to produce hybrids
resembling in their stripes, not their own parents, but other species of
the genus. To admit this view is, as it seems to me, to reject a real for
an unreal, or at least for an unknown, cause. It makes the works of God a
mere mockery and deception; I would almost as soon believe with the old
and ignorant cosmogonists, that fossil shells had never lived, but had
been created in stone so as to mock the shells now living on the
sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part
differs, more or less, from the same part in the parents. But whenever we
have the means of instituting a comparison, the same laws appear to have
acted in producing the lesser differences between varieties of the same
species, and the greater differences between species of the same genus.
The external conditions of life, as climate and food, etc., seem to have
induced some slight modifications. Habit in producing constitutional
differences, and use in strengthening, and disuse in weakening and
diminishing organs, seem to have been more potent in their effects.
Homologous parts tend to vary in the same way, and homologous parts tend
to cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed,
perhaps it tends to draw nourishment from the adjoining parts; and every
part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable to
understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to any
particular function, so that their modifications have not been closely
checked by natural selection. It is probably from this same cause that
organic beings low in the scale of nature are more variable than those
which have their whole organisation more specialised, and are higher in
the scale. Rudimentary organs, from being useless, will be disregarded by
natural selection, and hence probably are variable. Specific characters—that
is, the characters which have come to differ since the several species of
the same genus branched off from a common parent—are more variable
than generic characters, or those which have long been inherited, and have
not differed within this same period. In these remarks we have referred to
special parts or organs being still variable, because they have recently
varied and thus come to differ; but we have also seen in the second
Chapter that the same principle applies to the whole individual; for in a
district where many species of any genus are found—that is, where
there has been much former variation and differentiation, or where the
manufactory of new specific forms has been actively at work—there,
on an average, we now find most varieties or incipient species. Secondary
sexual characters are highly variable, and such characters differ much in
the species of the same group. Variability in the same parts of the
organisation has generally been taken advantage of in giving secondary
sexual differences to the sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in
comparison with the same part or organ in the allied species, must have
gone through an extraordinary amount of modification since the genus
arose; and thus we can understand why it should often still be variable in
a much higher degree than other parts; for variation is a long-continued
and slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants—which on my view must be a very slow process, requiring
a long lapse of time—in this case, natural selection may readily
have succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly the
same constitution from a common parent and exposed to similar influences
will naturally tend to present analogous variations, and these same
species may occasionally revert to some of the characters of their ancient
progenitors. Although new and important modifications may not arise from
reversion and analogous variation, such modifications will add to the
beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the offspring from
their parents—and a cause for each must exist—it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face of
this earth are enabled to struggle with each other, and the best adapted
to survive.
6. DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification. Transitions.
Absence or rarity of transitional varieties. Transitions in habits of
life. Diversified habits in the same species. Species with habits widely
different from those of their allies. Organs of extreme perfection. Means
of transition. Cases of difficulty. Natura non facit saltum. Organs of
small importance. Organs not in all cases absolutely perfect. The law of
Unity of Type and of the Conditions of Existence embraced by the theory of
Natural Selection.
Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so grave
that to this day I can never reflect on them without being staggered; but,
to the best of my judgment, the greater number are only apparent, and
those that are real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following
heads:—
Firstly, why, if species have descended from other species by insensibly
fine gradations, do we not everywhere see innumerable transitional forms?
Why is not all nature in confusion instead of the species being, as we see
them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modification
of some animal with wholly different habits? Can we believe that natural
selection could produce, on the one hand, organs of trifling importance,
such as the tail of a giraffe, which serves as a fly-flapper, and, on the
other hand, organs of such wonderful structure, as the eye, of which we
hardly as yet fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to so marvellous an instinct as that which leads the bee
to make cells, which have practically anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?
The two first heads shall be here discussed—Instinct and Hybridism
in separate chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved parent or
other less-favoured forms with which it comes into competition. Thus
extinction and natural selection will, as we have seen, go hand in hand.
Hence, if we look at each species as descended from some other unknown
form, both the parent and all the transitional varieties will generally
have been exterminated by the very process of formation and perfection of
the new form.
But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be much more convenient to discuss this question in the
chapter on the Imperfection of the geological record; and I will here only
state that I believe the answer mainly lies in the record being
incomparably less perfect than is generally supposed; the imperfection of
the record being chiefly due to organic beings not inhabiting profound
depths of the sea, and to their remains being embedded and preserved to a
future age only in masses of sediment sufficiently thick and extensive to
withstand an enormous amount of future degradation; and such fossiliferous
masses can be accumulated only where much sediment is deposited on the
shallow bed of the sea, whilst it slowly subsides. These contingencies
will concur only rarely, and after enormously long intervals. Whilst the
bed of the sea is stationary or is rising, or when very little sediment is
being deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural collections have been made
only at intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabit the
same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north to
south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the
same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and rarer,
the other becomes more and more frequent, till the one replaces the other.
But if we compare these species where they intermingle, they are generally
as absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species have descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent and
all the transitional varieties between its past and present states. Hence
we ought not to expect at the present time to meet with numerous
transitional varieties in each region, though they must have existed
there, and may be embedded there in a fossil condition. But in the
intermediate region, having intermediate conditions of life, why do we not
now find closely-linking intermediate varieties? This difficulty for a
long time quite confounded me. But I think it can be in large part
explained.
In the first place we should be extremely cautious in inferring, because
an area is now continuous, that it has been continuous during a long
period. Geology would lead us to believe that almost every continent has
been broken up into islands even during the later tertiary periods; and in
such islands distinct species might have been separately formed without
the possibility of intermediate varieties existing in the intermediate
zones. By changes in the form of the land and of climate, marine areas now
continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over
this way of escaping from the difficulty; for I believe that many
perfectly defined species have been formed on strictly continuous areas;
though I do not doubt that the formerly broken condition of areas now
continuous has played an important part in the formation of new species,
more especially with freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and
height or depth graduate away insensibly. But when we bear in mind that
almost every species, even in its metropolis, would increase immensely in
numbers, were it not for other competing species; that nearly all either
prey on or serve as prey for others; in short, that each organic being is
either directly or indirectly related in the most important manner to
other organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species, on which
it depends, or by which it is destroyed, or with which it comes into
competition; and as these species are already defined objects (however
they may have become so), not blending one into another by insensible
gradations, the range of any one species, depending as it does on the
range of others, will tend to be sharply defined. Moreover, each species
on the confines of its range, where it exists in lessened numbers, will,
during fluctuations in the number of its enemies or of its prey, or in the
seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each has a
wide range, with a comparatively narrow neutral territory between them, in
which they become rather suddenly rarer and rarer; then, as varieties do
not essentially differ from species, the same rule will probably apply to
both; and if we in imagination adapt a varying species to a very large
area, we shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur, they
are much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that
varieties linking two other varieties together have generally existed in
lesser numbers than the forms which they connect, then, I think, we can
understand why intermediate varieties should not endure for very long
periods;—why as a general rule they should be exterminated and
disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently
liable to the inroads of closely allied forms existing on both sides of
it. But a far more important consideration, as I believe, is that, during
the process of further modification, by which two varieties are supposed
on my theory to be converted and perfected into two distinct species, the
two which exist in larger numbers from inhabiting larger areas, will have
a great advantage over the intermediate variety, which exists in smaller
numbers in a narrow and intermediate zone. For forms existing in larger
numbers will always have a better chance, within any given period, of
presenting further favourable variations for natural selection to seize
on, than will the rarer forms which exist in lesser numbers. Hence, the
more common forms, in the race for life, will tend to beat and supplant
the less common forms, for these will be more slowly modified and
improved. It is the same principle which, as I believe, accounts for the
common species in each country, as shown in the second chapter, presenting
on an average a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three varieties of
sheep to be kept, one adapted to an extensive mountainous region; a second
to a comparatively narrow, hilly tract; and a third to wide plains at the
base; and that the inhabitants are all trying with equal steadiness and
skill to improve their stocks by selection; the chances in this case will
be strongly in favour of the great holders on the mountains or on the
plains improving their breeds more quickly than the small holders on the
intermediate narrow, hilly tract; and consequently the improved mountain
or plain breed will soon take the place of the less improved hill breed;
and thus the two breeds, which originally existed in greater numbers, will
come into close contact with each other, without the interposition of the
supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural selection
can do nothing until favourable variations chance to occur, and until a
place in the natural polity of the country can be better filled by some
modification of some one or more of its inhabitants. And such new places
will depend on slow changes of climate, or on the occasional immigration
of new inhabitants, and, probably, in a still more important degree, on
some of the old inhabitants becoming slowly modified, with the new forms
thus produced and the old ones acting and reacting on each other. So that,
in any one region and at any one time, we ought only to see a few species
presenting slight modifications of structure in some degree permanent; and
this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent
period in isolated portions, in which many forms, more especially amongst
the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several
representative species and their common parent, must formerly have existed
in each broken portion of the land, but these links will have been
supplanted and exterminated during the process of natural selection, so
that they will no longer exist in a living state.
Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is
probable, at first have been formed in the intermediate zones, but they
will generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the
actual distribution of closely allied or representative species, and
likewise of acknowledged varieties), exist in the intermediate zones in
lesser numbers than the varieties which they tend to connect. From this
cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted by
the forms which they connect; for these from existing in greater numbers
will, in the aggregate, present more variation, and thus be further
improved through natural selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so often
remarked, to exterminate the parent forms and the intermediate links.
Consequently evidence of their former existence could be found only
amongst fossil remains, which are preserved, as we shall in a future
chapter attempt to show, in an extremely imperfect and intermittent
record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for
instance, a land carnivorous animal could have been converted into one
with aquatic habits; for how could the animal in its transitional state
have subsisted? It would be easy to show that within the same group
carnivorous animals exist having every intermediate grade between truly
aquatic and strictly terrestrial habits; and as each exists by a struggle
for life, it is clear that each is well adapted in its habits to its place
in nature. Look at the Mustela vison of North America, which has webbed
feet and which resembles an otter in its fur, short legs, and form of
tail; during summer this animal dives for and preys on fish, but during
the long winter it leaves the frozen waters, and preys like other polecats
on mice and land animals. If a different case had been taken, and it had
been asked how an insectivorous quadruped could possibly have been
converted into a flying bat, the question would have been far more
difficult, and I could have given no answer. Yet I think such difficulties
have very little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in closely allied species
of the same genus; and of diversified habits, either constant or
occasional, in the same species. And it seems to me that nothing less than
a long list of such cases is sufficient to lessen the difficulty in any
particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called
flying squirrels; and flying squirrels have their limbs and even the base
of the tail united by a broad expanse of skin, which serves as a parachute
and allows them to glide through the air to an astonishing distance from
tree to tree. We cannot doubt that each structure is of use to each kind
of squirrel in its own country, by enabling it to escape birds or beasts
of prey, or to collect food more quickly, or, as there is reason to
believe, by lessening the danger from occasional falls. But it does not
follow from this fact that the structure of each squirrel is the best that
it is possible to conceive under all natural conditions. Let the climate
and vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some at least of the squirrels would decrease in numbers or
become exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until by the accumulated
effects of this process of natural selection, a perfect so-called flying
squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was falsely
ranked amongst bats. It has an extremely wide flank-membrane, stretching
from the corners of the jaw to the tail, and including the limbs and the
elongated fingers: the flank membrane is, also, furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding
through the air, now connect the Galeopithecus with the other Lemuridae,
yet I can see no difficulty in supposing that such links formerly existed,
and that each had been formed by the same steps as in the case of the less
perfectly gliding squirrels; and that each grade of structure had been
useful to its possessor. Nor can I see any insuperable difficulty in
further believing it possible that the membrane-connected fingers and
fore-arm of the Galeopithecus might be greatly lengthened by natural
selection; and this, as far as the organs of flight are concerned, would
convert it into a bat. In bats which have the wing-membrane extended from
the top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through the
air rather than for flight.
If about a dozen genera of birds had become extinct or were unknown, who
would have ventured to have surmised that birds might have existed which
used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed, for each
has to live by a struggle; but it is not necessarily the best possible
under all possible conditions. It must not be inferred from these remarks
that any of the grades of wing-structure here alluded to, which perhaps
may all have resulted from disuse, indicate the natural steps by which
birds have acquired their perfect power of flight; but they serve, at
least, to show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land, and seeing that we have
flying birds and mammals, flying insects of the most diversified types,
and formerly had flying reptiles, it is conceivable that flying-fish,
which now glide far through the air, slightly rising and turning by the
aid of their fluttering fins, might have been modified into perfectly
winged animals. If this had been effected, who would have ever imagined
that in an early transitional state they had been inhabitants of the open
ocean, and had used their incipient organs of flight exclusively, as far
as we know, to escape being devoured by other fish?
When we see any structure highly perfected for any particular habit, as
the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom continue
to exist to the present day, for they will have been supplanted by the
very process of perfection through natural selection. Furthermore, we may
conclude that transitional grades between structures fitted for very
different habits of life will rarely have been developed at an early
period in great numbers and under many subordinate forms. Thus, to return
to our imaginary illustration of the flying-fish, it does not seem
probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in many ways,
on the land and in the water, until their organs of flight had come to a
high stage of perfection, so as to have given them a decided advantage
over other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition will
always be less, from their having existed in lesser numbers, than in the
case of species with fully developed structures.
I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case occurs, it
would be easy for natural selection to fit the animal, by some
modification of its structure, for its changed habits, or exclusively for
one of its several different habits. But it is difficult to tell, and
immaterial for us, whether habits generally change first and structure
afterwards; or whether slight modifications of structure lead to changed
habits; both probably often change almost simultaneously. Of cases of
changed habits it will suffice merely to allude to that of the many
British insects which now feed on exotic plants, or exclusively on
artificial substances. Of diversified habits innumerable instances could
be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot and then proceeding
to another, like a kestrel, and at other times standing stationary on the
margin of water, and then dashing like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing branches,
almost like a creeper; it often, like a shrike, kills small birds by blows
on the head; and I have many times seen and heard it hammering the seeds
of the yew on a branch, and thus breaking them like a nuthatch. In North
America the black bear was seen by Hearne swimming for hours with widely
open mouth, thus catching, like a whale, insects in the water. Even in so
extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can see
no difficulty in a race of bears being rendered, by natural selection,
more and more aquatic in their structure and habits, with larger and
larger mouths, till a creature was produced as monstrous as a whale.
As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other species of
the same genus, we might expect, on my theory, that such individuals would
occasionally have given rise to new species, having anomalous habits, and
with their structure either slightly or considerably modified from that of
their proper type. And such instances do occur in nature. Can a more
striking instance of adaptation be given than that of a woodpecker for
climbing trees and for seizing insects in the chinks of the bark? Yet in
North America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing; and on the
plains of La Plata, where not a tree grows, there is a woodpecker, which
in every essential part of its organisation, even in its colouring, in the
harsh tone of its voice, and undulatory flight, told me plainly of its
close blood-relationship to our common species; yet it is a woodpecker
which never climbs a tree!
Petrels are the most aerial and oceanic of birds, yet in the quiet Sounds
of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, its manner of swimming, and of flying when
unwillingly it takes flight, would be mistaken by any one for an auk or
grebe; nevertheless, it is essentially a petrel, but with many parts of
its organisation profoundly modified. On the other hand, the acutest
observer by examining the dead body of the water-ouzel would never have
suspected its sub-aquatic habits; yet this anomalous member of the
strictly terrestrial thrush family wholly subsists by diving,—grasping
the stones with its feet and using its wings under water.
He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not at all in agreement. What can be plainer than
that the webbed feet of ducks and geese are formed for swimming? yet there
are upland geese with webbed feet which rarely or never go near the water;
and no one except Audubon has seen the frigate-bird, which has all its
four toes webbed, alight on the surface of the sea. On the other hand,
grebes and coots are eminently aquatic, although their toes are only
bordered by membrane. What seems plainer than that the long toes of
grallatores are formed for walking over swamps and floating plants, yet
the water-hen is nearly as aquatic as the coot; and the landrail nearly as
terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
rudimentary in function, though not in structure. In the frigate-bird, the
deeply-scooped membrane between the toes shows that structure has begun to
change.
He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle
for existence and in the principle of natural selection, will acknowledge
that every organic being is constantly endeavouring to increase in
numbers; and that if any one being vary ever so little, either in habits
or structure, and thus gain an advantage over some other inhabitant of the
country, it will seize on the place of that inhabitant, however different
it may be from its own place. Hence it will cause him no surprise that
there should be geese and frigate-birds with webbed feet, either living on
the dry land or most rarely alighting on the water; that there should be
long-toed corncrakes living in meadows instead of in swamps; that there
should be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I freely
confess, absurd in the highest possible degree. Yet reason tells me, that
if numerous gradations from a perfect and complex eye to one very
imperfect and simple, each grade being useful to its possessor, can be
shown to exist; if further, the eye does vary ever so slightly, and the
variations be inherited, which is certainly the case; and if any variation
or modification in the organ be ever useful to an animal under changing
conditions of life, then the difficulty of believing that a perfect and
complex eye could be formed by natural selection, though insuperable by
our imagination, can hardly be considered real. How a nerve comes to be
sensitive to light, hardly concerns us more than how life itself first
originated; but I may remark that several facts make me suspect that any
sensitive nerve may be rendered sensitive to light, and likewise to those
coarser vibrations of the air which produce sound.
In looking for the gradations by which an organ in any species has been
perfected, we ought to look exclusively to its lineal ancestors; but this
is scarcely ever possible, and we are forced in each case to look to
species of the same group, that is to the collateral descendants from the
same original parent-form, in order to see what gradations are possible,
and for the chance of some gradations having been transmitted from the
earlier stages of descent, in an unaltered or little altered condition.
Amongst existing Vertebrata, we find but a small amount of gradation in
the structure of the eye, and from fossil species we can learn nothing on
this head. In this great class we should probably have to descend far
beneath the lowest known fossiliferous stratum to discover the earlier
stages, by which the eye has been perfected.
In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this low
stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for instance,
there is a double cornea, the inner one divided into facets, within each
of which there is a lens-shaped swelling. In other crustaceans the
transparent cones which are coated by pigment, and which properly act only
by excluding lateral pencils of light, are convex at their upper ends and
must act by convergence; and at their lower ends there seems to be an
imperfect vitreous substance. With these facts, here far too briefly and
imperfectly given, which show that there is much graduated diversity in
the eyes of living crustaceans, and bearing in mind how small the number
of living animals is in proportion to those which have become extinct, I
can see no very great difficulty (not more than in the case of many other
structures) in believing that natural selection has converted the simple
apparatus of an optic nerve merely coated with pigment and invested by
transparent membrane, into an optical instrument as perfect as is
possessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing this treatise that large
bodies of facts, otherwise inexplicable, can be explained by the theory of
descent, ought not to hesitate to go further, and to admit that a
structure even as perfect as the eye of an eagle might be formed by
natural selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination; though I
have felt the difficulty far too keenly to be surprised at any degree of
hesitation in extending the principle of natural selection to such
startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We know
that this instrument has been perfected by the long-continued efforts of
the highest human intellects; and we naturally infer that the eye has been
formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with a nerve sensitive to light beneath, and then
suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and
thicknesses, placed at different distances from each other, and with the
surfaces of each layer slowly changing in form. Further we must suppose
that there is a power always intently watching each slight accidental
alteration in the transparent layers; and carefully selecting each
alteration which, under varied circumstances, may in any way, or in any
degree, tend to produce a distincter image. We must suppose each new state
of the instrument to be multiplied by the million; and each to be
preserved till a better be produced, and then the old ones to be
destroyed. In living bodies, variation will cause the slight alterations,
generation will multiply them almost infinitely, and natural selection
will pick out with unerring skill each improvement. Let this process go on
for millions on millions of years; and during each year on millions of
individuals of many kinds; and may we not believe that a living optical
instrument might thus be formed as superior to one of glass, as the works
of the Creator are to those of man?
If it could be demonstrated that any complex organ existed, which could
not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find out
no such case. No doubt many organs exist of which we do not know the
transitional grades, more especially if we look to much-isolated species,
round which, according to my theory, there has been much extinction. Or
again, if we look to an organ common to all the members of a large class,
for in this latter case the organ must have been first formed at an
extremely remote period, since which all the many members of the class
have been developed; and in order to discover the early transitional
grades through which the organ has passed, we should have to look to very
ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could
be given amongst the lower animals of the same organ performing at the
same time wholly distinct functions; thus the alimentary canal respires,
digests, and excretes in the larva of the dragon-fly and in the fish
Cobites. In the Hydra, the animal may be turned inside out, and the
exterior surface will then digest and the stomach respire. In such cases
natural selection might easily specialise, if any advantage were thus
gained, a part or organ, which had performed two functions, for one
function alone, and thus wholly change its nature by insensible steps. Two
distinct organs sometimes perform simultaneously the same function in the
same individual; to give one instance, there are fish with gills or
branchiae that breathe the air dissolved in the water, at the same time
that they breathe free air in their swimbladders, this latter organ having
a ductus pneumaticus for its supply, and being divided by highly vascular
partitions. In these cases, one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then this
other organ might be modified for some other and quite distinct purpose,
or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a wholly different purpose, namely respiration. The swimbladder has,
also, been worked in as an accessory to the auditory organs of certain
fish, or, for I do not know which view is now generally held, a part of
the auditory apparatus has been worked in as a complement to the
swimbladder. All physiologists admit that the swimbladder is homologous,
or "ideally similar," in position and structure with the lungs of the
higher vertebrate animals: hence there seems to me to be no great
difficulty in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true lungs
have descended by ordinary generation from an ancient prototype, of which
we know nothing, furnished with a floating apparatus or swimbladder. We
can thus, as I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle of food and
drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared—the slits on the sides of the neck
and the loop-like course of the arteries still marking in the embryo their
former position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some quite
distinct purpose: in the same manner as, on the view entertained by some
naturalists that the branchiae and dorsal scales of Annelids are
homologous with the wings and wing-covers of insects, it is probable that
organs which at a very ancient period served for respiration have been
actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give one more instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means of
a sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiae, the whole surface of the body
and sack, including the small frena, serving for respiration. The
Balanidae or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, in the
well-enclosed shell; but they have large folded branchiae. Now I think no
one will dispute that the ovigerous frena in the one family are strictly
homologous with the branchiae of the other family; indeed, they graduate
into each other. Therefore I do not doubt that little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes had
become extinct, and they have already suffered far more extinction than
have sessile cirripedes, who would ever have imagined that the branchiae
in this latter family had originally existed as organs for preventing the
ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ could
not possibly have been produced by successive transitional gradations,
yet, undoubtedly, grave cases of difficulty occur, some of which will be
discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; it is impossible to conceive by
what steps these wondrous organs have been produced; but, as Owen and
others have remarked, their intimate structure closely resembles that of
common muscle; and as it has lately been shown that Rays have an organ
closely analogous to the electric apparatus, and yet do not, as Matteuchi
asserts, discharge any electricity, we must own that we are far too
ignorant to argue that no transition of any kind is possible.
The electric organs offer another and even more serious difficulty; for
they occur in only about a dozen fishes, of which several are widely
remote in their affinities. Generally when the same organ appears in
several members of the same class, especially if in members having very
different habits of life, we may attribute its presence to inheritance
from a common ancestor; and its absence in some of the members to its loss
through disuse or natural selection. But if the electric organs had been
inherited from one ancient progenitor thus provided, we might have
expected that all electric fishes would have been specially related to
each other. Nor does geology at all lead to the belief that formerly most
fishes had electric organs, which most of their modified descendants have
lost. The presence of luminous organs in a few insects, belonging to
different families and orders, offers a parallel case of difficulty. Other
cases could be given; for instance in plants, the very curious contrivance
of a mass of pollen-grains, borne on a foot-stalk with a sticky gland at
the end, is the same in Orchis and Asclepias,—genera almost as
remote as possible amongst flowering plants. In all these cases of two
very distinct species furnished with apparently the same anomalous organ,
it should be observed that, although the general appearance and function
of the organ may be the same, yet some fundamental difference can
generally be detected. I am inclined to believe that in nearly the same
way as two men have sometimes independently hit on the very same
invention, so natural selection, working for the good of each being and
taking advantage of analogous variations, has sometimes modified in very
nearly the same manner two parts in two organic beings, which owe but
little of their structure in common to inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the extinct
and unknown is very small, I have been astonished how rarely an organ can
be named, towards which no transitional grade is known to lead. The truth
of this remark is indeed shown by that old canon in natural history of
"Natura non facit saltum." We meet with this admission in the writings of
almost every experienced naturalist; or, as Milne Edwards has well
expressed it, nature is prodigal in variety, but niggard in innovation.
Why, on the theory of Creation, should this be so? Why should all the
parts and organs of many independent beings, each supposed to have been
separately created for its proper place in nature, be so invariably linked
together by graduated steps? Why should not Nature have taken a leap from
structure to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection can act only by
taking advantage of slight successive variations; she can never take a
leap, but must advance by the shortest and slowest steps.
ORGANS OF LITTLE APPARENT IMPORTANCE.
As natural selection acts by life and death,—by the preservation of
individuals with any favourable variation, and by the destruction of those
with any unfavourable deviation of structure,—I have sometimes felt
much difficulty in understanding the origin of simple parts, of which the
importance does not seem sufficient to cause the preservation of
successively varying individuals. I have sometimes felt as much
difficulty, though of a very different kind, on this head, as in the case
of an organ as perfect and complex as the eye.
In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
most trifling characters, such as the down on fruit and the colour of the
flesh, which, from determining the attacks of insects or from being
correlated with constitutional differences, might assuredly be acted on by
natural selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this could
have been adapted for its present purpose by successive slight
modifications, each better and better, for so trifling an object as
driving away flies; yet we should pause before being too positive even in
this case, for we know that the distribution and existence of cattle and
other animals in South America absolutely depends on their power of
resisting the attacks of insects: so that individuals which could by any
means defend themselves from these small enemies, would be able to range
into new pastures and thus gain a great advantage. It is not that the
larger quadrupeds are actually destroyed (except in some rare cases) by
the flies, but they are incessantly harassed and their strength reduced,
so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted in nearly the same state,
although now become of very slight use; and any actually injurious
deviations in their structure will always have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as with the dog, though the aid must
be slight, for the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters
which are really of very little importance, and which have originated from
quite secondary causes, independently of natural selection. We should
remember that climate, food, etc., probably have some little direct
influence on the organisation; that characters reappear from the law of
reversion; that correlation of growth will have had a most important
influence in modifying various structures; and finally, that sexual
selection will often have largely modified the external characters of
animals having a will, to give one male an advantage in fighting with
another or in charming the females. Moreover when a modification of
structure has primarily arisen from the above or other unknown causes, it
may at first have been of no advantage to the species, but may
subsequently have been taken advantage of by the descendants of the
species under new conditions of life and with newly acquired habits.
To give a few instances to illustrate these latter remarks. If green
woodpeckers alone had existed, and we did not know that there were many
black and pied kinds, I dare say that we should have thought that the
green colour was a beautiful adaptation to hide this tree-frequenting bird
from its enemies; and consequently that it was a character of importance
and might have been acquired through natural selection; as it is, I have
no doubt that the colour is due to some quite distinct cause, probably to
sexual selection. A trailing bamboo in the Malay Archipelago climbs the
loftiest trees by the aid of exquisitely constructed hooks clustered
around the ends of the branches, and this contrivance, no doubt, is of the
highest service to the plant; but as we see nearly similar hooks on many
trees which are not climbers, the hooks on the bamboo may have arisen from
unknown laws of growth, and have been subsequently taken advantage of by
the plant undergoing further modification and becoming a climber. The
naked skin on the head of a vulture is generally looked at as a direct
adaptation for wallowing in putridity; and so it may be, or it may
possibly be due to the direct action of putrid matter; but we should be
very cautious in drawing any such inference, when we see that the skin on
the head of the clean-feeding male turkey is likewise naked. The sutures
in the skulls of young mammals have been advanced as a beautiful
adaptation for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and unimportant
variations; and we are immediately made conscious of this by reflecting on
the differences in the breeds of our domesticated animals in different
countries,—more especially in the less civilized countries where
there has been but little artificial selection. Careful observers are
convinced that a damp climate affects the growth of the hair, and that
with the hair the horns are correlated. Mountain breeds always differ from
lowland breeds; and a mountainous country would probably affect the hind
limbs from exercising them more, and possibly even the form of the pelvis;
and then by the law of homologous variation, the front limbs and even the
head would probably be affected. The shape, also, of the pelvis might
affect by pressure the shape of the head of the young in the womb. The
laborious breathing necessary in high regions would, we have some reason
to believe, increase the size of the chest; and again correlation would
come into play. Animals kept by savages in different countries often have
to struggle for their own subsistence, and would be exposed to a certain
extent to natural selection, and individuals with slightly different
constitutions would succeed best under different climates; and there is
reason to believe that constitution and colour are correlated. A good
observer, also, states that in cattle susceptibility to the attacks of
flies is correlated with colour, as is the liability to be poisoned by
certain plants; so that colour would be thus subjected to the action of
natural selection. But we are far too ignorant to speculate on the
relative importance of the several known and unknown laws of variation;
and I have here alluded to them only to show that, if we are unable to
account for the characteristic differences of our domestic breeds, which
nevertheless we generally admit to have arisen through ordinary
generation, we ought not to lay too much stress on our ignorance of the
precise cause of the slight analogous differences between species. I might
have adduced for this same purpose the differences between the races of
man, which are so strongly marked; I may add that some little light can
apparently be thrown on the origin of these differences, chiefly through
sexual selection of a particular kind, but without here entering on
copious details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor. They
believe that very many structures have been created for beauty in the eyes
of man, or for mere variety. This doctrine, if true, would be absolutely
fatal to my theory. Yet I fully admit that many structures are of no
direct use to their possessors. Physical conditions probably have had some
little effect on structure, quite independently of any good thus gained.
Correlation of growth has no doubt played a most important part, and a
useful modification of one part will often have entailed on other parts
diversified changes of no direct use. So again characters which formerly
were useful, or which formerly had arisen from correlation of growth, or
from other unknown cause, may reappear from the law of reversion, though
now of no direct use. The effects of sexual selection, when displayed in
beauty to charm the females, can be called useful only in rather a forced
sense. But by far the most important consideration is that the chief part
of the organisation of every being is simply due to inheritance; and
consequently, though each being assuredly is well fitted for its place in
nature, many structures now have no direct relation to the habits of life
of each species. Thus, we can hardly believe that the webbed feet of the
upland goose or of the frigate-bird are of special use to these birds; we
cannot believe that the same bones in the arm of the monkey, in the fore
leg of the horse, in the wing of the bat, and in the flipper of the seal,
are of special use to these animals. We may safely attribute these
structures to inheritance. But to the progenitor of the upland goose and
of the frigate-bird, webbed feet no doubt were as useful as they now are
to the most aquatic of existing birds. So we may believe that the
progenitor of the seal had not a flipper, but a foot with five toes fitted
for walking or grasping; and we may further venture to believe that the
several bones in the limbs of the monkey, horse, and bat, which have been
inherited from a common progenitor, were formerly of more special use to
that progenitor, or its progenitors, than they now are to these animals
having such widely diversified habits. Therefore we may infer that these
several bones might have been acquired through natural selection,
subjected formerly, as now, to the several laws of inheritance, reversion,
correlation of growth, etc. Hence every detail of structure in every
living creature (making some little allowance for the direct action of
physical conditions) may be viewed, either as having been of special use
to some ancestral form, or as being now of special use to the descendants
of this form—either directly, or indirectly through the complex laws
of growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang
of the adder, and in the ovipositor of the ichneumon, by which its eggs
are deposited in the living bodies of other insects. If it could be proved
that any part of the structure of any one species had been formed for the
exclusive good of another species, it would annihilate my theory, for such
could not have been produced through natural selection. Although many
statements may be found in works on natural history to this effect, I
cannot find even one which seems to me of any weight. It is admitted that
the rattlesnake has a poison-fang for its own defence and for the
destruction of its prey; but some authors suppose that at the same time
this snake is furnished with a rattle for its own injury, namely, to warn
its prey to escape. I would almost as soon believe that the cat curls the
end of its tail when preparing to spring, in order to warn the doomed
mouse. But I have not space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each. No
organ will be formed, as Paley has remarked, for the purpose of causing
pain or for doing an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will be found on the
whole advantageous. After the lapse of time, under changing conditions of
life, if any part comes to be injurious, it will be modified; or if it be
not so, the being will become extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than, the other inhabitants of the same country with
which it has to struggle for existence. And we see that this is the degree
of perfection attained under nature. The endemic productions of New
Zealand, for instance, are perfect one compared with another; but they are
now rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said,
on high authority, not to be perfect even in that most perfect organ, the
eye. If our reason leads us to admire with enthusiasm a multitude of
inimitable contrivances in nature, this same reason tells us, though we
may easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the wasp or of the bee as perfect,
which, when used against many attacking animals, cannot be withdrawn,
owing to the backward serratures, and so inevitably causes the death of
the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so
many members of the same great order, and which has been modified but not
perfected for its present purpose, with the poison originally adapted to
cause galls subsequently intensified, we can perhaps understand how it is
that the use of the sting should so often cause the insect's own death:
for if on the whole the power of stinging be useful to the community, it
will fulfil all the requirements of natural selection, though it may cause
the death of some few members. If we admire the truly wonderful power of
scent by which the males of many insects find their females, can we admire
the production for this single purpose of thousands of drones, which are
utterly useless to the community for any other end, and which are
ultimately slaughtered by their industrious and sterile sisters? It may be
difficult, but we ought to admire the savage instinctive hatred of the
queen-bee, which urges her instantly to destroy the young queens her
daughters as soon as born, or to perish herself in the combat; for
undoubtedly this is for the good of the community; and maternal love or
maternal hatred, though the latter fortunately is most rare, is all the
same to the inexorable principle of natural selection. If we admire the
several ingenious contrivances, by which the flowers of the orchis and of
many other plants are fertilised through insect agency, can we consider as
equally perfect the elaboration by our fir-trees of dense clouds of
pollen, in order that a few granules may be wafted by a chance breeze on
to the ovules?
SUMMARY OF CHAPTER.
We have in this chapter discussed some of the difficulties and objections
which may be urged against my theory. Many of them are very grave; but I
think that in the discussion light has been thrown on several facts, which
on the theory of independent acts of creation are utterly obscure. We have
seen that species at any one period are not indefinitely variable, and are
not linked together by a multitude of intermediate gradations, partly
because the process of natural selection will always be very slow, and
will act, at any one time, only on a very few forms; and partly because
the very process of natural selection almost implies the continual
supplanting and extinction of preceding and intermediate gradations.
Closely allied species, now living on a continuous area, must often have
been formed when the area was not continuous, and when the conditions of
life did not insensibly graduate away from one part to another. When two
varieties are formed in two districts of a continuous area, an
intermediate variety will often be formed, fitted for an intermediate
zone; but from reasons assigned, the intermediate variety will usually
exist in lesser numbers than the two forms which it connects; consequently
the two latter, during the course of further modification, from existing
in greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting and
exterminating it.
We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first could only glide through the air.
We have seen that a species may under new conditions of life change its
habits, or have diversified habits, with some habits very unlike those of
its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one; yet
in the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing conditions
of life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases
in which we know of no intermediate or transitional states, we should be
very cautious in concluding that none could have existed, for the
homologies of many organs and their intermediate states show that
wonderful metamorphoses in function are at least possible. For instance, a
swim-bladder has apparently been converted into an air-breathing lung. The
same organ having performed simultaneously very different functions, and
then having been specialised for one function; and two very distinct
organs having performed at the same time the same function, the one having
been perfected whilst aided by the other, must often have largely
facilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assert
that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly accumulated
by means of natural selection. But we may confidently believe that many
modifications, wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken advantage of by
the still further modified descendants of this species. We may, also,
believe that a part formerly of high importance has often been retained
(as the tail of an aquatic animal by its terrestrial descendants), though
it has become of such small importance that it could not, in its present
state, have been acquired by natural selection,—a power which acts
solely by the preservation of profitable variations in the struggle for
life.
Natural selection will produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the owner.
Natural selection in each well-stocked country, must act chiefly through
the competition of the inhabitants one with another, and consequently will
produce perfection, or strength in the battle for life, only according to
the standard of that country. Hence the inhabitants of one country,
generally the smaller one, will often yield, as we see they do yield, to
the inhabitants of another and generally larger country. For in the larger
country there will have existed more individuals, and more diversified
forms, and the competition will have been severer, and thus the standard
of perfection will have been rendered higher. Natural selection will not
necessarily produce absolute perfection; nor, as far as we can judge by
our limited faculties, can absolute perfection be everywhere found.
On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look only to the present inhabitants of the world, is
not strictly correct, but if we include all those of past times, it must
by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed on
two great laws—Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure, which we
see in organic beings of the same class, and which is quite independent of
their habits of life. On my theory, unity of type is explained by unity of
descent. The expression of conditions of existence, so often insisted on
by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying
parts of each being to its organic and inorganic conditions of life; or by
having adapted them during long-past periods of time: the adaptations
being aided in some cases by use and disuse, being slightly affected by
the direct action of the external conditions of life, and being in all
cases subjected to the several laws of growth. Hence, in fact, the law of
the Conditions of Existence is the higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type.
7. INSTINCT.
Instincts comparable with habits, but different in their origin. Instincts
graduated. Aphides and ants. Instincts variable. Domestic instincts, their
origin. Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the
theory of the Natural Selection of instincts. Neuter or sterile insects.
Summary.
The subject of instinct might have been worked into the previous chapters;
but I have thought that it would be more convenient to treat the subject
separately, especially as so wonderful an instinct as that of the hive-bee
making its cells will probably have occurred to many readers, as a
difficulty sufficient to overthrow my whole theory. I must premise, that I
have nothing to do with the origin of the primary mental powers, any more
than I have with that of life itself. We are concerned only with the
diversities of instinct and of the other mental qualities of animals
within the same class.
I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
action, which we ourselves should require experience to enable us to
perform, when performed by an animal, more especially by a very young one,
without any experience, and when performed by many individuals in the same
way, without their knowing for what purpose it is performed, is usually
said to be instinctive. But I could show that none of these characters of
instinct are universal. A little dose, as Pierre Huber expresses it, of
judgment or reason, often comes into play, even in animals very low in the
scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably accurate
notion of the frame of mind under which an instinctive action is
performed, but not of its origin. How unconsciously many habitual actions
are performed, indeed not rarely in direct opposition to our conscious
will! yet they may be modified by the will or reason. Habits easily become
associated with other habits, and with certain periods of time and states
of the body. When once acquired, they often remain constant throughout
life. Several other points of resemblance between instincts and habits
could be pointed out. As in repeating a well-known song, so in instincts,
one action follows another by a sort of rhythm; if a person be interrupted
in a song, or in repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought: so P. Huber found it was
with a caterpillar, which makes a very complicated hammock; for if he took
a caterpillar which had completed its hammock up to, say, the sixth stage
of construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already done
for it, far from feeling the benefit of this, it was much embarrassed,
and, in order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the already
finished work.
If we suppose any habitual action to become inherited—and
I think it can be shown that this does sometimes happen—then the
resemblance between what originally was a habit and an instinct becomes so
close as not to be distinguished. If Mozart, instead of playing the
pianoforte at three years old with wonderfully little practice, had played
a tune with no practice at all, he might truly be said to have done so
instinctively. But it would be the most serious error to suppose that the
greater number of instincts have been acquired by habit in one generation,
and then transmitted by inheritance to succeeding generations. It can be
clearly shown that the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants, could not
possibly have been thus acquired.
It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may be
profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what may
be called accidental variations of instincts;—that is of variations
produced by the same unknown causes which produce slight deviations of
bodily structure.
No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired—for these could be found
only in the lineal ancestors of each species—but we ought to find in
the collateral lines of descent some evidence of such gradations; or we
ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed except in Europe and North America, and for no instinct being
known amongst extinct species, how very generally gradations, leading to
the most complex instincts, can be discovered. The canon of "Natura non
facit saltum" applies with almost equal force to instincts as to bodily
organs. Changes of instinct may sometimes be facilitated by the same
species having different instincts at different periods of life, or at
different seasons of the year, or when placed under different
circumstances, etc.; in which case either one or the other instinct might
be preserved by natural selection. And such instances of diversity of
instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has never, as
far as we can judge, been produced for the exclusive good of others. One
of the strongest instances of an animal apparently performing an action
for the sole good of another, with which I am acquainted, is that of
aphides voluntarily yielding their sweet excretion to ants: that they do
so voluntarily, the following facts show. I removed all the ants from a
group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that the
aphides would want to excrete. I watched them for some time through a
lens, but not one excreted; I then tickled and stroked them with a hair in
the same manner, as well as I could, as the ants do with their antennae;
but not one excreted. Afterwards I allowed an ant to visit them, and it
immediately seemed, by its eager way of running about, to be well aware
what a rich flock it had discovered; it then began to play with its
antennae on the abdomen first of one aphis and then of another; and each
aphis, as soon as it felt the antennae, immediately lifted up its abdomen
and excreted a limpid drop of sweet juice, which was eagerly devoured by
the ant. Even the quite young aphides behaved in this manner, showing that
the action was instinctive, and not the result of experience. But as the
excretion is extremely viscid, it is probably a convenience to the aphides
to have it removed; and therefore probably the aphides do not
instinctively excrete for the sole good of the ants. Although I do not
believe that any animal in the world performs an action for the exclusive
good of another of a distinct species, yet each species tries to take
advantage of the instincts of others, as each takes advantage of the
weaker bodily structure of others. So again, in some few cases, certain
instincts cannot be considered as absolutely perfect; but as details on
this and other such points are not indispensable, they may be here passed
over.
As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to have been here
given; but want of space prevents me. I can only assert, that instincts
certainly do vary—for instance, the migratory instinct, both in
extent and direction, and in its total loss. So it is with the nests of
birds, which vary partly in dependence on the situations chosen, and on
the nature and temperature of the country inhabited, but often from causes
wholly unknown to us: Audubon has given several remarkable cases of
differences in nests of the same species in the northern and southern
United States. Fear of any particular enemy is certainly an instinctive
quality, as may be seen in nestling birds, though it is strengthened by
experience, and by the sight of fear of the same enemy in other animals.
But fear of man is slowly acquired, as I have elsewhere shown, by various
animals inhabiting desert islands; and we may see an instance of this,
even in England, in the greater wildness of all our large birds than of
our small birds; for the large birds have been most persecuted by man. We
may safely attribute the greater wildness of our large birds to this
cause; for in uninhabited islands large birds are not more fearful than
small; and the magpie, so wary in England, is tame in Norway, as is the
hooded crow in Egypt.
That the general disposition of individuals of the same species, born in a
state of nature, is extremely diversified, can be shown by a multitude of
facts. Several cases also, could be given, of occasional and strange
habits in certain species, which might, if advantageous to the species,
give rise, through natural selection, to quite new instincts. But I am
well aware that these general statements, without facts given in detail,
can produce but a feeble effect on the reader's mind. I can only repeat my
assurance, that I do not speak without good evidence.
The possibility, or even probability, of inherited variations of instinct
in a state of nature will be strengthened by briefly considering a few
cases under domestication. We shall thus also be enabled to see the
respective parts which habit and the selection of so-called accidental
variations have played in modifying the mental qualities of our domestic
animals. A number of curious and authentic instances could be given of the
inheritance of all shades of disposition and tastes, and likewise of the
oddest tricks, associated with certain frames of mind or periods of time.
But let us look to the familiar case of the several breeds of dogs: it
cannot be doubted that young pointers (I have myself seen a striking
instance) will sometimes point and even back other dogs the very first
time that they are taken out; retrieving is certainly in some degree
inherited by retrievers; and a tendency to run round, instead of at, a
flock of sheep, by shepherd-dogs. I cannot see that these actions,
performed without experience by the young, and in nearly the same manner
by each individual, performed with eager delight by each breed, and
without the end being known,—for the young pointer can no more know
that he points to aid his master, than the white butterfly knows why she
lays her eggs on the leaf of the cabbage,—I cannot see that these
actions differ essentially from true instincts. If we were to see one kind
of wolf, when young and without any training, as soon as it scented its
prey, stand motionless like a statue, and then slowly crawl forward with a
peculiar gait; and another kind of wolf rushing round, instead of at, a
herd of deer, and driving them to a distant point, we should assuredly
call these actions instinctive. Domestic instincts, as they may be called,
are certainly far less fixed or invariable than natural instincts; but
they have been acted on by far less rigorous selection, and have been
transmitted for an incomparably shorter period, under less fixed
conditions of life.
How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when
thus tested by crossing, resemble natural instincts, which in a like
manner become curiously blended together, and for a long period exhibit
traces of the instincts of either parent: for example, Le Roy describes a
dog, whose great-grandfather was a wolf, and this dog showed a trace of
its wild parentage only in one way, by not coming in a straight line to
his master when called.
Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this, I
think, is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble,—an action
which, as I have witnessed, is performed by young birds, that have never
seen a pigeon tumble. We may believe that some one pigeon showed a slight
tendency to this strange habit, and that the long-continued selection of
the best individuals in successive generations made tumblers what they now
are; and near Glasgow there are house-tumblers, as I hear from Mr. Brent,
which cannot fly eighteen inches high without going head over heels. It
may be doubted whether any one would have thought of training a dog to
point, had not some one dog naturally shown a tendency in this line; and
this is known occasionally to happen, as I once saw in a pure terrier.
When the first tendency was once displayed, methodical selection and the
inherited effects of compulsory training in each successive generation
would soon complete the work; and unconscious selection is still at work,
as each man tries to procure, without intending to improve the breed, dogs
which will stand and hunt best. On the other hand, habit alone in some
cases has sufficed; no animal is more difficult to tame than the young of
the wild rabbit; scarcely any animal is tamer than the young of the tame
rabbit; but I do not suppose that domestic rabbits have ever been selected
for tameness; and I presume that we must attribute the whole of the
inherited change from extreme wildness to extreme tameness, simply to
habit and long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how universally and largely the minds of our domestic
animals have been modified by domestication. It is scarcely possible to
doubt that the love of man has become instinctive in the dog. All wolves,
foxes, jackals, and species of the cat genus, when kept tame, are most
eager to attack poultry, sheep, and pigs; and this tendency has been found
incurable in dogs which have been brought home as puppies from countries,
such as Tierra del Fuego and Australia, where the savages do not keep
these domestic animals. How rarely, on the other hand, do our civilised
dogs, even when quite young, require to be taught not to attack poultry,
sheep, and pigs! No doubt they occasionally do make an attack, and are
then beaten; and if not cured, they are destroyed; so that habit, with
some degree of selection, has probably concurred in civilising by
inheritance our dogs. On the other hand, young chickens have lost, wholly
by habit, that fear of the dog and cat which no doubt was originally
instinctive in them, in the same way as it is so plainly instinctive in
young pheasants, though reared under a hen. It is not that chickens have
lost all fear, but fear only of dogs and cats, for if the hen gives the
danger-chuckle, they will run (more especially young turkeys) from under
her, and conceal themselves in the surrounding grass or thickets; and this
is evidently done for the instinctive purpose of allowing, as we see in
wild ground-birds, their mother to fly away. But this instinct retained by
our chickens has become useless under domestication, for the mother-hen
has almost lost by disuse the power of flight.
Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar mental
habits and actions, which at first appeared from what we must in our
ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce such inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most cases,
probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of nature have
become modified by selection, by considering a few cases. I will select
only three, out of the several which I shall have to discuss in my future
work,—namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain ants; and the
comb-making power of the hive-bee: these two latter instincts have
generally, and most justly, been ranked by naturalists as the most
wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of the
cuckoo's instinct is, that she lays her eggs, not daily, but at intervals
of two or three days; so that, if she were to make her own nest and sit on
her own eggs, those first laid would have to be left for some time
unincubated, or there would be eggs and young birds of different ages in
the same nest. If this were the case, the process of laying and hatching
might be inconveniently long, more especially as she has to migrate at a
very early period; and the first hatched young would probably have to be
fed by the male alone. But the American cuckoo is in this predicament; for
she makes her own nest and has eggs and young successively hatched, all at
the same time. It has been asserted that the American cuckoo occasionally
lays her eggs in other birds' nests; but I hear on the high authority of
Dr. Brewer, that this is a mistake. Nevertheless, I could give several
instances of various birds which have been known occasionally to lay their
eggs in other birds' nests. Now let us suppose that the ancient progenitor
of our European cuckoo had the habits of the American cuckoo; but that
occasionally she laid an egg in another bird's nest. If the old bird
profited by this occasional habit, or if the young were made more vigorous
by advantage having been taken of the mistaken maternal instinct of
another bird, than by their own mother's care, encumbered as she can
hardly fail to be by having eggs and young of different ages at the same
time; then the old birds or the fostered young would gain an advantage.
And analogy would lead me to believe, that the young thus reared would be
apt to follow by inheritance the occasional and aberrant habit of their
mother, and in their turn would be apt to lay their eggs in other birds'
nests, and thus be successful in rearing their young. By a continued
process of this nature, I believe that the strange instinct of our cuckoo
could be, and has been, generated. I may add that, according to Dr. Gray
and to some other observers, the European cuckoo has not utterly lost all
maternal love and care for her own offspring.
The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with the
Gallinaceae; and this perhaps explains the origin of a singular instinct
in the allied group of ostriches. For several hen ostriches, at least in
the case of the American species, unite and lay first a few eggs in one
nest and then in another; and these are hatched by the males. This
instinct may probably be accounted for by the fact of the hens laying a
large number of eggs; but, as in the case of the cuckoo, at intervals of
two or three days. This instinct, however, of the American ostrich has not
as yet been perfected; for a surprising number of eggs lie strewed over
the plains, so that in one day's hunting I picked up no less than twenty
lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of bees of
other kinds. This case is more remarkable than that of the cuckoo; for
these bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would be necessary if they had to store
food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown
good reason for believing that although the Tachytes nigra generally makes
its own burrow and stores it with paralysed prey for its own larvae to
feed on, yet that when this insect finds a burrow already made and stored
by another sphex, it takes advantage of the prize, and becomes for the
occasion parasitic. In this case, as with the supposed case of the cuckoo,
I can see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose nest
and stored food are thus feloniously appropriated, be not thus
exterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their aid,
the species would certainly become extinct in a single year. The males and
fertile females do no work. The workers or sterile females, though most
energetic and courageous in capturing slaves, do no other work. They are
incapable of making their own nests, or of feeding their own larvae. When
the old nest is found inconvenient, and they have to migrate, it is the
slaves which determine the migration, and actually carry their masters in
their jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvae and pupae to stimulate them to work, they
did nothing; they could not even feed themselves, and many perished of
hunger. Huber then introduced a single slave (F. fusca), and she instantly
set to work, fed and saved the survivors; made some cells and tended the
larvae, and put all to rights. What can be more extraordinary than these
well-ascertained facts? If we had not known of any other slave-making ant,
it would have been hopeless to have speculated how so wonderful an
instinct could have been perfected.
Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of England,
and its habits have been attended to by Mr. F. Smith, of the British
Museum, to whom I am much indebted for information on this and other
subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as
any one may well be excused for doubting the truth of so extraordinary and
odious an instinct as that of making slaves. Hence I will give the
observations which I have myself made, in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves in all. Males and
fertile females of the slave-species are found only in their own proper
communities, and have never been observed in the nests of F. sanguinea.
The slaves are black and not above half the size of their red masters, so
that the contrast in their appearance is very great. When the nest is
slightly disturbed, the slaves occasionally come out, and like their
masters are much agitated and defend the nest: when the nest is much
disturbed and the larvae and pupae are exposed, the slaves work
energetically with their masters in carrying them away to a place of
safety. Hence, it is clear, that the slaves feel quite at home. During the
months of June and July, on three successive years, I have watched for
many hours several nests in Surrey and Sussex, and never saw a slave
either leave or enter a nest. As, during these months, the slaves are very
few in number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June and August, both in Surrey and Hampshire,
and has never seen the slaves, though present in large numbers in August,
either leave or enter the nest. Hence he considers them as strictly
household slaves. The masters, on the other hand, may be constantly seen
bringing in materials for the nest, and food of all kinds. During the
present year, however, in the month of July, I came across a community
with an unusually large stock of slaves, and I observed a few slaves
mingled with their masters leaving the nest, and marching along the same
road to a tall Scotch-fir-tree, twenty-five yards distant, which they
ascended together, probably in search of aphides or cocci. According to
Huber, who had ample opportunities for observation, in Switzerland the
slaves habitually work with their masters in making the nest, and they
alone open and close the doors in the morning and evening; and, as Huber
expressly states, their principal office is to search for aphides. This
difference in the usual habits of the masters and slaves in the two
countries, probably depends merely on the slaves being captured in greater
numbers in Switzerland than in England.
One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their jaws.
Another day my attention was struck by about a score of the slave-makers
haunting the same spot, and evidently not in search of food; they
approached and were vigorously repulsed by an independent community of the
slave species (F. fusca); sometimes as many as three of these ants
clinging to the legs of the slave-making F. sanguinea. The latter
ruthlessly killed their small opponents, and carried their dead bodies as
food to their nest, twenty-nine yards distant; but they were prevented
from getting any pupae to rear as slaves. I then dug up a small parcel of
the pupae of F. fusca from another nest, and put them down on a bare spot
near the place of combat; they were eagerly seized, and carried off by the
tyrants, who perhaps fancied that, after all, they had been victorious in
their late combat.
At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of the nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage. Now I was
curious to ascertain whether F. sanguinea could distinguish the pupae of
F. fusca, which they habitually make into slaves, from those of the little
and furious F. flava, which they rarely capture, and it was evident that
they did at once distinguish them: for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth from the nest of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they took heart and
carried off the pupae.
One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of F.
fusca (showing that it was not a migration) and numerous pupae. I traced
the returning file burthened with booty, for about forty yards, to a very
thick clump of heath, whence I saw the last individual of F. sanguinea
emerge, carrying a pupa; but I was not able to find the desolated nest in
the thick heath. The nest, however, must have been close at hand, for two
or three individuals of F. fusca were rushing about in the greatest
agitation, and one was perched motionless with its own pupa in its mouth
on the top of a spray of heath over its ravaged home.
Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the
F. rufescens. The latter does not build its own nest, does not determine
its own migrations, does not collect food for itself or its young, and
cannot even feed itself: it is absolutely dependent on its numerous
slaves. Formica sanguinea, on the other hand, possesses much fewer slaves,
and in the early part of the summer extremely few. The masters determine
when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves seem
to have the exclusive care of the larvae, and the masters alone go on
slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly
the slaves, tend, and milk as it may be called, their aphides; and thus
both collect food for the community. In England the masters alone usually
leave the nest to collect building materials and food for themselves,
their slaves and larvae. So that the masters in this country receive much
less service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not pretend
to conjecture. But as ants, which are not slave-makers, will, as I have
seen, carry off pupae of other species, if scattered near their nests, it
is possible that pupae originally stored as food might become developed;
and the ants thus unintentionally reared would then follow their proper
instincts, and do what work they could. If their presence proved useful to
the species which had seized them—if it were more advantageous to
this species to capture workers than to procreate them—the habit of
collecting pupae originally for food might by natural selection be
strengthened and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if carried out to a
much less extent even than in our British F. sanguinea, which, as we have
seen, is less aided by its slaves than the same species in Switzerland, I
can see no difficulty in natural selection increasing and modifying the
instinct—always supposing each modification to be of use to the
species—until an ant was formed as abjectly dependent on its slaves
as is the Formica rufescens.
CELL-MAKING INSTINCT OF THE HIVE-BEE.
I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite problem, and
have made their cells of the proper shape to hold the greatest possible
amount of honey, with the least possible consumption of precious wax in
their construction. It has been remarked that a skilful workman, with
fitting tools and measures, would find it very difficult to make cells of
wax of the true form, though this is perfectly effected by a crowd of bees
working in a dark hive. Grant whatever instincts you please, and it seems
at first quite inconceivable how they can make all the necessary angles
and planes, or even perceive when they are correctly made. But the
difficulty is not nearly so great as it at first appears: all this
beautiful work can be shown, I think, to follow from a few very simple
instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered only
as a modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of
work. At one end of a short series we have humble-bees, which use their
old cocoons to hold honey, sometimes adding to them short tubes of wax,
and likewise making separate and very irregular rounded cells of wax. At
the other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well known, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join on to a pyramid,
formed of three rhombs. These rhombs have certain angles, and the three
which form the pyramidal base of a single cell on one side of the comb,
enter into the composition of the bases of three adjoining cells on the
opposite side. In the series between the extreme perfection of the cells
of the hive-bee and the simplicity of those of the humble-bee, we have the
cells of the Mexican Melipona domestica, carefully described and figured
by Pierre Huber. The Melipona itself is intermediate in structure between
the hive and humble bee, but more nearly related to the latter: it forms a
nearly regular waxen comb of cylindrical cells, in which the young are
hatched, and, in addition, some large cells of wax for holding honey.
These latter cells are nearly spherical and of nearly equal sizes, and are
aggregated into an irregular mass. But the important point to notice, is
that these cells are always made at that degree of nearness to each other,
that they would have intersected or broken into each other, if the spheres
had been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus tend to
intersect. Hence each cell consists of an outer spherical portion and of
two, three, or more perfectly flat surfaces, according as the cell adjoins
two, three or more other cells. When one cell comes into contact with
three other cells, which, from the spheres being nearly of the same size,
is very frequently and necessarily the case, the three flat surfaces are
united into a pyramid; and this pyramid, as Huber has remarked, is
manifestly a gross imitation of the three-sided pyramidal basis of the
cell of the hive-bee. As in the cells of the hive-bee, so here, the three
plane surfaces in any one cell necessarily enter into the construction of
three adjoining cells. It is obvious that the Melipona saves wax by this
manner of building; for the flat walls between the adjoining cells are not
double, but are of the same thickness as the outer spherical portions, and
yet each flat portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would probably have been as perfect as the comb of the
hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this
geometer has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:—
If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius
x the square root of 2 or radius x 1.41421 (or at some lesser distance),
from the centres of the six surrounding spheres in the same layer; and at
the same distance from the centres of the adjoining spheres in the other
and parallel layer; then, if planes of intersection between the several
spheres in both layers be formed, there will result a double layer of
hexagonal prisms united together by pyramidal bases formed of three
rhombs; and the rhombs and the sides of the hexagonal prisms will have
every angle identically the same with the best measurements which have
been made of the cells of the hive-bee.
Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as that
of the hive-bee. We must suppose the Melipona to make her cells truly
spherical, and of equal sizes; and this would not be very surprising,
seeing that she already does so to a certain extent, and seeing what
perfectly cylindrical burrows in wood many insects can make, apparently by
turning round on a fixed point. We must suppose the Melipona to arrange
her cells in level layers, as she already does her cylindrical cells; and
we must further suppose, and this is the greatest difficulty, that she can
somehow judge accurately at what distance to stand from her
fellow-labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her spheres
so as to intersect largely; and then she unites the points of intersection
by perfectly flat surfaces. We have further to suppose, but this is no
difficulty, that after hexagonal prisms have been formed by the
intersection of adjoining spheres in the same layer, she can prolong the
hexagon to any length requisite to hold the stock of honey; in the same
way as the rude humble-bee adds cylinders of wax to the circular mouths of
her old cocoons. By such modifications of instincts in themselves not very
wonderful,—hardly more wonderful than those which guide a bird to
make its nest,—I believe that the hive-bee has acquired, through
natural selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following the example of Mr.
Tegetmeier, I separated two combs, and put between them a long, thick,
square strip of wax: the bees instantly began to excavate minute circular
pits in it; and as they deepened these little pits, they made them wider
and wider until they were converted into shallow basins, appearing to the
eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to me to observe that wherever several bees
had begun to excavate these basins near together, they had begun their
work at such a distance from each other, that by the time the basins had
acquired the above stated width (i.e. about the width of an ordinary
cell), and were in depth about one sixth of the diameter of the sphere of
which they formed a part, the rims of the basins intersected or broke into
each other. As soon as this occurred, the bees ceased to excavate, and
began to build up flat walls of wax on the lines of intersection between
the basins, so that each hexagonal prism was built upon the festooned edge
of a smooth basin, instead of on the straight edges of a three-sided
pyramid as in the case of ordinary cells.
I then put into the hive, instead of a thick, square piece of wax, a thin
and narrow, knife-edged ridge, coloured with vermilion. The bees instantly
began on both sides to excavate little basins near to each other, in the
same way as before; but the ridge of wax was so thin, that the bottoms of
the basins, if they had been excavated to the same depth as in the former
experiment, would have broken into each other from the opposite sides. The
bees, however, did not suffer this to happen, and they stopped their
excavations in due time; so that the basins, as soon as they had been a
little deepened, came to have flat bottoms; and these flat bottoms, formed
by thin little plates of the vermilion wax having been left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the
ridge of wax. In parts, only little bits, in other parts, large portions
of a rhombic plate had been left between the opposed basins, but the work,
from the unnatural state of things, had not been neatly performed. The
bees must have worked at very nearly the same rate on the opposite sides
of the ridge of vermilion wax, as they circularly gnawed away and deepened
the basins on both sides, in order to have succeeded in thus leaving flat
plates between the basins, by stopping work along the intermediate planes
or planes of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side, where the bees had worked less quickly. In one well-marked instance,
I put the comb back into the hive, and allowed the bees to go on working
for a short time, and again examined the cell, and I found that the
rhombic plate had been completed, and had become PERFECTLY FLAT: it was
absolutely impossible, from the extreme thinness of the little rhombic
plate, that they could have effected this by gnawing away the convex side;
and I suspect that the bees in such cases stand in the opposed cells and
push and bend the ductile and warm wax (which as I have tried is easily
done) into its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly see that
if the bees were to build for themselves a thin wall of wax, they could
make their cells of the proper shape, by standing at the proper distance
from each other, by excavating at the same rate, and by endeavouring to
make equal spherical hollows, but never allowing the spheres to break into
each other. Now bees, as may be clearly seen by examining the edge of a
growing comb, do make a rough, circumferential wall or rim all round the
comb; and they gnaw into this from the opposite sides, always working
circularly as they deepen each cell. They do not make the whole
three-sided pyramidal base of any one cell at the same time, but only the
one rhombic plate which stands on the extreme growing margin, or the two
plates, as the case may be; and they never complete the upper edges of the
rhombic plates, until the hexagonal walls are commenced. Some of these
statements differ from those made by the justly celebrated elder Huber,
but I am convinced of their accuracy; and if I had space, I could show
that they are conformable with my theory.
Huber's statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of wax;
but I will not here enter on these details. We see how important a part
excavation plays in the construction of the cells; but it would be a great
error to suppose that the bees cannot build up a rough wall of wax in the
proper position—that is, along the plane of intersection between two
adjoining spheres. I have several specimens showing clearly that they can
do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But
the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten to twenty times
thicker than the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing masons
first to pile up a broad ridge of cement, and then to begin cutting it
away equally on both sides near the ground, till a smooth, very thin wall
is left in the middle; the masons always piling up the cut-away cement,
and adding fresh cement, on the summit of the ridge. We shall thus have a
thin wall steadily growing upward; but always crowned by a gigantic
coping. From all the cells, both those just commenced and those completed,
being thus crowned by a strong coping of wax, the bees can cluster and
crawl over the comb without injuring the delicate hexagonal walls, which
are only about one four-hundredth of an inch in thickness; the plates of
the pyramidal basis being about twice as thick. By this singular manner of
building, strength is continually given to the comb, with the utmost
ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after
working a short time at one cell going to another, so that, as Huber has
stated, a score of individuals work even at the commencement of the first
cell. I was able practically to show this fact, by covering the edges of
the hexagonal walls of a single cell, or the extreme margin of the
circumferential rim of a growing comb, with an extremely thin layer of
melted vermilion wax; and I invariably found that the colour was most
delicately diffused by the bees—as delicately as a painter could
have done with his brush—by atoms of the coloured wax having been
taken from the spot on which it had been placed, and worked into the
growing edges of the cells all round. The work of construction seems to be
a sort of balance struck between many bees, all instinctively standing at
the same relative distance from each other, all trying to sweep equal
spheres, and then building up, or leaving ungnawed, the planes of
intersection between these spheres. It was really curious to note in cases
of difficulty, as when two pieces of comb met at an angle, how often the
bees would entirely pull down and rebuild in different ways the same cell,
sometimes recurring to a shape which they had at first rejected.
When bees have a place on which they can stand in their proper positions
for working,—for instance, on a slip of wood, placed directly under
the middle of a comb growing downwards so that the comb has to be built
over one face of the slip—in this case the bees can lay the
foundations of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that the bees
should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate between
two adjoining spheres; but, as far as I have seen, they never gnaw away
and finish off the angles of a cell till a large part both of that cell
and of the adjoining cells has been built. This capacity in bees of laying
down under certain circumstances a rough wall in its proper place between
two just-commenced cells, is important, as it bears on a fact, which seems
at first quite subversive of the foregoing theory; namely, that the cells
on the extreme margin of wasp-combs are sometimes strictly hexagonal; but
I have not space here to enter on this subject. Nor does there seem to me
any great difficulty in a single insect (as in the case of a queen-wasp)
making hexagonal cells, if she work alternately on the inside and outside
of two or three cells commenced at the same time, always standing at the
proper relative distance from the parts of the cells just begun, sweeping
spheres or cylinders, and building up intermediate planes. It is even
conceivable that an insect might, by fixing on a point at which to
commence a cell, and then moving outside, first to one point, and then to
five other points, at the proper relative distances from the central point
and from each other, strike the planes of intersection, and so make an
isolated hexagon: but I am not aware that any such case has been observed;
nor would any good be derived from a single hexagon being built, as in its
construction more materials would be required than for a cylinder.
As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its
conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the
present perfect plan of construction, could have profited the progenitors
of the hive-bee? I think the answer is not difficult: it is known that
bees are often hard pressed to get sufficient nectar; and I am informed by
Mr. Tegetmeier that it has been experimentally found that no less than
from twelve to fifteen pounds of dry sugar are consumed by a hive of bees
for the secretion of each pound of wax; so that a prodigious quantity of
fluid nectar must be collected and consumed by the bees in a hive for the
secretion of the wax necessary for the construction of their combs.
Moreover, many bees have to remain idle for many days during the process
of secretion. A large store of honey is indispensable to support a large
stock of bees during the winter; and the security of the hive is known
mainly to depend on a large number of bees being supported. Hence the
saving of wax by largely saving honey must be a most important element of
success in any family of bees. Of course the success of any species of bee
may be dependent on the number of its parasites or other enemies, or on
quite distinct causes, and so be altogether independent of the quantity of
honey which the bees could collect. But let us suppose that this latter
circumstance determined, as it probably often does determine, the numbers
of a humble-bee which could exist in a country; and let us further suppose
that the community lived throughout the winter, and consequently required
a store of honey: there can in this case be no doubt that it would be an
advantage to our humble-bee, if a slight modification of her instinct led
her to make her waxen cells near together, so as to intersect a little;
for a wall in common even to two adjoining cells, would save some little
wax. Hence it would continually be more and more advantageous to our
humble-bee, if she were to make her cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona; for
in this case a large part of the bounding surface of each cell would serve
to bound other cells, and much wax would be saved. Again, from the same
cause, it would be advantageous to the Melipona, if she were to make her
cells closer together, and more regular in every way than at present; for
then, as we have seen, the spherical surfaces would wholly disappear, and
would all be replaced by plane surfaces; and the Melipona would make a
comb as perfect as that of the hive-bee. Beyond this stage of perfection
in architecture, natural selection could not lead; for the comb of the
hive-bee, as far as we can see, is absolutely perfect in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having by slow degrees, more and more perfectly, led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection. The
bees, of course, no more knowing that they swept their spheres at one
particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates. The motive
power of the process of natural selection having been economy of wax; that
individual swarm which wasted least honey in the secretion of wax, having
succeeded best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had the
best chance of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection,—cases, in which we cannot see how
an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically the
same in animals so remote in the scale of nature, that we cannot account
for their similarity by inheritance from a common parent, and must
therefore believe that they have been acquired by independent acts of
natural selection. I will not here enter on these several cases, but will
confine myself to one special difficulty, which at first appeared to me
insuperable, and actually fatal to my whole theory. I allude to the
neuters or sterile females in insect-communities: for these neuters often
differ widely in instinct and in structure from both the males and fertile
females, and yet, from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that
some insects and other articulate animals in a state of nature
occasionally become sterile; and if such insects had been social, and it
had been profitable to the community that a number should have been
annually born capable of work, but incapable of procreation, I can see no
very great difficulty in this being effected by natural selection. But I
must pass over this preliminary difficulty. The great difficulty lies in
the working ants differing widely from both the males and the fertile
females in structure, as in the shape of the thorax and in being destitute
of wings and sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the prodigious difference in this respect between the
workers and the perfect females, would have been far better exemplified by
the hive-bee. If a working ant or other neuter insect had been an animal
in the ordinary state, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by
an individual having been born with some slight profitable modification of
structure, this being inherited by its offspring, which again varied and
were again selected, and so onwards. But with the working ant we have an
insect differing greatly from its parents, yet absolutely sterile; so that
it could never have transmitted successively acquired modifications of
structure or instinct to its progeny. It may well be asked how is it
possible to reconcile this case with the theory of natural selection?
First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts
of differences of structure which have become correlated to certain ages,
and to either sex. We have differences correlated not only to one sex, but
to that short period alone when the reproductive system is active, as in
the nuptial plumage of many birds, and in the hooked jaws of the male
salmon. We have even slight differences in the horns of different breeds
of cattle in relation to an artificially imperfect state of the male sex;
for oxen of certain breeds have longer horns than in other breeds, in
comparison with the horns of the bulls or cows of these same breeds. Hence
I can see no real difficulty in any character having become correlated
with the sterile condition of certain members of insect-communities: the
difficulty lies in understanding how such correlated modifications of
structure could have been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied
to the family, as well as to the individual, and may thus gain the desired
end. Thus, a well-flavoured vegetable is cooked, and the individual is
destroyed; but the horticulturist sows seeds of the same stock, and
confidently expects to get nearly the same variety; breeders of cattle
wish the flesh and fat to be well marbled together; the animal has been
slaughtered, but the breeder goes with confidence to the same family. I
have such faith in the powers of selection, that I do not doubt that a
breed of cattle, always yielding oxen with extraordinarily long horns,
could be slowly formed by carefully watching which individual bulls and
cows, when matched, produced oxen with the longest horns; and yet no one
ox could ever have propagated its kind. Thus I believe it has been with
social insects: a slight modification of structure, or instinct,
correlated with the sterile condition of certain members of the community,
has been advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their fertile
offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, until
that prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many social
insects.
But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes. The
castes, moreover, do not generally graduate into each other, but are
perfectly well defined; being as distinct from each other, as are any two
species of the same genus, or rather as any two genera of the same family.
Thus in Eciton, there are working and soldier neuters, with jaws and
instincts extraordinarily different: in Cryptocerus, the workers of one
caste alone carry a wonderful sort of shield on their heads, the use of
which is quite unknown: in the Mexican Myrmecocystus, the workers of one
caste never leave the nest; they are fed by the workers of another caste,
and they have an enormously developed abdomen which secretes a sort of
honey, supplying the place of that excreted by the aphides, or the
domestic cattle as they may be called, which our European ants guard or
imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,—in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few alone;
and that by the long-continued selection of the fertile parents which
produced most neuters with the profitable modification, all the neuters
ultimately came to have the desired character. On this view we ought
occasionally to find neuter-insects of the same species, in the same nest,
presenting gradations of structure; and this we do find, even often,
considering how few neuter-insects out of Europe have been carefully
examined. Mr. F. Smith has shown how surprisingly the neuters of several
British ants differ from each other in size and sometimes in colour; and
that the extreme forms can sometimes be perfectly linked together by
individuals taken out of the same nest: I have myself compared perfect
gradations of this kind. It often happens that the larger or the smaller
sized workers are the most numerous; or that both large and small are
numerous, with those of an intermediate size scanty in numbers. Formica
flava has larger and smaller workers, with some of intermediate size; and,
in this species, as Mr. F. Smith has observed, the larger workers have
simple eyes (ocelli), which though small can be plainly distinguished,
whereas the smaller workers have their ocelli rudimentary. Having
carefully dissected several specimens of these workers, I can affirm that
the eyes are far more rudimentary in the smaller workers than can be
accounted for merely by their proportionally lesser size; and I fully
believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate condition.
So that we here have two bodies of sterile workers in the same nest,
differing not only in size, but in their organs of vision, yet connected
by some few members in an intermediate condition. I may digress by adding,
that if the smaller workers had been the most useful to the community, and
those males and females had been continually selected, which produced more
and more of the smaller workers, until all the workers had come to be in
this condition; we should then have had a species of ant with neuters very
nearly in the same condition with those of Myrmica. For the workers of
Myrmica have not even rudiments of ocelli, though the male and female ants
of this genus have well-developed ocelli.
I may give one other case: so confidently did I expect to find gradations
in important points of structure between the different castes of neuters
in the same species, that I gladly availed myself of Mr. F. Smith's offer
of numerous specimens from the same nest of the driver ant (Anomma) of
West Africa. The reader will perhaps best appreciate the amount of
difference in these workers, by my giving not the actual measurements, but
a strictly accurate illustration: the difference was the same as if we
were to see a set of workmen building a house of whom many were five feet
four inches high, and many sixteen feet high; but we must suppose that the
larger workmen had heads four instead of three times as big as those of
the smaller men, and jaws nearly five times as big. The jaws, moreover, of
the working ants of the several sizes differed wonderfully in shape, and
in the form and number of the teeth. But the important fact for us is,
that though the workers can be grouped into castes of different sizes, yet
they graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as Mr.
Lubbock made drawings for me with the camera lucida of the jaws which I
had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting on
the fertile parents, could form a species which should regularly produce
neuters, either all of large size with one form of jaw, or all of small
size with jaws having a widely different structure; or lastly, and this is
our climax of difficulty, one set of workers of one size and structure,
and simultaneously another set of workers of a different size and
structure;—a graduated series having been first formed, as in the
case of the driver ant, and then the extreme forms, from being the most
useful to the community, having been produced in greater and greater
numbers through the natural selection of the parents which generated them;
until none with an intermediate structure were produced.
Thus, as I believe, the wonderful fact of two distinctly defined castes of
sterile workers existing in the same nest, both widely different from each
other and from their parents, has originated. We can see how useful their
production may have been to a social community of insects, on the same
principle that the division of labour is useful to civilised man. As ants
work by inherited instincts and by inherited tools or weapons, and not by
acquired knowledge and manufactured instruments, a perfect division of
labour could be effected with them only by the workers being sterile; for
had they been fertile, they would have intercrossed, and their instincts
and structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants, by
the means of natural selection. But I am bound to confess, that, with all
my faith in this principle, I should never have anticipated that natural
selection could have been efficient in so high a degree, had not the case
of these neuter insects convinced me of the fact. I have, therefore,
discussed this case, at some little but wholly insufficient length, in
order to show the power of natural selection, and likewise because this is
by far the most serious special difficulty, which my theory has
encountered. The case, also, is very interesting, as it proves that with
animals, as with plants, any amount of modification in structure can be
effected by the accumulation of numerous, slight, and as we must call them
accidental, variations, which are in any manner profitable, without
exercise or habit having come into play. For no amount of exercise, or
habit, or volition, in the utterly sterile members of a community could
possibly have affected the structure or instincts of the fertile members,
which alone leave descendants. I am surprised that no one has advanced
this demonstrative case of neuter insects, against the well-known doctrine
of Lamarck.
SUMMARY.
I have endeavoured briefly in this chapter to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of
the highest importance to each animal. Therefore I can see no difficulty,
under changing conditions of life, in natural selection accumulating
slight modifications of instinct to any extent, in any useful direction.
In some cases habit or use and disuse have probably come into play. I do
not pretend that the facts given in this chapter strengthen in any great
degree my theory; but none of the cases of difficulty, to the best of my
judgment, annihilate it. On the other hand, the fact that instincts are
not always absolutely perfect and are liable to mistakes;—that no
instinct has been produced for the exclusive good of other animals, but
that each animal takes advantage of the instincts of others;—that
the canon in natural history, of "natura non facit saltum" is applicable
to instincts as well as to corporeal structure, and is plainly explicable
on the foregoing views, but is otherwise inexplicable,—all tend to
corroborate the theory of natural selection.
This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining
nearly the same instincts. For instance, we can understand on the
principle of inheritance, how it is that the thrush of South America lines
its nest with mud, in the same peculiar manner as does our British thrush:
how it is that the male wrens (Troglodytes) of North America, build
"cock-nests," to roost in, like the males of our distinct Kitty-wrens,—a
habit wholly unlike that of any other known bird. Finally, it may not be a
logical deduction, but to my imagination it is far more satisfactory to
look at such instincts as the young cuckoo ejecting its foster-brothers,—ants
making slaves,—the larvae of ichneumonidae feeding within the live
bodies of caterpillars,—not as specially endowed or created
instincts, but as small consequences of one general law, leading to the
advancement of all organic beings, namely, multiply, vary, let the
strongest live and the weakest die.
8. HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication. Laws governing the sterility of
hybrids. Sterility not a special endowment, but incidental on other
differences. Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of
their fertility. Summary.
The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility,
in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country
could hardly have kept distinct had they been capable of crossing freely.
The importance of the fact that hybrids are very generally sterile, has, I
think, been much underrated by some late writers. On the theory of natural
selection the case is especially important, inasmuch as the sterility of
hybrids could not possibly be of any advantage to them, and therefore
could not have been acquired by the continued preservation of successive
profitable degrees of sterility. I hope, however, to be able to show that
sterility is not a specially acquired or endowed quality, but is
incidental on other acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together; namely,
the sterility of two species when first crossed, and the sterility of the
hybrids produced from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the organs themselves are perfect in structure,
as far as the microscope reveals. In the first case the two sexual
elements which go to form the embryo are perfect; in the second case they
are either not at all developed, or are imperfectly developed. This
distinction is important, when the cause of the sterility, which is common
to the two cases, has to be considered. The distinction has probably been
slurred over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to have
descended from common parents, when intercrossed, and likewise the
fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad and
clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility. Kolreuter
makes the rule universal; but then he cuts the knot, for in ten cases in
which he found two forms, considered by most authors as distinct species,
quite fertile together, he unhesitatingly ranks them as varieties.
Gartner, also, makes the rule equally universal; and he disputes the
entire fertility of Kolreuter's ten cases. But in these and in many other
cases, Gartner is obliged carefully to count the seeds, in order to show
that there is any degree of sterility. He always compares the maximum
number of seeds produced by two species when crossed and by their hybrid
offspring, with the average number produced by both pure parent-species in
a state of nature. But a serious cause of error seems to me to be here
introduced: a plant to be hybridised must be castrated, and, what is often
more important, must be secluded in order to prevent pollen being brought
to it by insects from other plants. Nearly all the plants experimentised
on by Gartner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gartner gives in his table about a score of
cases of plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover, as
Gartner during several years repeatedly crossed the primrose and cowslip,
which we have such good reason to believe to be varieties, and only once
or twice succeeded in getting fertile seed; as he found the common red and
blue pimpernels (Anagallis arvensis and coerulea), which the best
botanists rank as varieties, absolutely sterile together; and as he came
to the same conclusion in several other analogous cases; it seems to me
that we may well be permitted to doubt whether many other species are
really so sterile, when intercrossed, as Gartner believes.
It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and,
on the other hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical purposes it is
most difficult to say where perfect fertility ends and sterility begins. I
think no better evidence of this can be required than that the two most
experienced observers who have ever lived, namely, Kolreuter and Gartner,
should have arrived at diametrically opposite conclusions in regard to the
very same species. It is also most instructive to compare—but I have
not space here to enter on details—the evidence advanced by our best
botanists on the question whether certain doubtful forms should be ranked
as species or varieties, with the evidence from fertility adduced by
different hybridisers, or by the same author, from experiments made during
different years. It can thus be shown that neither sterility nor fertility
affords any clear distinction between species and varieties; but that the
evidence from this source graduates away, and is doubtful in the same
degree as is the evidence derived from other constitutional and structural
differences.
In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in the
first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing that close interbreeding lessens fertility, and, on the
other hand, that an occasional cross with a distinct individual or variety
increases fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids
will generally be fertilised during each generation by their own
individual pollen; and I am convinced that this would be injurious to
their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly made
by Gartner, namely, that if even the less fertile hybrids be artificially
fertilised with hybrid pollen of the same kind, their fertility,
notwithstanding the frequent ill effects of manipulation, sometimes
decidedly increases, and goes on increasing. Now, in artificial
fertilisation pollen is as often taken by chance (as I know from my own
experience) from the anthers of another flower, as from the anthers of the
flower itself which is to be fertilised; so that a cross between two
flowers, though probably on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in progress, so careful an
observer as Gartner would have castrated his hybrids, and this would have
insured in each generation a cross with the pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of the increase of fertility in the
successive generations of ARTIFICIALLY FERTILISED hybrids may, I believe,
be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Honourable and Reverend W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile—as
fertile as the pure parent-species—as are Kolreuter and Gartner that
some degree of sterility between distinct species is a universal law of
nature. He experimentised on some of the very same species as did Gartner.
The difference in their results may, I think, be in part accounted for by
Herbert's great horticultural skill, and by his having hothouses at his
command. Of his many important statements I will here give only a single
one as an example, namely, that "every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which (he says) I never saw
to occur in a case of its natural fecundation." So that we here have
perfect, or even more than commonly perfect, fertility in a first cross
between two distinct species.
This case of the Crinum leads me to refer to a most singular fact, namely,
that there are individual plants, as with certain species of Lobelia, and
with all the species of the genus Hippeastrum, which can be far more
easily fertilised by the pollen of another and distinct species, than by
their own pollen. For these plants have been found to yield seed to the
pollen of a distinct species, though quite sterile with their own pollen,
notwithstanding that their own pollen was found to be perfectly good, for
it fertilised distinct species. So that certain individual plants and all
the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance, a bulb of
Hippeastrum aulicum produced four flowers; three were fertilised by
Herbert with their own pollen, and the fourth was subsequently fertilised
by the pollen of a compound hybrid descended from three other and distinct
species: the result was that "the ovaries of the three first flowers soon
ceased to grow, and after a few days perished entirely, whereas the pod
impregnated by the pollen of the hybrid made vigorous growth and rapid
progress to maturity, and bore good seed, which vegetated freely." In a
letter to me, in 1839, Mr. Herbert told me that he had then tried the
experiment during five years, and he continued to try it during several
subsequent years, and always with the same result. This result has, also,
been confirmed by other observers in the case of Hippeastrum with its
sub-genera, and in the case of some other genera, as Lobelia, Passiflora
and Verbascum. Although the plants in these experiments appeared perfectly
healthy, and although both the ovules and pollen of the same flower were
perfectly good with respect to other species, yet as they were
functionally imperfect in their mutual self-action, we must infer that the
plants were in an unnatural state. Nevertheless these facts show on what
slight and mysterious causes the lesser or greater fertility of species
when crossed, in comparison with the same species when self-fertilised,
sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, "reproduced itself as perfectly as if it had been a natural species
from the mountains of Chile." I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhododendron Ponticum and Catawbiense, and that this hybrid "seeds
as freely as it is possible to imagine." Had hybrids, when fairly treated,
gone on decreasing in fertility in each successive generation, as Gartner
believes to be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone are
fairly treated, for by insect agency the several individuals of the same
hybrid variety are allowed to freely cross with each other, and the
injurious influence of close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-agency by examining
the flowers of the more sterile kinds of hybrid rhododendrons, which
produce no pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that is
if the genera of animals are as distinct from each other, as are the
genera of plants, then we may infer that animals more widely separated in
the scale of nature can be more easily crossed than in the case of plants;
but the hybrids themselves are, I think, more sterile. I doubt whether any
case of a perfectly fertile hybrid animal can be considered as thoroughly
well authenticated. It should, however, be borne in mind that, owing to
few animals breeding freely under confinement, few experiments have been
fairly tried: for instance, the canary-bird has been crossed with nine
other finches, but as not one of these nine species breeds freely in
confinement, we have no right to expect that the first crosses between
them and the canary, or that their hybrids, should be perfectly fertile.
Again, with respect to the fertility in successive generations of the more
fertile hybrid animals, I hardly know of an instance in which two families
of the same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each
successive generation, in opposition to the constantly repeated admonition
of every breeder. And in this case, it is not at all surprising that the
inherent sterility in the hybrids should have gone on increasing. If we
were to act thus, and pair brothers and sisters in the case of any pure
animal, which from any cause had the least tendency to sterility, the
breed would assuredly be lost in a very few generations.
Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
with P. torquatus and with P. versicolor are perfectly fertile. The
hybrids from the common and Chinese geese (A. cygnoides), species which
are so different that they are generally ranked in distinct genera, have
often bred in this country with either pure parent, and in one single
instance they have bred inter se. This was effected by Mr. Eyton, who
raised two hybrids from the same parents but from different hatches; and
from these two birds he raised no less than eight hybrids (grandchildren
of the pure geese) from one nest. In India, however, these cross-bred
geese must be far more fertile; for I am assured by two eminently capable
judges, namely Mr. Blyth and Capt. Hutton, that whole flocks of these
crossed geese are kept in various parts of the country; and as they are
kept for profit, where neither pure parent-species exists, they must
certainly be highly fertile.
A doctrine which originated with Pallas, has been largely accepted by
modern naturalists; namely, that most of our domestic animals have
descended from two or more aboriginal species, since commingled by
intercrossing. On this view, the aboriginal species must either at first
have produced quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe in
its truth, although it rests on no direct evidence. I believe, for
instance, that our dogs have descended from several wild stocks; yet, with
perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; and analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again there is
reason to believe that our European and the humped Indian cattle are quite
fertile together; but from facts communicated to me by Mr. Blyth, I think
they must be considered as distinct species. On this view of the origin of
many of our domestic animals, we must either give up the belief of the
almost universal sterility of distinct species of animals when crossed; or
we must look at sterility, not as an indelible characteristic, but as one
capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result; but
that it cannot, under our present state of knowledge, be considered as
absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the circumstances and rules
governing the sterility of first crosses and of hybrids. Our chief object
will be to see whether or not the rules indicate that species have
specially been endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following rules and
conclusions are chiefly drawn up from Gartner's admirable work on the
hybridisation of plants. I have taken much pains to ascertain how far the
rules apply to animals, and considering how scanty our knowledge is in
regard to hybrid animals, I have been surprised to find how generally the
same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown to exist;
but only the barest outline of the facts can here be given. When pollen
from a plant of one family is placed on the stigma of a plant of a
distinct family, it exerts no more influence than so much inorganic dust.
From this absolute zero of fertility, the pollen of different species of
the same genus applied to the stigma of some one species, yields a perfect
gradation in the number of seeds produced, up to nearly complete or even
quite complete fertility; and, as we have seen, in certain abnormal cases,
even to an excess of fertility, beyond that which the plant's own pollen
will produce. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of either
pure parent, a single fertile seed: but in some of these cases a first
trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than it
otherwise would have done; and the early withering of the flower is well
known to be a sign of incipient fertilisation. From this extreme degree of
sterility we have self-fertilised hybrids producing a greater and greater
number of seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which
are generally confounded together—is by no means strict. There are
many cases, in which two pure species can be united with unusual facility,
and produce numerous hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very
rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus, for
instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable; for it
is not always the same when the same two species are crossed under the
same circumstances, but depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it is
with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same
capsule and exposed to exactly the same conditions.
By the term systematic affinity is meant, the resemblance between species
in structure and in constitution, more especially in the structure of
parts which are of high physiological importance and which differ little
in the allied species. Now the fertility of first crosses between species,
and of the hybrids produced from them, is largely governed by their
systematic affinity. This is clearly shown by hybrids never having been
raised between species ranked by systematists in distinct families; and on
the other hand, by very closely allied species generally uniting with
facility. But the correspondence between systematic affinity and the
facility of crossing is by no means strict. A multitude of cases could be
given of very closely allied species which will not unite, or only with
extreme difficulty; and on the other hand of very distinct species which
unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single hybrid. Even
within the limits of the same genus, we meet with this same difference;
for instance, the many species of Nicotiana have been more largely crossed
than the species of almost any other genus; but Gartner found that N.
acuminata, which is not a particularly distinct species, obstinately
failed to fertilise, or to be fertilised by, no less than eight other
species of Nicotiana. Very many analogous facts could be given.
No one has been able to point out what kind, or what amount, of difference
in any recognisable character is sufficient to prevent two species
crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part
of the flower, even in the pollen, in the fruit, and in the cotyledons,
can be crossed. Annual and perennial plants, deciduous and evergreen
trees, plants inhabiting different stations and fitted for extremely
different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance,
of a stallion-horse being first crossed with a female-ass, and then a
male-ass with a mare: these two species may then be said to have been
reciprocally crossed. There is often the widest possible difference in the
facility of making reciprocal crosses. Such cases are highly important,
for they prove that the capacity in any two species to cross is often
completely independent of their systematic affinity, or of any
recognisable difference in their whole organisation. On the other hand,
these cases clearly show that the capacity for crossing is connected with
constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalappa can easily be fertilised by the pollen of
M. longiflora, and the hybrids thus produced are sufficiently fertile; but
Kolreuter tried more than two hundred times, during eight following years,
to fertilise reciprocally M. longiflora with the pollen of M. jalappa, and
utterly failed. Several other equally striking cases could be given.
Thuret has observed the same fact with certain sea-weeds or Fuci. Gartner,
moreover, found that this difference of facility in making reciprocal
crosses is extremely common in a lesser degree. He has observed it even
between forms so closely related (as Matthiola annua and glabra) that many
botanists rank them only as varieties. It is also a remarkable fact, that
hybrids raised from reciprocal crosses, though of course compounded of the
very same two species, the one species having first been used as the
father and then as the mother, generally differ in fertility in a small,
and occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all
necessarily go together. There are certain hybrids which instead of
having, as is usual, an intermediate character between their two parents,
always closely resemble one of them; and such hybrids, though externally
so like one of their pure parent-species, are with rare exceptions
extremely sterile. So again amongst hybrids which are usually intermediate
in structure between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure parents; and
these hybrids are almost always utterly sterile, even when the other
hybrids raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely fertility in the hybrid is
independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in the
first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they resemble
in external appearance either parent. And lastly, that the facility of
making a first cross between any two species is not always governed by
their systematic affinity or degree of resemblance to each other. This
latter statement is clearly proved by reciprocal crosses between the same
two species, for according as the one species or the other is used as the
father or the mother, there is generally some difference, and occasionally
the widest possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often differ in
fertility.
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending together?
Why should the degree of sterility be innately variable in the individuals
of the same species? Why should some species cross with facility, and yet
produce very sterile hybrids; and other species cross with extreme
difficulty, and yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids
been permitted? to grant to species the special power of producing
hybrids, and then to stop their further propagation by different degrees
of sterility, not strictly related to the facility of the first union
between their parents, seems to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility both of first crosses and of hybrids is simply
incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature, that, in reciprocal crosses
between two species the male sexual element of the one will often freely
act on the female sexual element of the other, but not in a reversed
direction. It will be advisable to explain a little more fully by an
example what I mean by sterility being incidental on other differences,
and not a specially endowed quality. As the capacity of one plant to be
grafted or budded on another is so entirely unimportant for its welfare in
a state of nature, I presume that no one will suppose that this capacity
is a SPECIALLY endowed quality, but will admit that it is incidental on
differences in the laws of growth of the two plants. We can sometimes see
the reason why one tree will not take on another, from differences in
their rate of growth, in the hardness of their wood, in the period of the
flow or nature of their sap, etc.; but in a multitude of cases we can
assign no reason whatever. Great diversity in the size of two plants, one
being woody and the other herbaceous, one being evergreen and the other
deciduous, and adaptation to widely different climates, does not always
prevent the two grafting together. As in hybridisation, so with grafting,
the capacity is limited by systematic affinity, for no one has been able
to graft trees together belonging to quite distinct families; and, on the
other hand, closely allied species, and varieties of the same species, can
usually, but not invariably, be grafted with ease. But this capacity, as
in hybridisation, is by no means absolutely governed by systematic
affinity. Although many distinct genera within the same family have been
grafted together, in other cases species of the same genus will not take
on each other. The pear can be grafted far more readily on the quince,
which is ranked as a distinct genus, than on the apple, which is a member
of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of
the apricot and peach on certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same two
species in being grafted together. As in reciprocal crosses, the facility
of effecting an union is often very far from equal, so it sometimes is in
grafting; the common gooseberry, for instance, cannot be grafted on the
currant, whereas the currant will take, though with difficulty, on the
gooseberry.
We have seen that the sterility of hybrids, which have their reproductive
organs in an imperfect condition, is a very different case from the
difficulty of uniting two pure species, which have their reproductive
organs perfect; yet these two distinct cases run to a certain extent
parallel. Something analogous occurs in grafting; for Thouin found that
three species of Robinia, which seeded freely on their own roots, and
which could be grafted with no great difficulty on another species, when
thus grafted were rendered barren. On the other hand, certain species of
Sorbus, when grafted on other species, yielded twice as much fruit as when
on their own roots. We are reminded by this latter fact of the
extraordinary case of Hippeastrum, Lobelia, etc., which seeded much more
freely when fertilised with the pollen of distinct species, than when
self-fertilised with their own pollen.
We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male and
female elements in the act of reproduction, yet that there is a rude
degree of parallelism in the results of grafting and of crossing distinct
species. And as we must look at the curious and complex laws governing the
facility with which trees can be grafted on each other as incidental on
unknown differences in their vegetative systems, so I believe that the
still more complex laws governing the facility of first crosses, are
incidental on unknown differences, chiefly in their reproductive systems.
These differences, in both cases, follow to a certain extent, as might
have been expected, systematic affinity, by which every kind of
resemblance and dissimilarity between organic beings is attempted to be
expressed. The facts by no means seem to me to indicate that the greater
or lesser difficulty of either grafting or crossing together various
species has been a special endowment; although in the case of crossing,
the difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.
CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We may now look a little closer at the probable causes of the sterility of
first crosses and of hybrids. These two cases are fundamentally different,
for, as just remarked, in the union of two pure species the male and
female sexual elements are perfect, whereas in hybrids they are imperfect.
Even in first crosses, the greater or lesser difficulty in effecting a
union apparently depends on several distinct causes. There must sometimes
be a physical impossibility in the male element reaching the ovule, as
would be the case with a plant having a pistil too long for the
pollen-tubes to reach the ovarium. It has also been observed that when
pollen of one species is placed on the stigma of a distantly allied
species, though the pollen-tubes protrude, they do not penetrate the
stigmatic surface. Again, the male element may reach the female element,
but be incapable of causing an embryo to be developed, as seems to have
been the case with some of Thuret's experiments on Fuci. No explanation
can be given of these facts, any more than why certain trees cannot be
grafted on others. Lastly, an embryo may be developed, and then perish at
an early period. This latter alternative has not been sufficiently
attended to; but I believe, from observations communicated to me by Mr.
Hewitt, who has had great experience in hybridising gallinaceous birds,
that the early death of the embryo is a very frequent cause of sterility
in first crosses. I was at first very unwilling to believe in this view;
as hybrids, when once born, are generally healthy and long-lived, as we
see in the case of the common mule. Hybrids, however, are differently
circumstanced before and after birth: when born and living in a country
where their two parents can live, they are generally placed under suitable
conditions of life. But a hybrid partakes of only half of the nature and
constitution of its mother, and therefore before birth, as long as it is
nourished within its mother's womb or within the egg or seed produced by
the mother, it may be exposed to conditions in some degree unsuitable, and
consequently be liable to perish at an early period; more especially as
all very young beings seem eminently sensitive to injurious or unnatural
conditions of life.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once
alluded to a large body of facts, which I have collected, showing that
when animals and plants are removed from their natural conditions, they
are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of animals.
Between the sterility thus superinduced and that of hybrids, there are
many points of similarity. In both cases the sterility is independent of
general health, and is often accompanied by excess of size or great
luxuriance. In both cases, the sterility occurs in various degrees; in
both, the male element is the most liable to be affected; but sometimes
the female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants
are rendered impotent by the same unnatural conditions; and whole groups
of species tend to produce sterile hybrids. On the other hand, one species
in a group will sometimes resist great changes of conditions with
unimpaired fertility; and certain species in a group will produce
unusually fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement or any plant seed freely
under culture; nor can he tell, till he tries, whether any two species of
a genus will produce more or less sterile hybrids. Lastly, when organic
beings are placed during several generations under conditions not natural
to them, they are extremely liable to vary, which is due, as I believe, to
their reproductive systems having been specially affected, though in a
lesser degree than when sterility ensues. So it is with hybrids, for
hybrids in successive generations are eminently liable to vary, as every
experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of two
species, the reproductive system, independently of the general state of
health, is affected by sterility in a very similar manner. In the one
case, the conditions of life have been disturbed, though often in so
slight a degree as to be inappreciable by us; in the other case, or that
of hybrids, the external conditions have remained the same, but the
organisation has been disturbed by two different structures and
constitutions having been blended into one. For it is scarcely possible
that two organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or mutual
relation of the different parts and organs one to another, or to the
conditions of life. When hybrids are able to breed inter se, they transmit
to their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of hybrids;
for instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation is
offered why an organism, when placed under unnatural conditions, is
rendered sterile. All that I have attempted to show, is that in two cases,
in some respects allied, sterility is the common result,—in the one
case from the conditions of life having been disturbed, in the other case
from the organisation having been disturbed by two organisations having
been compounded into one.
It may seem fanciful, but I suspect that a similar parallelism extends to
an allied yet very different class of facts. It is an old and almost
universal belief, founded, I think, on a considerable body of evidence,
that slight changes in the conditions of life are beneficial to all living
things. We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, etc., from one soil or climate to another, and
back again. During the convalescence of animals, we plainly see that great
benefit is derived from almost any change in the habits of life. Again,
both with plants and animals, there is abundant evidence, that a cross
between very distinct individuals of the same species, that is between
members of different strains or sub-breeds, gives vigour and fertility to
the offspring. I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and sterility
in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions of
life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same species
which have varied and become slightly different, give vigour and fertility
to the offspring. But we have seen that greater changes, or changes of a
particular nature, often render organic beings in some degree sterile; and
that greater crosses, that is crosses between males and females which have
become widely or specifically different, produce hybrids which are
generally sterile in some degree. I cannot persuade myself that this
parallelism is an accident or an illusion. Both series of facts seem to be
connected together by some common but unknown bond, which is essentially
related to the principle of life.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.
It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there must
be some error in all the foregoing remarks, inasmuch as varieties, however
much they may differ from each other in external appearance, cross with
perfect facility, and yield perfectly fertile offspring. I fully admit
that this is almost invariably the case. But if we look to varieties
produced under nature, we are immediately involved in hopeless
difficulties; for if two hitherto reputed varieties be found in any degree
sterile together, they are at once ranked by most naturalists as species.
For instance, the blue and red pimpernel, the primrose and cowslip, which
are considered by many of our best botanists as varieties, are said by
Gartner not to be quite fertile when crossed, and he consequently ranks
them as undoubted species. If we thus argue in a circle, the fertility of
all varieties produced under nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily than
other dogs with foxes, or that certain South American indigenous domestic
dogs do not readily cross with European dogs, the explanation which will
occur to everyone, and probably the true one, is that these dogs have
descended from several aboriginally distinct species. Nevertheless the
perfect fertility of so many domestic varieties, differing widely from
each other in appearance, for instance of the pigeon or of the cabbage, is
a remarkable fact; more especially when we reflect how many species there
are, which, though resembling each other most closely, are utterly sterile
when intercrossed. Several considerations, however, render the fertility
of domestic varieties less remarkable than at first appears. It can, in
the first place, be clearly shown that mere external dissimilarity between
two species does not determine their greater or lesser degree of sterility
when crossed; and we may apply the same rule to domestic varieties. In the
second place, some eminent naturalists believe that a long course of
domestication tends to eliminate sterility in the successive generations
of hybrids, which were at first only slightly sterile; and if this be so,
we surely ought not to expect to find sterility both appearing and
disappearing under nearly the same conditions of life. Lastly, and this
seems to me by far the most important consideration, new races of animals
and plants are produced under domestication by man's methodical and
unconscious power of selection, for his own use and pleasure: he neither
wishes to select, nor could select, slight differences in the reproductive
system, or other constitutional differences correlated with the
reproductive system. He supplies his several varieties with the same food;
treats them in nearly the same manner, and does not wish to alter their
general habits of life. Nature acts uniformly and slowly during vast
periods of time on the whole organisation, in any way which may be for
each creature's own good; and thus she may, either directly, or more
probably indirectly, through correlation, modify the reproductive system
in the several descendants from any one species. Seeing this difference in
the process of selection, as carried on by man and nature, we need not be
surprised at some difference in the result.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility in
the few following cases, which I will briefly abstract. The evidence is at
least as good as that from which we believe in the sterility of a
multitude of species. The evidence is, also, derived from hostile
witnesses, who in all other cases consider fertility and sterility as safe
criterions of specific distinction. Gartner kept during several years a
dwarf kind of maize with yellow seeds, and a tall variety with red seeds,
growing near each other in his garden; and although these plants have
separated sexes, they never naturally crossed. He then fertilised thirteen
flowers of the one with the pollen of the other; but only a single head
produced any seed, and this one head produced only five grains.
Manipulation in this case could not have been injurious, as the plants
have separated sexes. No one, I believe, has suspected that these
varieties of maize are distinct species; and it is important to notice
that the hybrid plants thus raised were themselves PERFECTLY fertile; so
that even Gartner did not venture to consider the two varieties as
specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual fertilisation
is by so much the less easy as their differences are greater. How far
these experiments may be trusted, I know not; but the forms experimentised
on, are ranked by Sagaret, who mainly founds his classification by the
test of infertility, as varieties.
The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of experiments
made during many years on nine species of Verbascum, by so good an
observer and so hostile a witness, as Gartner: namely, that yellow and
white varieties of the same species of Verbascum when intercrossed produce
less seed, than do either coloured varieties when fertilised with pollen
from their own coloured flowers. Moreover, he asserts that when yellow and
white varieties of one species are crossed with yellow and white varieties
of a DISTINCT species, more seed is produced by the crosses between the
same coloured flowers, than between those which are differently coloured.
Yet these varieties of Verbascum present no other difference besides the
mere colour of the flower; and one variety can sometimes be raised from
the seed of the other.
From observations which I have made on certain varieties of hollyhock, I
am inclined to suspect that they present analogous facts.
Kolreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact, that one variety of the common tobacco is
more fertile, when crossed with a widely distinct species, than are the
other varieties. He experimentised on five forms, which are commonly
reputed to be varieties, and which he tested by the severest trial,
namely, by reciprocal crosses, and he found their mongrel offspring
perfectly fertile. But one of these five varieties, when used either as
father or mother, and crossed with the Nicotiana glutinosa, always yielded
hybrids not so sterile as those which were produced from the four other
varieties when crossed with N. glutinosa. Hence the reproductive system of
this one variety must have been in some manner and in some degree
modified.
From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety if
infertile in any degree would generally be ranked as species; from man
selecting only external characters in the production of the most distinct
domestic varieties, and from not wishing or being able to produce
recondite and functional differences in the reproductive system; from
these several considerations and facts, I do not think that the very
general fertility of varieties can be proved to be of universal
occurrence, or to form a fundamental distinction between varieties and
species. The general fertility of varieties does not seem to me sufficient
to overthrow the view which I have taken with respect to the very general,
but not invariable, sterility of first crosses and of hybrids, namely,
that it is not a special endowment, but is incidental on slowly acquired
modifications, more especially in the reproductive systems of the forms
which are crossed.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of species when
crossed and of varieties when crossed may be compared in several other
respects. Gartner, whose strong wish was to draw a marked line of
distinction between species and varieties, could find very few and, as it
seems to me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of varieties.
And, on the other hand, they agree most closely in very many important
respects.
I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable
than hybrids; but Gartner admits that hybrids from species which have long
been cultivated are often variable in the first generation; and I have
myself seen striking instances of this fact. Gartner further admits that
hybrids between very closely allied species are more variable than those
from very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations an extreme amount of
variability in their offspring is notorious; but some few cases both of
hybrids and mongrels long retaining uniformity of character could be
given. The variability, however, in the successive generations of mongrels
is, perhaps, greater than in hybrids.
This greater variability of mongrels than of hybrids does not seem to me
at all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies in most cases that there has been recent
variability; and therefore we might expect that such variability would
often continue and be super-added to that arising from the mere act of
crossing. The slight degree of variability in hybrids from the first cross
or in the first generation, in contrast with their extreme variability in
the succeeding generations, is a curious fact and deserves attention. For
it bears on and corroborates the view which I have taken on the cause of
ordinary variability; namely, that it is due to the reproductive system
being eminently sensitive to any change in the conditions of life, being
thus often rendered either impotent or at least incapable of its proper
function of producing offspring identical with the parent-form. Now
hybrids in the first generation are descended from species (excluding
those long cultivated) which have not had their reproductive systems in
any way affected, and they are not variable; but hybrids themselves have
their reproductive systems seriously affected, and their descendants are
highly variable.
But to return to our comparison of mongrels and hybrids: Gartner states
that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in
degree. Gartner further insists that when any two species, although most
closely allied to each other, are crossed with a third species, the
hybrids are widely different from each other; whereas if two very distinct
varieties of one species are crossed with another species, the hybrids do
not differ much. But this conclusion, as far as I can make out, is founded
on a single experiment; and seems directly opposed to the results of
several experiments made by Kolreuter.
These alone are the unimportant differences, which Gartner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents, more
especially in hybrids produced from nearly related species, follows
according to Gartner the same laws. When two species are crossed, one has
sometimes a prepotent power of impressing its likeness on the hybrid; and
so I believe it to be with varieties of plants. With animals one variety
certainly often has this prepotent power over another variety. Hybrid
plants produced from a reciprocal cross, generally resemble each other
closely; and so it is with mongrels from a reciprocal cross. Both hybrids
and mongrels can be reduced to either pure parent-form, by repeated
crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency in
transmitting likeness running more strongly in one sex than in the other,
both when one species is crossed with another, and when one variety is
crossed with another variety. For instance, I think those authors are
right, who maintain that the ass has a prepotent power over the horse, so
that both the mule and the hinny more resemble the ass than the horse; but
that the prepotency runs more strongly in the male-ass than in the female,
so that the mule, which is the offspring of the male-ass and mare, is more
like an ass, than is the hinny, which is the offspring of the female-ass
and stallion.
Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but it
can be shown that this does sometimes occur with hybrids; yet I grant much
less frequently with hybrids than with mongrels. Looking to the cases
which I have collected of cross-bred animals closely resembling one
parent, the resemblances seem chiefly confined to characters almost
monstrous in their nature, and which have suddenly appeared—such as
albinism, melanism, deficiency of tail or horns, or additional fingers and
toes; and do not relate to characters which have been slowly acquired by
selection. Consequently, sudden reversions to the perfect character of
either parent would be more likely to occur with mongrels, which are
descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly and
naturally produced. On the whole I entirely agree with Dr. Prosper Lucas,
who, after arranging an enormous body of facts with respect to animals,
comes to the conclusion, that the laws of resemblance of the child to its
parents are the same, whether the two parents differ much or little from
each other, namely in the union of individuals of the same variety, or of
different varieties, or of distinct species.
Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as
having been specially created, and at varieties as having been produced by
secondary laws, this similarity would be an astonishing fact. But it
harmonises perfectly with the view that there is no essential distinction
between species and varieties.
SUMMARY OF CHAPTER.
First crosses between forms sufficiently distinct to be ranked as species,
and their hybrids, are very generally, but not universally, sterile. The
sterility is of all degrees, and is often so slight that the two most
careful experimentalists who have ever lived, have come to diametrically
opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently
susceptible of favourable and unfavourable conditions. The degree of
sterility does not strictly follow systematic affinity, but is governed by
several curious and complex laws. It is generally different, and sometimes
widely different, in reciprocal crosses between the same two species. It
is not always equal in degree in a first cross and in the hybrid produced
from this cross.
In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown differences
in their vegetative systems, so in crossing, the greater or less facility
of one species to unite with another, is incidental on unknown differences
in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to
prevent them crossing and blending in nature, than to think that trees
have been specially endowed with various and somewhat analogous degrees of
difficulty in being grafted together in order to prevent them becoming
inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have
had this system and their whole organisation disturbed by being compounded
of two distinct species, seems closely allied to that sterility which so
frequently affects pure species, when their natural conditions of life
have been disturbed. This view is supported by a parallelism of another
kind;—namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour
and fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct
causes; for both depend on the amount of difference of some kind between
the species which are crossed. Nor is it surprising that the facility of
effecting a first cross, the fertility of the hybrids produced, and the
capacity of being grafted together—though this latter capacity
evidently depends on widely different circumstances—should all run,
to a certain extent, parallel with the systematic affinity of the forms
which are subjected to experiment; for systematic affinity attempts to
express all kinds of resemblance between all species.
First crosses between forms known to be varieties, or sufficiently alike
to be considered as varieties, and their mongrel offspring, are very
generally, but not quite universally, fertile. Nor is this nearly general
and perfect fertility surprising, when we remember how liable we are to
argue in a circle with respect to varieties in a state of nature; and when
we remember that the greater number of varieties have been produced under
domestication by the selection of mere external differences, and not of
differences in the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between hybrids and
mongrels. Finally, then, the facts briefly given in this chapter do not
seem to me opposed to, but even rather to support the view, that there is
no fundamental distinction between species and varieties.
9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day. On the nature
of extinct intermediate varieties; on their number. On the vast lapse of
time, as inferred from the rate of deposition and of denudation. On the
poorness of our palaeontological collections. On the intermittence of
geological formations. On the absence of intermediate varieties in any one
formation. On the sudden appearance of groups of species. On their sudden
appearance in the lowest known fossiliferous strata.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific forms,
and their not being blended together by innumerable transitional links, is
a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day, under the circumstances apparently most
favourable for their presence, namely on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life
of each species depends in a more important manner on the presence of
other already defined organic forms, than on climate; and, therefore, that
the really governing conditions of life do not graduate away quite
insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms
which they connect, will generally be beaten out and exterminated during
the course of further modification and improvement. The main cause,
however, of innumerable intermediate links not now occurring everywhere
throughout nature depends on the very process of natural selection,
through which new varieties continually take the places of and exterminate
their parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so must the number of
intermediate varieties, which have formerly existed on the earth, be truly
enormous. Why then is not every geological formation and every stratum
full of such intermediate links? Geology assuredly does not reveal any
such finely graduated organic chain; and this, perhaps, is the most
obvious and gravest objection which can be urged against my theory. The
explanation lies, as I believe, in the extreme imperfection of the
geological record.
In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have found
it difficult, when looking at any two species, to avoid picturing to
myself, forms DIRECTLY intermediate between them. But this is a wholly
false view; we should always look for forms intermediate between each
species and a common but unknown progenitor; and the progenitor will
generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter pigeons
have both descended from the rock-pigeon; if we possessed all the
intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds. These
two breeds, moreover, have become so much modified, that if we had no
historical or indirect evidence regarding their origin, it would not have
been possible to have determined from a mere comparison of their structure
with that of the rock-pigeon, whether they had descended from this species
or from some other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for instance
to the horse and tapir, we have no reason to suppose that links ever
existed directly intermediate between them, but between each and an
unknown common parent. The common parent will have had in its whole
organisation much general resemblance to the tapir and to the horse; but
in some points of structure may have differed considerably from both, even
perhaps more than they differ from each other. Hence in all such cases, we
should be unable to recognise the parent-form of any two or more species,
even if we closely compared the structure of the parent with that of its
modified descendants, unless at the same time we had a nearly perfect
chain of the intermediate links.
It is just possible by my theory, that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this
case DIRECT intermediate links will have existed between them. But such a
case would imply that one form had remained for a very long period
unaltered, whilst its descendants had undergone a vast amount of change;
and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the
new and improved forms of life will tend to supplant the old and
unimproved forms.
By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the varieties of the same species at the present day; and
these parent-species, now generally extinct, have in their turn been
similarly connected with more ancient species; and so on backwards, always
converging to the common ancestor of each great class. So that the number
of intermediate and transitional links, between all living and extinct
species, must have been inconceivably great. But assuredly, if this theory
be true, such have lived upon this earth.
ON THE LAPSE OF TIME.
Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected, that time will not have
sufficed for so great an amount of organic change, all changes having been
effected very slowly through natural selection. It is hardly possible for
me even to recall to the reader, who may not be a practical geologist, the
facts leading the mind feebly to comprehend the lapse of time. He who can
read Sir Charles Lyell's grand work on the Principles of Geology, which
the future historian will recognise as having produced a revolution in
natural science, yet does not admit how incomprehensibly vast have been
the past periods of time, may at once close this volume. Not that it
suffices to study the Principles of Geology, or to read special treatises
by different observers on separate formations, and to mark how each author
attempts to give an inadequate idea of the duration of each formation or
even each stratum. A man must for years examine for himself great piles of
superimposed strata, and watch the sea at work grinding down old rocks and
making fresh sediment, before he can hope to comprehend anything of the
lapse of time, the monuments of which we see around us.
It is good to wander along lines of sea-coast, when formed of moderately
hard rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat into
them only when they are charged with sand or pebbles; for there is reason
to believe that pure water can effect little or nothing in wearing away
rock. At last the base of the cliff is undermined, huge fragments fall
down, and these remaining fixed, have to be worn away, atom by atom, until
reduced in size they can be rolled about by the waves, and then are more
quickly ground into pebbles, sand, or mud. But how often do we see along
the bases of retreating cliffs rounded boulders, all thickly clothed by
marine productions, showing how little they are abraded and how seldom
they are rolled about! Moreover, if we follow for a few miles any line of
rocky cliff, which is undergoing degradation, we find that it is only here
and there, along a short length or round a promontory, that the cliffs are
at the present time suffering. The appearance of the surface and the
vegetation show that elsewhere years have elapsed since the waters washed
their base.
He who most closely studies the action of the sea on our shores, will, I
believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller, and by
that excellent observer Mr. Smith of Jordan Hill, are most impressive.
With the mind thus impressed, let any one examine beds of conglomerate
many thousand feet in thickness, which, though probably formed at a
quicker rate than many other deposits, yet, from being formed of worn and
rounded pebbles, each of which bears the stamp of time, are good to show
how slowly the mass has been accumulated. Let him remember Lyell's
profound remark, that the thickness and extent of sedimentary formations
are the result and measure of the degradation which the earth's crust has
elsewhere suffered. And what an amount of degradation is implied by the
sedimentary deposits of many countries! Professor Ramsay has given me the
maximum thickness, in most cases from actual measurement, in a few cases
from estimate, of each formation in different parts of Great Britain; and
this is the result:—
Feet
Palaeozoic strata (not including igneous beds)..57,154.
Secondary strata................................13,190.
Tertiary strata..................................2,240.
—making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness on
the Continent. Moreover, between each successive formation, we have, in
the opinion of most geologists, enormously long blank periods. So that the
lofty pile of sedimentary rocks in Britain, gives but an inadequate idea
of the time which has elapsed during their accumulation; yet what time
this must have consumed! Good observers have estimated that sediment is
deposited by the great Mississippi river at the rate of only 600 feet in a
hundred thousand years. This estimate may be quite erroneous; yet,
considering over what wide spaces very fine sediment is transported by the
currents of the sea, the process of accumulation in any one area must be
extremely slow.
But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I remember
having been much struck with the evidence of denudation, when viewing
volcanic islands, which have been worn by the waves and pared all round
into perpendicular cliffs of one or two thousand feet in height; for the
gentle slope of the lava-streams, due to their formerly liquid state,
showed at a glance how far the hard, rocky beds had once extended into the
open ocean. The same story is still more plainly told by faults,—those
great cracks along which the strata have been upheaved on one side, or
thrown down on the other, to the height or depth of thousands of feet; for
since the crust cracked, the surface of the land has been so completely
planed down by the action of the sea, that no trace of these vast
dislocations is externally visible.
The Craven fault, for instance, extends for upwards of 30 miles, and along
this line the vertical displacement of the strata has varied from 600 to
3000 feet. Professor Ramsay has published an account of a downthrow in
Anglesea of 2300 feet; and he informs me that he fully believes there is
one in Merionethshire of 12,000 feet; yet in these cases there is nothing
on the surface to show such prodigious movements; the pile of rocks on the
one or other side having been smoothly swept away. The consideration of
these facts impresses my mind almost in the same manner as does the vain
endeavour to grapple with the idea of eternity.
I am tempted to give one other case, the well-known one of the denudation
of the Weald. Though it must be admitted that the denudation of the Weald
has been a mere trifle, in comparison with that which has removed masses
of our palaeozoic strata, in parts ten thousand feet in thickness, as
shown in Professor Ramsay's masterly memoir on this subject. Yet it is an
admirable lesson to stand on the North Downs and to look at the distant
South Downs; for, remembering that at no great distance to the west the
northern and southern escarpments meet and close, one can safely picture
to oneself the great dome of rocks which must have covered up the Weald
within so limited a period as since the latter part of the Chalk
formation. The distance from the northern to the southern Downs is about
22 miles, and the thickness of the several formations is on an average
about 1100 feet, as I am informed by Professor Ramsay. But if, as some
geologists suppose, a range of older rocks underlies the Weald, on the
flanks of which the overlying sedimentary deposits might have accumulated
in thinner masses than elsewhere, the above estimate would be erroneous;
but this source of doubt probably would not greatly affect the estimate as
applied to the western extremity of the district. If, then, we knew the
rate at which the sea commonly wears away a line of cliff of any given
height, we could measure the time requisite to have denuded the Weald.
This, of course, cannot be done; but we may, in order to form some crude
notion on the subject, assume that the sea would eat into cliffs 500 feet
in height at the rate of one inch in a century. This will at first appear
much too small an allowance; but it is the same as if we were to assume a
cliff one yard in height to be eaten back along a whole line of coast at
the rate of one yard in nearly every twenty-two years. I doubt whether any
rock, even as soft as chalk, would yield at this rate excepting on the
most exposed coasts; though no doubt the degradation of a lofty cliff
would be more rapid from the breakage of the fallen fragments. On the
other hand, I do not believe that any line of coast, ten or twenty miles
in length, ever suffers degradation at the same time along its whole
indented length; and we must remember that almost all strata contain
harder layers or nodules, which from long resisting attrition form a
breakwater at the base. Hence, under ordinary circumstances, I conclude
that for a cliff 500 feet in height, a denudation of one inch per century
for the whole length would be an ample allowance. At this rate, on the
above data, the denudation of the Weald must have required 306,662,400
years; or say three hundred million years.
The action of fresh water on the gently inclined Wealden district, when
upraised, could hardly have been great, but it would somewhat reduce the
above estimate. On the other hand, during oscillations of level, which we
know this area has undergone, the surface may have existed for millions of
years as land, and thus have escaped the action of the sea: when deeply
submerged for perhaps equally long periods, it would, likewise, have
escaped the action of the coast-waves. So that in all probability a far
longer period than 300 million years has elapsed since the latter part of
the Secondary period.
I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During each of
these years, over the whole world, the land and the water has been peopled
by hosts of living forms. What an infinite number of generations, which
the mind cannot grasp, must have succeeded each other in the long roll of
years! Now turn to our richest geological museums, and what a paltry
display we behold!
ON THE POORNESS OF OUR PALAEONTOLOGICAL COLLECTIONS.
That our palaeontological collections are very imperfect, is admitted by
every one. The remark of that admirable palaeontologist, the late Edward
Forbes, should not be forgotten, namely, that numbers of our fossil
species are known and named from single and often broken specimens, or
from a few specimens collected on some one spot. Only a small portion of
the surface of the earth has been geologically explored, and no part with
sufficient care, as the important discoveries made every year in Europe
prove. No organism wholly soft can be preserved. Shells and bones will
decay and disappear when left on the bottom of the sea, where sediment is
not accumulating. I believe we are continually taking a most erroneous
view, when we tacitly admit to ourselves that sediment is being deposited
over nearly the whole bed of the sea, at a rate sufficiently quick to
embed and preserve fossil remains. Throughout an enormously large
proportion of the ocean, the bright blue tint of the water bespeaks its
purity. The many cases on record of a formation conformably covered, after
an enormous interval of time, by another and later formation, without the
underlying bed having suffered in the interval any wear and tear, seem
explicable only on the view of the bottom of the sea not rarely lying for
ages in an unaltered condition. The remains which do become embedded, if
in sand or gravel, will when the beds are upraised generally be dissolved
by the percolation of rain-water. I suspect that but few of the very many
animals which live on the beach between high and low watermark are
preserved. For instance, the several species of the Chthamalinae (a
sub-family of sessile cirripedes) coat the rocks all over the world in
infinite numbers: they are all strictly littoral, with the exception of a
single Mediterranean species, which inhabits deep water and has been found
fossil in Sicily, whereas not one other species has hitherto been found in
any tertiary formation: yet it is now known that the genus Chthamalus
existed during the chalk period. The molluscan genus Chiton offers a
partially analogous case.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with one exception discovered by Sir C. Lyell in the
carboniferous strata of North America. In regard to mammiferous remains, a
single glance at the historical table published in the Supplement to
Lyell's Manual, will bring home the truth, how accidental and rare is
their preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging to
the age of our secondary or palaeozoic formations.
But the imperfection in the geological record mainly results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of
time. When we see the formations tabulated in written works, or when we
follow them in nature, it is difficult to avoid believing that they are
closely consecutive. But we know, for instance, from Sir R. Murchison's
great work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other
parts of the world. The most skilful geologist, if his attention had been
exclusively confined to these large territories, would never have
suspected that during the periods which were blank and barren in his own
country, great piles of sediment, charged with new and peculiar forms of
life, had elsewhere been accumulated. And if in each separate territory,
hardly any idea can be formed of the length of time which has elapsed
between the consecutive formations, we may infer that this could nowhere
be ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in
the geography of the surrounding lands, whence the sediment has been
derived, accords with the belief of vast intervals of time having elapsed
between each formation.
But we can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short geological
period. Along the whole west coast, which is inhabited by a peculiar
marine fauna, tertiary beds are so scantily developed, that no record of
several successive and peculiar marine faunas will probably be preserved
to a distant age. A little reflection will explain why along the rising
coast of the western side of South America, no extensive formations with
recent or tertiary remains can anywhere be found, though the supply of
sediment must for ages have been great, from the enormous degradation of
the coast-rocks and from muddy streams entering the sea. The explanation,
no doubt, is, that the littoral and sub-littoral deposits are continually
worn away, as soon as they are brought up by the slow and gradual rising
of the land within the grinding action of the coast-waves.
We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of the sea, in which
case, judging from the researches of E. Forbes, we may conclude that the
bottom will be inhabited by extremely few animals, and the mass when
upraised will give a most imperfect record of the forms of life which then
existed; or, sediment may be accumulated to any thickness and extent over
a shallow bottom, if it continue slowly to subside. In this latter case,
as long as the rate of subsidence and supply of sediment nearly balance
each other, the sea will remain shallow and favourable for life, and thus
a fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed.
I am convinced that all our ancient formations, which are rich in fossils,
have thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of Geology, and have been
surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated during
subsidence. I may add, that the only ancient tertiary formation on the
west coast of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last to a
distant geological age, was certainly deposited during a downward
oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have
affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but only
where the supply of sediment was sufficient to keep the sea shallow and to
embed and preserve the remains before they had time to decay. On the other
hand, as long as the bed of the sea remained stationary, THICK deposits
could not have been accumulated in the shallow parts, which are the most
favourable to life. Still less could this have happened during the
alternate periods of elevation; or, to speak more accurately, the beds
which were then accumulated will have been destroyed by being upraised and
brought within the limits of the coast-action.
Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for they
are in strict accordance with the general principles inculcated by Sir C.
Lyell; and E. Forbes independently arrived at a similar conclusion.
One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased, and new stations will often be formed;—all circumstances
most favourable, as previously explained, for the formation of new
varieties and species; but during such periods there will generally be a
blank in the geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (excepting the
productions on the shores of a continent when first broken up into an
archipelago), and consequently during subsidence, though there will be
much extinction, fewer new varieties or species will be formed; and it is
during these very periods of subsidence, that our great deposits rich in
fossils have been accumulated. Nature may almost be said to have guarded
against the frequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes more difficult to understand,
why we do not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close. Some cases are
on record of the same species presenting distinct varieties in the upper
and lower parts of the same formation, but, as they are rare, they may be
here passed over. Although each formation has indisputably required a vast
number of years for its deposition, I can see several reasons why each
should not include a graduated series of links between the species which
then lived; but I can by no means pretend to assign due proportional
weight to the following considerations.
Although each formation may mark a very long lapse of years, each perhaps
is short compared with the period requisite to change one species into
another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long as the
average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us coming to any just conclusion on this head. When
we see a species first appearing in the middle of any formation, it would
be rash in the extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing before the uppermost
layers have been deposited, it would be equally rash to suppose that it
then became wholly extinct. We forget how small the area of Europe is
compared with the rest of the world; nor have the several stages of the
same formation throughout Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it only then
first immigrated into that area. It is well known, for instance, that
several species appeared somewhat earlier in the palaeozoic beds of North
America than in those of Europe; time having apparently been required for
their migration from the American to the European seas. In examining the
latest deposits of various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit, but
have become extinct in the immediately surrounding sea; or, conversely,
that some are now abundant in the neighbouring sea, but are rare or absent
in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the
Glacial period, which forms only a part of one whole geological period;
and likewise to reflect on the great changes of level, on the inordinately
great change of climate, on the prodigious lapse of time, all included
within this same glacial period. Yet it may be doubted whether in any
quarter of the world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have
gone on accumulating within the same area during the whole of this period.
It is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within that
limit of depth at which marine animals can flourish; for we know what vast
geographical changes occurred in other parts of America during this space
of time. When such beds as were deposited in shallow water near the mouth
of the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to geographical
changes. And in the distant future, a geologist examining these beds,
might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is extending from before the glacial
epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will generally
have to be a very thick one; and the species undergoing modification will
have had to live on the same area throughout this whole time. But we have
seen that a thick fossiliferous formation can only be accumulated during a
period of subsidence; and to keep the depth approximately the same, which
is necessary in order to enable the same species to live on the same
space, the supply of sediment must nearly have counterbalanced the amount
of subsidence. But this same movement of subsidence will often tend to
sink the area whence the sediment is derived, and thus diminish the supply
whilst the downward movement continues. In fact, this nearly exact
balancing between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of organic
remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of different mineralogical composition, we may reasonably suspect
that the process of deposition has been much interrupted, as a change in
the currents of the sea and a supply of sediment of a different nature
will generally have been due to geographical changes requiring much time.
Nor will the closest inspection of a formation give any idea of the time
which its deposition has consumed. Many instances could be given of beds
only a few feet in thickness, representing formations, elsewhere thousands
of feet in thickness, and which must have required an enormous period for
their accumulation; yet no one ignorant of this fact would have suspected
the vast lapse of time represented by the thinner formation. Many cases
could be given of the lower beds of a formation having been upraised,
denuded, submerged, and then re-covered by the upper beds of the same
formation,—facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have the
plainest evidence in great fossilised trees, still standing upright as
they grew, of many long intervals of time and changes of level during the
process of deposition, which would never even have been suspected, had not
the trees chanced to have been preserved: thus, Messrs. Lyell and Dawson
found carboniferous beds 1400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occur at the bottom,
middle, and top of a formation, the probability is that they have not
lived on the same spot during the whole period of deposition, but have
disappeared and reappeared, perhaps many times, during the same geological
period. So that if such species were to undergo a considerable amount of
modification during any one geological period, a section would not
probably include all the fine intermediate gradations which must on my
theory have existed between them, but abrupt, though perhaps very slight,
changes of form.
It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by close intermediate
gradations. And this from the reasons just assigned we can seldom hope to
effect in any one geological section. Supposing B and C to be two species,
and a third, A, to be found in an underlying bed; even if A were strictly
intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be most closely
connected with either one or both forms by intermediate varieties. Nor
should it be forgotten, as before explained, that A might be the actual
progenitor of B and C, and yet might not at all necessarily be strictly
intermediate between them in all points of structure. So that we might
obtain the parent-species and its several modified descendants from the
lower and upper beds of a formation, and unless we obtained numerous
transitional gradations, we should not recognise their relationship, and
should consequently be compelled to rank them all as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the very
fine species of D'Orbigny and others into the rank of varieties; and on
this view we do find the kind of evidence of change which on my theory we
ought to find. Moreover, if we look to rather wider intervals, namely, to
distinct but consecutive stages of the same great formation, we find that
the embedded fossils, though almost universally ranked as specifically
different, yet are far more closely allied to each other than are the
species found in more widely separated formations; but to this subject I
shall have to return in the following chapter.
One other consideration is worth notice: with animals and plants that can
propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally at
first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected in
some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine
animals have a wide range; and we have seen that with plants it is those
which have the widest range, that oftenest present varieties; so that with
shells and other marine animals, it is probably those which have had the
widest range, far exceeding the limits of the known geological formations
of Europe, which have oftenest given rise, first to local varieties and
ultimately to new species; and this again would greatly lessen the chance
of our being able to trace the stages of transition in any one geological
formation.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until many
specimens have been collected from many places; and in the case of fossil
species this could rarely be effected by palaeontologists. We shall,
perhaps, best perceive the improbability of our being enabled to connect
species by numerous, fine, intermediate, fossil links, by asking ourselves
whether, for instance, geologists at some future period will be able to
prove, that our different breeds of cattle, sheep, horses, and dogs have
descended from a single stock or from several aboriginal stocks; or,
again, whether certain sea-shells inhabiting the shores of North America,
which are ranked by some conchologists as distinct species from their
European representatives, and by other conchologists as only varieties,
are really varieties or are, as it is called, specifically distinct. This
could be effected only by the future geologist discovering in a fossil
state numerous intermediate gradations; and such success seems to me
improbable in the highest degree.
Geological research, though it has added numerous species to existing and
extinct genera, and has made the intervals between some few groups less
wide than they otherwise would have been, yet has done scarcely anything
in breaking down the distinction between species, by connecting them
together by numerous, fine, intermediate varieties; and this not having
been effected, is probably the gravest and most obvious of all the many
objections which may be urged against my views. Hence it will be worth
while to sum up the foregoing remarks, under an imaginary illustration.
The Malay Archipelago is of about the size of Europe from the North Cape
to the Mediterranean, and from Britain to Russia; and therefore equals all
the geological formations which have been examined with any accuracy,
excepting those of the United States of America. I fully agree with Mr.
Godwin-Austen, that the present condition of the Malay Archipelago, with
its numerous large islands separated by wide and shallow seas, probably
represents the former state of Europe, when most of our formations were
accumulating. The Malay Archipelago is one of the richest regions of the
whole world in organic beings; yet if all the species were to be collected
which have ever lived there, how imperfectly would they represent the
natural history of the world!
But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an excessively imperfect manner in
the formations which we suppose to be there accumulating. I suspect that
not many of the strictly littoral animals, or of those which lived on
naked submarine rocks, would be embedded; and those embedded in gravel or
sand, would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could be
preserved.
In our archipelago, I believe that fossiliferous formations could be
formed of sufficient thickness to last to an age, as distant in futurity
as the secondary formations lie in the past, only during periods of
subsidence. These periods of subsidence would be separated from each other
by enormous intervals, during which the area would be either stationary or
rising; whilst rising, each fossiliferous formation would be destroyed,
almost as soon as accumulated, by the incessant coast-action, as we now
see on the shores of South America. During the periods of subsidence there
would probably be much extinction of life; during the periods of
elevation, there would be much variation, but the geological record would
then be least perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would EXCEED the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully
preserved, transitional varieties would merely appear as so many distinct
species. It is, also, probable that each great period of subsidence would
be interrupted by oscillations of level, and that slight climatal changes
would intervene during such lengthy periods; and in these cases the
inhabitants of the archipelago would have to migrate, and no closely
consecutive record of their modifications could be preserved in any one
formation.
Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy leads me to believe that it
would be chiefly these far-ranging species which would oftenest produce
new varieties; and the varieties would at first generally be local or
confined to one place, but if possessed of any decided advantage, or when
further modified and improved, they would slowly spread and supplant their
parent-forms. When such varieties returned to their ancient homes, as they
would differ from their former state, in a nearly uniform, though perhaps
extremely slight degree, they would, according to the principles followed
by many palaeontologists, be ranked as new and distinct species.
If then, there be some degree of truth in these remarks, we have no right
to expect to find in our geological formations, an infinite number of
those fine transitional forms, which on my theory assuredly have connected
all the past and present species of the same group into one long and
branching chain of life. We ought only to look for a few links, some more
closely, some more distantly related to each other; and these links, let
them be ever so close, if found in different stages of the same formation,
would, by most palaeontologists, be ranked as distinct species. But I do
not pretend that I should ever have suspected how poor a record of the
mutations of life, the best preserved geological section presented, had
not the difficulty of our not discovering innumerable transitional links
between the species which appeared at the commencement and close of each
formation, pressed so hardly on my theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.
The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists, for
instance, by Agassiz, Pictet, and by none more forcibly than by Professor
Sedgwick, as a fatal objection to the belief in the transmutation of
species. If numerous species, belonging to the same genera or families,
have really started into life all at once, the fact would be fatal to the
theory of descent with slow modification through natural selection. For
the development of a group of forms, all of which have descended from some
one progenitor, must have been an extremely slow process; and the
progenitors must have lived long ages before their modified descendants.
But we continually over-rate the perfection of the geological record, and
falsely infer, because certain genera or families have not been found
beneath a certain stage, that they did not exist before that stage. We
continually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we forget
that groups of species may elsewhere have long existed and have slowly
multiplied before they invaded the ancient archipelagoes of Europe and of
the United States. We do not make due allowance for the enormous intervals
of time, which have probably elapsed between our consecutive formations,—longer
perhaps in some cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of
species from some one or some few parent-forms; and in the succeeding
formation such species will appear as if suddenly created.
I may here recall a remark formerly made, namely that it might require a
long succession of ages to adapt an organism to some new and peculiar line
of life, for instance to fly through the air; but that when this had been
effected, and a few species had thus acquired a great advantage over other
organisms, a comparatively short time would be necessary to produce many
divergent forms, which would be able to spread rapidly and widely
throughout the world.
I will now give a few examples to illustrate these remarks; and to show
how liable we are to error in supposing that whole groups of species have
suddenly been produced. I may recall the well-known fact that in
geological treatises, published not many years ago, the great class of
mammals was always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest known
accumulations of fossil mammals belongs to the middle of the secondary
series; and one true mammal has been discovered in the new red sandstone
at nearly the commencement of this great series. Cuvier used to urge that
no monkey occurred in any tertiary stratum; but now extinct species have
been discovered in India, South America, and in Europe even as far back as
the eocene stage. The most striking case, however, is that of the Whale
family; as these animals have huge bones, are marine, and range over the
world, the fact of not a single bone of a whale having been discovered in
any secondary formation, seemed fully to justify the belief that this
great and distinct order had been suddenly produced in the interval
between the latest secondary and earliest tertiary formation. But now we
may read in the Supplement to Lyell's 'Manual,' published in 1858, clear
evidence of the existence of whales in the upper greensand, some time
before the close of the secondary period.
I may give another instance, which from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I have
stated that, from the number of existing and extinct tertiary species;
from the extraordinary abundance of the individuals of many species all
over the world, from the Arctic regions to the equator, inhabiting various
zones of depths from the upper tidal limits to 50 fathoms; from the
perfect manner in which specimens are preserved in the oldest tertiary
beds; from the ease with which even a fragment of a valve can be
recognised; from all these circumstances, I inferred that had sessile
cirripedes existed during the secondary periods, they would certainly have
been preserved and discovered; and as not one species had been discovered
in beds of this age, I concluded that this great group had been suddenly
developed at the commencement of the tertiary series. This was a sore
trouble to me, adding as I thought one more instance of the abrupt
appearance of a great group of species. But my work had hardly been
published, when a skilful palaeontologist, M. Bosquet, sent me a drawing
of a perfect specimen of an unmistakeable sessile cirripede, which he had
himself extracted from the chalk of Belgium. And, as if to make the case
as striking as possible, this sessile cirripede was a Chthamalus, a very
common, large, and ubiquitous genus, of which not one specimen has as yet
been found even in any tertiary stratum. Hence we now positively know that
sessile cirripedes existed during the secondary period; and these
cirripedes might have been the progenitors of our many tertiary and
existing species.
The case most frequently insisted on by palaeontologists of the apparently
sudden appearance of a whole group of species, is that of the teleostean
fishes, low down in the Chalk period. This group includes the large
majority of existing species. Lately, Professor Pictet has carried their
existence one sub-stage further back; and some palaeontologists believe
that certain much older fishes, of which the affinities are as yet
imperfectly known, are really teleostean. Assuming, however, that the
whole of them did appear, as Agassiz believes, at the commencement of the
chalk formation, the fact would certainly be highly remarkable; but I
cannot see that it would be an insuperable difficulty on my theory, unless
it could likewise be shown that the species of this group appeared
suddenly and simultaneously throughout the world at this same period. It
is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's Palaeontology it
will be seen that very few species are known from several formations in
Europe. Some few families of fish now have a confined range; the
teleostean fish might formerly have had a similarly confined range, and
after having been largely developed in some one sea, might have spread
widely. Nor have we any right to suppose that the seas of the world have
always been so freely open from south to north as they are at present.
Even at this day, if the Malay Archipelago were converted into land, the
tropical parts of the Indian Ocean would form a large and perfectly
enclosed basin, in which any great group of marine animals might be
multiplied; and here they would remain confined, until some of the species
became adapted to a cooler climate, and were enabled to double the
southern capes of Africa or Australia, and thus reach other and distant
seas.
From these and similar considerations, but chiefly from our ignorance of
the geology of other countries beyond the confines of Europe and the
United States; and from the revolution in our palaeontological ideas on
many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic beings throughout the world, as it would be for a
naturalist to land for five minutes on some one barren point in Australia,
and then to discuss the number and range of its productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much graver. I allude to
the manner in which numbers of species of the same group, suddenly appear
in the lowest known fossiliferous rocks. Most of the arguments which have
convinced me that all the existing species of the same group have
descended from one progenitor, apply with nearly equal force to the
earliest known species. For instance, I cannot doubt that all the Silurian
trilobites have descended from some one crustacean, which must have lived
long before the Silurian age, and which probably differed greatly from any
known animal. Some of the most ancient Silurian animals, as the Nautilus,
Lingula, etc., do not differ much from living species; and it cannot on my
theory be supposed, that these old species were the progenitors of all the
species of the orders to which they belong, for they do not present
characters in any degree intermediate between them. If, moreover, they had
been the progenitors of these orders, they would almost certainly have
been long ago supplanted and exterminated by their numerous and improved
descendants.
Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as,
or probably far longer than, the whole interval from the Silurian age to
the present day; and that during these vast, yet quite unknown, periods of
time, the world swarmed with living creatures.
To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir R. Murchison at their head, are convinced that we see
in the organic remains of the lowest Silurian stratum the dawn of life on
this planet. Other highly competent judges, as Lyell and the late E.
Forbes, dispute this conclusion. We should not forget that only a small
portion of the world is known with accuracy. M. Barrande has lately added
another and lower stage to the Silurian system, abounding with new and
peculiar species. Traces of life have been detected in the Longmynd beds
beneath Barrande's so-called primordial zone. The presence of phosphatic
nodules and bituminous matter in some of the lowest azoic rocks, probably
indicates the former existence of life at these periods. But the
difficulty of understanding the absence of vast piles of fossiliferous
strata, which on my theory no doubt were somewhere accumulated before the
Silurian epoch, is very great. If these most ancient beds had been wholly
worn away by denudation, or obliterated by metamorphic action, we ought to
find only small remnants of the formations next succeeding them in age,
and these ought to be very generally in a metamorphosed condition. But the
descriptions which we now possess of the Silurian deposits over immense
territories in Russia and in North America, do not support the view, that
the older a formation is, the more it has suffered the extremity of
denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as a
valid argument against the views here entertained. To show that it may
hereafter receive some explanation, I will give the following hypothesis.
From the nature of the organic remains, which do not appear to have
inhabited profound depths, in the several formations of Europe and of the
United States; and from the amount of sediment, miles in thickness, of
which the formations are composed, we may infer that from first to last
large islands or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the existing continents of Europe and North
America. But we do not know what was the state of things in the intervals
between the successive formations; whether Europe and the United States
during these intervals existed as dry land, or as a submarine surface near
land, on which sediment was not deposited, or again as the bed of an open
and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the land,
we see them studded with many islands; but not one oceanic island is as
yet known to afford even a remnant of any palaeozoic or secondary
formation. Hence we may perhaps infer, that during the palaeozoic and
secondary periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed there, palaeozoic and
secondary formations would in all probability have been accumulated from
sediment derived from their wear and tear; and would have been at least
partially upheaved by the oscillations of level, which we may fairly
conclude must have intervened during these enormously long periods. If
then we may infer anything from these facts, we may infer that where our
oceans now extend, oceans have extended from the remotest period of which
we have any record; and on the other hand, that where continents now
exist, large tracts of land have existed, subjected no doubt to great
oscillations of level, since the earliest silurian period. The coloured
map appended to my volume on Coral Reefs, led me to conclude that the
great oceans are still mainly areas of subsidence, the great archipelagoes
still areas of oscillations of level, and the continents areas of
elevation. But have we any right to assume that things have thus remained
from eternity? Our continents seem to have been formed by a preponderance,
during many oscillations of level, of the force of elevation; but may not
the areas of preponderant movement have changed in the lapse of ages? At a
period immeasurably antecedent to the silurian epoch, continents may have
existed where oceans are now spread out; and clear and open oceans may
have existed where our continents now stand. Nor should we be justified in
assuming that if, for instance, the bed of the Pacific Ocean were now
converted into a continent, we should there find formations older than the
silurian strata, supposing such to have been formerly deposited; for it
might well happen that strata which had subsided some miles nearer to the
centre of the earth, and which had been pressed on by an enormous weight
of superincumbent water, might have undergone far more metamorphic action
than strata which have always remained nearer to the surface. The immense
areas in some parts of the world, for instance in South America, of bare
metamorphic rocks, which must have been heated under great pressure, have
always seemed to me to require some special explanation; and we may
perhaps believe that we see in these large areas, the many formations long
anterior to the silurian epoch in a completely metamorphosed condition.
The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between the
many species which now exist or have existed; the sudden manner in which
whole groups of species appear in our European formations; the almost
entire absence, as at present known, of fossiliferous formations beneath
the Silurian strata, are all undoubtedly of the gravest nature. We see
this in the plainest manner by the fact that all the most eminent
palaeontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer, E.
Forbes, etc., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But I have reason to believe that one great
authority, Sir Charles Lyell, from further reflexion entertains grave
doubts on this subject. I feel how rash it is to differ from these great
authorities, to whom, with others, we owe all our knowledge. Those who
think the natural geological record in any degree perfect, and who do not
attach much weight to the facts and arguments of other kinds given in this
volume, will undoubtedly at once reject my theory. For my part, following
out Lyell's metaphor, I look at the natural geological record, as a
history of the world imperfectly kept, and written in a changing dialect;
of this history we possess the last volume alone, relating only to two or
three countries. Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there a few lines. Each
word of the slowly-changing language, in which the history is supposed to
be written, being more or less different in the interrupted succession of
chapters, may represent the apparently abruptly changed forms of life,
entombed in our consecutive, but widely separated formations. On this
view, the difficulties above discussed are greatly diminished, or even
disappear.
10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species. On their different
rates of change. Species once lost do not reappear. Groups of species
follow the same general rules in their appearance and disappearance as do
single species. On Extinction. On simultaneous changes in the forms of
life throughout the world. On the affinities of extinct species to each
other and to living species. On the state of development of ancient forms.
On the succession of the same types within the same areas. Summary of
preceding and present chapters.
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between them, and to make the
percentage system of lost and new forms more gradual. In some of the most
recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are lost forms, and only one or two are new forms,
having here appeared for the first time, either locally, or, as far as we
know, on the face of the earth. If we may trust the observations of
Philippi in Sicily, the successive changes in the marine inhabitants of
that island have been many and most gradual. The secondary formations are
more broken; but, as Bronn has remarked, neither the appearance nor
disappearance of their many now extinct species has been simultaneous in
each separate formation.
Species of different genera and classes have not changed at the same rate,
or in the same degree. In the oldest tertiary beds a few living shells may
still be found in the midst of a multitude of extinct forms. Falconer has
given a striking instance of a similar fact, in an existing crocodile
associated with many strange and lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to change at a quicker rate than those of the sea, of which a
striking instance has lately been observed in Switzerland. There is some
reason to believe that organisms, considered high in the scale of nature,
change more quickly than those that are low: though there are exceptions
to this rule. The amount of organic change, as Pictet has remarked, does
not strictly correspond with the succession of our geological formations;
so that between each two consecutive formations, the forms of life have
seldom changed in exactly the same degree. Yet if we compare any but the
most closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from the face
of the earth, we have reason to believe that the same identical form never
reappears. The strongest apparent exception to this latter rule, is that
of the so-called "colonies" of M. Barrande, which intrude for a period in
the midst of an older formation, and then allow the pre-existing fauna to
reappear; but Lyell's explanation, namely, that it is a case of temporary
migration from a distinct geographical province, seems to me satisfactory.
These several facts accord well with my theory. I believe in no fixed law
of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be taken
advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,—on the variability being of a beneficial
nature, on the power of intercrossing, on the rate of breeding, on the
slowly changing physical conditions of the country, and more especially on
the nature of the other inhabitants with which the varying species comes
into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, that it should change less. We see the same fact in geographical
distribution; for instance, in the land-shells and coleopterous insects of
Madeira having come to differ considerably from their nearest allies on
the continent of Europe, whereas the marine shells and birds have remained
unaltered. We can perhaps understand the apparently quicker rate of change
in terrestrial and in more highly organised productions compared with
marine and lower productions, by the more complex relations of the higher
beings to their organic and inorganic conditions of life, as explained in
a former chapter. When many of the inhabitants of a country have become
modified and improved, we can understand, on the principle of competition,
and on that of the many all-important relations of organism to organism,
that any form which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species in
the same region do at last, if we look to wide enough intervals of time,
become modified; for those which do not change will become extinct.
In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is not
equal. Each formation, on this view, does not mark a new and complete act
of creation, but only an occasional scene, taken almost at hazard, in a
slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the
exact place of another species in the economy of nature, and thus supplant
it; yet the two forms—the old and the new—would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors. For instance, it is just
possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and exterminated
by its improved offspring, it is quite incredible that a fantail,
identical with the existing breed, could be raised from any other species
of pigeon, or even from the other well-established races of the domestic
pigeon, for the newly-formed fantail would be almost sure to inherit from
its new progenitor some slight characteristic differences.
Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group does not
reappear after it has once disappeared; or its existence, as long as it
lasts, is continuous. I am aware that there are some apparent exceptions
to this rule, but the exceptions are surprisingly few, so few, that E.
Forbes, Pictet, and Woodward (though all strongly opposed to such views as
I maintain) admit its truth; and the rule strictly accords with my theory.
For as all the species of the same group have descended from some one
species, it is clear that as long as any species of the group have
appeared in the long succession of ages, so long must its members have
continuously existed, in order to have generated either new and modified
or the same old and unmodified forms. Species of the genus Lingula, for
instance, must have continuously existed by an unbroken succession of
generations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner
or later, it gradually decreases. If the number of the species of a genus,
or the number of the genera of a family, be represented by a vertical line
of varying thickness, crossing the successive geological formations in
which the species are found, the line will sometimes falsely appear to
begin at its lower end, not in a sharp point, but abruptly; it then
gradually thickens upwards, sometimes keeping for a space of equal
thickness, and ultimately thins out in the upper beds, marking the
decrease and final extinction of the species. This gradual increase in
number of the species of a group is strictly conformable with my theory;
as the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of modification
and the production of a number of allied forms must be slow and gradual,—one
species giving rise first to two or three varieties, these being slowly
converted into species, which in their turn produce by equally slow steps
other species, and so on, like the branching of a great tree from a single
stem, till the group becomes large.
ON EXTINCTION.
We have as yet spoken only incidentally of the disappearance of species
and of groups of species. On the theory of natural selection the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away at successive periods by catastrophes, is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them
to this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. Both single species and whole groups
of species last for very unequal periods; some groups, as we have seen,
having endured from the earliest known dawn of life to the present day;
some having disappeared before the close of the palaeozoic period. No
fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the
complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a
group of species be represented, as before, by a vertical line of varying
thickness, the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its lower end, which
marks the first appearance and increase in numbers of the species. In some
cases, however, the extermination of whole groups of beings, as of
ammonites towards the close of the secondary period, has been wonderfully
sudden.
The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a very
late geological period, I was filled with astonishment; for seeing that
the horse, since its introduction by the Spaniards into South America, has
run wild over the whole country and has increased in numbers at an
unparalleled rate, I asked myself what could so recently have exterminated
the former horse under conditions of life apparently so favourable. But
how utterly groundless was my astonishment! Professor Owen soon perceived
that the tooth, though so like that of the existing horse, belonged to an
extinct species. Had this horse been still living, but in some degree
rare, no naturalist would have felt the least surprise at its rarity; for
rarity is the attribute of a vast number of species of all classes, in all
countries. If we ask ourselves why this or that species is rare, we answer
that something is unfavourable in its conditions of life; but what that
something is, we can hardly ever tell. On the supposition of the fossil
horse still existing as a rare species, we might have felt certain from
the analogy of all other mammals, even of the slow-breeding elephant, and
from the history of the naturalisation of the domestic horse in South
America, that under more favourable conditions it would in a very few
years have stocked the whole continent. But we could not have told what
the unfavourable conditions were which checked its increase, whether some
one or several contingencies, and at what period of the horse's life, and
in what degree, they severally acted. If the conditions had gone on,
however slowly, becoming less and less favourable, we assuredly should not
have perceived the fact, yet the fossil horse would certainly have become
rarer and rarer, and finally extinct;—its place being seized on by
some more successful competitor.
It is most difficult always to remember that the increase of every living
being is constantly being checked by unperceived injurious agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. We see in many cases in the more recent tertiary
formations, that rarity precedes extinction; and we know that this has
been the progress of events with those animals which have been
exterminated, either locally or wholly, through man's agency. I may repeat
what I published in 1845, namely, that to admit that species generally
become rare before they become extinct—to feel no surprise at the
rarity of a species, and yet to marvel greatly when it ceases to exist, is
much the same as to admit that sickness in the individual is the
forerunner of death—to feel no surprise at sickness, but when the
sick man dies, to wonder and to suspect that he died by some unknown deed
of violence.
The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the less
improved varieties in the same neighbourhood; when much improved it is
transported far and near, like our short-horn cattle, and takes the place
of other breeds in other countries. Thus the appearance of new forms and
the disappearance of old forms, both natural and artificial, are bound
together. In certain flourishing groups, the number of new specific forms
which have been produced within a given time is probably greater than that
of the old forms which have been exterminated; but we know that the number
of species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may believe
that the production of new forms has caused the extinction of about the
same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group will have seized on the place occupied
by a species belonging to a distinct group, and thus caused its
extermination; and if many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
allied forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct class,
which yield their places to other species which have been modified and
improved, a few of the sufferers may often long be preserved, from being
fitted to some peculiar line of life, or from inhabiting some distant and
isolated station, where they have escaped severe competition. For
instance, a single species of Trigonia, a great genus of shells in the
secondary formations, survives in the Australian seas; and a few members
of the great and almost extinct group of Ganoid fishes still inhabit our
fresh waters. Therefore the utter extinction of a group is generally, as
we have seen, a slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of a
new area, they will have exterminated in a correspondingly rapid manner
many of the old inhabitants; and the forms which thus yield their places
will commonly be allied, for they will partake of some inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies, on which the existence of each species depends. If
we forget for an instant, that each species tends to increase
inordinately, and that some check is always in action, yet seldom
perceived by us, the whole economy of nature will be utterly obscured.
Whenever we can precisely say why this species is more abundant in
individuals than that; why this species and not another can be naturalised
in a given country; then, and not till then, we may justly feel surprise
why we cannot account for the extinction of this particular species or
group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.
Scarcely any palaeontological discovery is more striking than the fact,
that the forms of life change almost simultaneously throughout the world.
Thus our European Chalk formation can be recognised in many distant parts
of the world, under the most different climates, where not a fragment of
the mineral chalk itself can be found; namely, in North America, in
equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
and in the peninsula of India. For at these distant points, the organic
remains in certain beds present an unmistakeable degree of resemblance to
those of the Chalk. It is not that the same species are met with; for in
some cases not one species is identically the same, but they belong to the
same families, genera, and sections of genera, and sometimes are similarly
characterised in such trifling points as mere superficial sculpture.
Moreover other forms, which are not found in the Chalk of Europe, but
which occur in the formations either above or below, are similarly absent
at these distant points of the world. In the several successive palaeozoic
formations of Russia, Western Europe and North America, a similar
parallelism in the forms of life has been observed by several authors: so
it is, according to Lyell, with the several European and North American
tertiary deposits. Even if the few fossil species which are common to the
Old and New Worlds be kept wholly out of view, the general parallelism in
the successive forms of life, in the stages of the widely separated
palaeozoic and tertiary periods, would still be manifest, and the several
formations could be easily correlated.
These observations, however, relate to the marine inhabitants of distant
parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in the
same parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
from La Plata, without any information in regard to their geological
position, no one would have suspected that they had coexisted with still
living sea-shells; but as these anomalous monsters coexisted with the
Mastodon and Horse, it might at least have been inferred that they had
lived during one of the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year, or
even that it has a very strict geological sense; for if all the marine
animals which live at the present day in Europe, and all those that lived
in Europe during the pleistocene period (an enormously remote period as
measured by years, including the whole glacial epoch), were to be compared
with those now living in South America or in Australia, the most skilful
naturalist would hardly be able to say whether the existing or the
pleistocene inhabitants of Europe resembled most closely those of the
southern hemisphere. So, again, several highly competent observers believe
that the existing productions of the United States are more closely
related to those which lived in Europe during certain later tertiary
stages, than to those which now live here; and if this be so, it is
evident that fossiliferous beds deposited at the present day on the shores
of North America would hereafter be liable to be classed with somewhat
older European beds. Nevertheless, looking to a remotely future epoch,
there can, I think, be little doubt that all the more modern MARINE
formations, namely, the upper pliocene, the pleistocene and strictly
modern beds, of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not including
those forms which are only found in the older underlying deposits, would
be correctly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously, in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in
marine currents or other causes more or less local and temporary, but
depend on general laws which govern the whole animal kingdom." M. Barrande
has made forcible remarks to precisely the same effect. It is, indeed,
quite futile to look to changes of currents, climate, or other physical
conditions, as the cause of these great mutations in the forms of life
throughout the world, under the most different climates. We must, as
Barrande has remarked, look to some special law. We shall see this more
clearly when we treat of the present distribution of organic beings, and
find how slight is the relation between the physical conditions of various
countries, and the nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by new varieties arising, which have some advantage over older
forms; and those forms, which are already dominant, or have some advantage
over the other forms in their own country, would naturally oftenest give
rise to new varieties or incipient species; for these latter must be
victorious in a still higher degree in order to be preserved and to
survive. We have distinct evidence on this head, in the plants which are
dominant, that is, which are commonest in their own homes, and are most
widely diffused, having produced the greatest number of new varieties. It
is also natural that the dominant, varying, and far-spreading species,
which already have invaded to a certain extent the territories of other
species, should be those which would have the best chance of spreading
still further, and of giving rise in new countries to new varieties and
species. The process of diffusion may often be very slow, being dependent
on climatal and geographical changes, or on strange accidents, but in the
long run the dominant forms will generally succeed in spreading. The
diffusion would, it is probable, be slower with the terrestrial
inhabitants of distinct continents than with the marine inhabitants of the
continuous sea. We might therefore expect to find, as we apparently do
find, a less strict degree of parallel succession in the productions of
the land than of the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations,
and that severe competition with many already existing forms, would be
highly favourable, as would be the power of spreading into new
territories. A certain amount of isolation, recurring at long intervals of
time, would probably be also favourable, as before explained. One quarter
of the world may have been most favourable for the production of new and
dominant species on the land, and another for those in the waters of the
sea. If two great regions had been for a long period favourably
circumstanced in an equal degree, whenever their inhabitants met, the
battle would be prolonged and severe; and some from one birthplace and
some from the other might be victorious. But in the course of time, the
forms dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the extinction
of other and inferior forms; and as these inferior forms would be allied
in groups by inheritance, whole groups would tend slowly to disappear;
though here and there a single member might long be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant owing to inheritance, and to having
already had some advantage over their parents or over other species; these
again spreading, varying, and producing new species. The forms which are
beaten and which yield their places to the new and victorious forms, will
generally be allied in groups, from inheriting some inferiority in common;
and therefore as new and improved groups spread throughout the world, old
groups will disappear from the world; and the succession of forms in both
ways will everywhere tend to correspond.
There is one other remark connected with this subject worth making. I have
given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of the
sea was either stationary or rising, and likewise when sediment was not
thrown down quickly enough to embed and preserve organic remains. During
these long and blank intervals I suppose that the inhabitants of each
region underwent a considerable amount of modification and extinction, and
that there was much migration from other parts of the world. As we have
reason to believe that large areas are affected by the same movement, it
is probable that strictly contemporaneous formations have often been
accumulated over very wide spaces in the same quarter of the world; but we
are far from having any right to conclude that this has invariably been
the case, and that large areas have invariably been affected by the same
movements. When two formations have been deposited in two regions during
nearly, but not exactly the same period, we should find in both, from the
causes explained in the foregoing paragraphs, the same general succession
in the forms of life; but the species would not exactly correspond; for
there will have been a little more time in the one region than in the
other for modification, extinction, and immigration.
I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England and
France, is able to draw a close general parallelism between the successive
stages in the two countries; but when he compares certain stages in
England with those in France, although he finds in both a curious
accordance in the numbers of the species belonging to the same genera, yet
the species themselves differ in a manner very difficult to account for,
considering the proximity of the two areas,—unless, indeed, it be
assumed that an isthmus separated two seas inhabited by distinct, but
contemporaneous, faunas. Lyell has made similar observations on some of
the later tertiary formations. Barrande, also, shows that there is a
striking general parallelism in the successive Silurian deposits of
Bohemia and Scandinavia; nevertheless he finds a surprising amount of
difference in the species. If the several formations in these regions have
not been deposited during the same exact periods,—a formation in one
region often corresponding with a blank interval in the other,—and
if in both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals of
time between them; in this case, the several formations in the two regions
could be arranged in the same order, in accordance with the general
succession of the form of life, and the order would falsely appear to be
strictly parallel; nevertheless the species would not all be the same in
the apparently corresponding stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.
Let us now look to the mutual affinities of extinct and living species.
They all fall into one grand natural system; and this fact is at once
explained on the principle of descent. The more ancient any form is, the
more, as a general rule, it differs from living forms. But, as Buckland
long ago remarked, all fossils can be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the wide intervals between existing genera, families, and orders, cannot
be disputed. For if we confine our attention either to the living or to
the extinct alone, the series is far less perfect than if we combine both
into one general system. With respect to the Vertebrata, whole pages could
be filled with striking illustrations from our great palaeontologist,
Owen, showing how extinct animals fall in between existing groups. Cuvier
ranked the Ruminants and Pachyderms, as the two most distinct orders of
mammals; but Owen has discovered so many fossil links, that he has had to
alter the whole classification of these two orders; and has placed certain
pachyderms in the same sub-order with ruminants: for example, he dissolves
by fine gradations the apparently wide difference between the pig and the
camel. In regard to the Invertebrata, Barrande, and a higher authority
could not be named, asserts that he is every day taught that palaeozoic
animals, though belonging to the same orders, families, or genera with
those living at the present day, were not at this early epoch limited in
such distinct groups as they now are.
Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If by
this term it is meant that an extinct form is directly intermediate in all
its characters between two living forms, the objection is probably valid.
But I apprehend that in a perfectly natural classification many fossil
species would have to stand between living species, and some extinct
genera between living genera, even between genera belonging to distinct
families. The most common case, especially with respect to very distinct
groups, such as fish and reptiles, seems to be, that supposing them to be
distinguished at the present day from each other by a dozen characters,
the ancient members of the same two groups would be distinguished by a
somewhat lesser number of characters, so that the two groups, though
formerly quite distinct, at that period made some small approach to each
other.
It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now
and then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the more recent members of the same classes, we
must admit that there is some truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters represent genera, and the dotted
lines diverging from them the species in each genus. The diagram is much
too simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form
a small family; b14 and f14 a closely allied family or sub-family; and
o14, e14, m14, a third family. These three families, together with the
many extinct genera on the several lines of descent diverging from the
parent-form A, will form an order; for all will have inherited something
in common from their ancient and common progenitor. On the principle of
the continued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is, the more it will
generally differ from its ancient progenitor. Hence we can understand the
rule that the most ancient fossils differ most from existing forms. We
must not, however, assume that divergence of character is a necessary
contingency; it depends solely on the descendants from a species being
thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case of
some Silurian forms, that a species might go on being slightly modified in
relation to its slightly altered conditions of life, and yet retain
throughout a vast period the same general characteristics. This is
represented in the diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these
three families would be so closely linked together that they probably
would have to be united into one great family, in nearly the same manner
as has occurred with ruminants and pachyderms. Yet he who objected to call
the extinct genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations—for instance, above Number VI.—but none from
beneath this line, then only the two families on the left hand (namely,
a14, etc., and b14, etc.) would have to be united into one family; and the
two other families (namely, a14 to f14 now including five genera, and o14
to m14) would yet remain distinct. These two families, however, would be
less distinct from each other than they were before the discovery of the
fossils. If, for instance, we suppose the existing genera of the two
families to differ from each other by a dozen characters, in this case the
genera, at the early period marked VI., would differ by a lesser number of
characters; for at this early stage of descent they have not diverged in
character from the common progenitor of the order, nearly so much as they
subsequently diverged. Thus it comes that ancient and extinct genera are
often in some slight degree intermediate in character between their
modified descendants, or between their collateral relations.
In nature the case will be far more complicated than is represented in the
diagram; for the groups will have been more numerous, they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in very rare cases, to fill up wide intervals in the natural
system, and thus unite distinct families or orders. All that we have a
right to expect, is that those groups, which have within known geological
periods undergone much modification, should in the older formations make
some slight approach to each other; so that the older members should
differ less from each other in some of their characters than do the
existing members of the same groups; and this by the concurrent evidence
of our best palaeontologists seems frequently to be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory manner.
And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period in
the earth's history will be intermediate in general character between that
which preceded and that which succeeded it. Thus, the species which lived
at the sixth great stage of descent in the diagram are the modified
offspring of those which lived at the fifth stage, and are the parents of
those which became still more modified at the seventh stage; hence they
could hardly fail to be nearly intermediate in character between the forms
of life above and below. We must, however, allow for the entire extinction
of some preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long and
blank intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas. I
need give only one instance, namely, the manner in which the fossils of
the Devonian system, when this system was first discovered, were at once
recognised by palaeontologists as intermediate in character between those
of the overlying carboniferous, and underlying Silurian system. But each
fauna is not necessarily exactly intermediate, as unequal intervals of
time have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, mastodons and elephants, when arranged by Dr.
Falconer in two series, first according to their mutual affinities and
then according to their periods of existence, do not accord in
arrangement. The species extreme in character are not the oldest, or the
most recent; nor are those which are intermediate in character,
intermediate in age. But supposing for an instant, in this and other such
cases, that the record of the first appearance and disappearance of the
species was perfect, we have no reason to believe that forms successively
produced necessarily endure for corresponding lengths of time: a very
ancient form might occasionally last much longer than a form elsewhere
subsequently produced, especially in the case of terrestrial productions
inhabiting separated districts. To compare small things with great: if the
principal living and extinct races of the domestic pigeon were arranged as
well as they could be in serial affinity, this arrangement would not
closely accord with the order in time of their production, and still less
with the order of their disappearance; for the parent rock-pigeon now
lives; and many varieties between the rock-pigeon and the carrier have
become extinct; and carriers which are extreme in the important character
of length of beak originated earlier than short-beaked tumblers, which are
at the opposite end of the series in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his firm belief in the immutability of species. He who
is acquainted with the distribution of existing species over the globe,
will not attempt to account for the close resemblance of the distinct
species in closely consecutive formations, by the physical conditions of
the ancient areas having remained nearly the same. Let it be remembered
that the forms of life, at least those inhabiting the sea, have changed
almost simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious vicissitudes of
climate during the pleistocene period, which includes the whole glacial
period, and note how little the specific forms of the inhabitants of the
sea have been affected.
On the theory of descent, the full meaning of the fact of fossil remains
from closely consecutive formations, though ranked as distinct species,
being closely related, is obvious. As the accumulation of each formation
has often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as I
attempted to show in the last chapter, in any one or two formations all
the intermediate varieties between the species which appeared at the
commencement and close of these periods; but we ought to find after
intervals, very long as measured by years, but only moderately long as
measured geologically, closely allied forms, or, as they have been called
by some authors, representative species; and these we assuredly do find.
We find, in short, such evidence of the slow and scarcely sensible
mutation of specific forms, as we have a just right to expect to find.
ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS.
There has been much discussion whether recent forms are more highly
developed than ancient. I will not here enter on this subject, for
naturalists have not as yet defined to each other's satisfaction what is
meant by high and low forms. But in one particular sense the more recent
forms must, on my theory, be higher than the more ancient; for each new
species is formed by having had some advantage in the struggle for life
over other and preceding forms. If under a nearly similar climate, the
eocene inhabitants of one quarter of the world were put into competition
with the existing inhabitants of the same or some other quarter, the
eocene fauna or flora would certainly be beaten and exterminated; as would
a secondary fauna by an eocene, and a palaeozoic fauna by a secondary
fauna. I do not doubt that this process of improvement has affected in a
marked and sensible manner the organisation of the more recent and
victorious forms of life, in comparison with the ancient and beaten forms;
but I can see no way of testing this sort of progress. Crustaceans, for
instance, not the highest in their own class, may have beaten the highest
molluscs. From the extraordinary manner in which European productions have
recently spread over New Zealand, and have seized on places which must
have been previously occupied, we may believe, if all the animals and
plants of Great Britain were set free in New Zealand, that in the course
of time a multitude of British forms would become thoroughly naturalized
there, and would exterminate many of the natives. On the other hand, from
what we see now occurring in New Zealand, and from hardly a single
inhabitant of the southern hemisphere having become wild in any part of
Europe, we may doubt, if all the productions of New Zealand were set free
in Great Britain, whether any considerable number would be enabled to
seize on places now occupied by our native plants and animals. Under this
point of view, the productions of Great Britain may be said to be higher
than those of New Zealand. Yet the most skilful naturalist from an
examination of the species of the two countries could not have foreseen
this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and Huxley
in thinking that the truth of this doctrine is very far from proved. Yet I
fully expect to see it hereafter confirmed, at least in regard to
subordinate groups, which have branched off from each other within
comparatively recent times. For this doctrine of Agassiz accords well with
the theory of natural selection. In a future chapter I shall attempt to
show that the adult differs from its embryo, owing to variations
supervening at a not early age, and being inherited at a corresponding
age. This process, whilst it leaves the embryo almost unaltered,
continually adds, in the course of successive generations, more and more
difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fish strictly
belong to their own proper classes, though some of these old forms are in
a slight degree less distinct from each other than are the typical members
of the same groups at the present day, it would be vain to look for
animals having the common embryological character of the Vertebrata, until
beds far beneath the lowest Silurian strata are discovered—a
discovery of which the chance is very small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even to
an uneducated eye, in the gigantic pieces of armour like those of the
armadillo, found in several parts of La Plata; and Professor Owen has
shown in the most striking manner that most of the fossil mammals, buried
there in such numbers, are related to South American types. This
relationship is even more clearly seen in the wonderful collection of
fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so
much impressed with these facts that I strongly insisted, in 1839 and
1845, on this "law of the succession of types,"—on "this wonderful
relationship in the same continent between the dead and the living."
Professor Owen has subsequently extended the same generalisation to the
mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see it
also in the birds of the caves of Brazil. Mr. Woodward has shown that the
same law holds good with sea-shells, but from the wide distribution of
most genera of molluscs, it is not well displayed by them. Other cases
could be added, as the relation between the extinct and living land-shells
of Madeira; and between the extinct and living brackish-water shells of
the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under the
same latitude, would attempt to account, on the one hand, by dissimilar
physical conditions for the dissimilarity of the inhabitants of these two
continents, and, on the other hand, by similarity of conditions, for the
uniformity of the same types in each during the later tertiary periods.
Nor can it be pretended that it is an immutable law that marsupials should
have been chiefly or solely produced in Australia; or that Edentata and
other American types should have been solely produced in South America.
For we know that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above alluded to, that in
America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly of
the present character of the southern half of the continent; and the
southern half was formerly more closely allied, than it is at present, to
the northern half. In a similar manner we know from Falconer and Cautley's
discoveries, that northern India was formerly more closely related in its
mammals to Africa than it is at the present time. Analogous facts could be
given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time and after great geographical changes, permitting much
inter-migration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the laws of past and present
distribution.
It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become wholly
extinct, and have left no progeny. But in the caves of Brazil, there are
many extinct species which are closely allied in size and in other
characters to the species still living in South America; and some of these
fossils may be the actual progenitors of living species. It must not be
forgotten that, on my theory, all the species of the same genus have
descended from some one species; so that if six genera, each having eight
species, be found in one geological formation, and in the next succeeding
formation there be six other allied or representative genera with the same
number of species, then we may conclude that only one species of each of
the six older genera has left modified descendants, constituting the six
new genera. The other seven species of the old genera have all died out
and have left no progeny. Or, which would probably be a far commoner case,
two or three species of two or three alone of the six older genera will
have been the parents of the six new genera; the other old species and the
other whole genera having become utterly extinct. In failing orders, with
the genera and species decreasing in numbers, as apparently is the case of
the Edentata of South America, still fewer genera and species will have
left modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geologically
explored with care; that only certain classes of organic beings have been
largely preserved in a fossil state; that the number both of specimens and
of species, preserved in our museums, is absolutely as nothing compared
with the incalculable number of generations which must have passed away
even during a single formation; that, owing to subsidence being necessary
for the accumulation of fossiliferous deposits thick enough to resist
future degradation, enormous intervals of time have elapsed between the
successive formations; that there has probably been more extinction during
the periods of subsidence, and more variation during the periods of
elevation, and during the latter the record will have been least perfectly
kept; that each single formation has not been continuously deposited; that
the duration of each formation is, perhaps, short compared with the
average duration of specific forms; that migration has played an important
part in the first appearance of new forms in any one area and formation;
that widely ranging species are those which have varied most, and have
oftenest given rise to new species; and that varieties have at first often
been local. All these causes taken conjointly, must have tended to make
the geological record extremely imperfect, and will to a large extent
explain why we do not find interminable varieties, connecting together all
the extinct and existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages of
the same great formation. He may disbelieve in the enormous intervals of
time which have elapsed between our consecutive formations; he may
overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent, sudden
coming in of whole groups of species. He may ask where are the remains of
those infinitely numerous organisms which must have existed long before
the first bed of the Silurian system was deposited: I can answer this
latter question only hypothetically, by saying that as far as we can see,
where our oceans now extend they have for an enormous period extended, and
where our oscillating continents now stand they have stood ever since the
Silurian epoch; but that long before that period, the world may have
presented a wholly different aspect; and that the older continents, formed
of formations older than any known to us, may now all be in a
metamorphosed condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence
of the production of new forms. We can understand why when a species has
once disappeared it never reappears. Groups of species increase in numbers
slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species of the larger dominant groups tend to
leave many modified descendants, and thus new sub-groups and groups are
formed. As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of the
earth. But the utter extinction of a whole group of species may often be a
very slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.
We can understand how the spreading of the dominant forms of life, which
are those that oftenest vary, will in the long run tend to people the
world with allied, but modified, descendants; and these will generally
succeed in taking the places of those groups of species which are their
inferiors in the struggle for existence. Hence, after long intervals of
time, the productions of the world will appear to have changed
simultaneously.
We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to divergence
of character, why the more ancient a form is, the more it generally
differs from those now living. Why ancient and extinct forms often tend to
fill up gaps between existing forms, sometimes blending two groups
previously classed as distinct into one; but more commonly only bringing
them a little closer together. The more ancient a form is, the more often,
apparently, it displays characters in some degree intermediate between
groups now distinct; for the more ancient a form is, the more nearly it
will be related to, and consequently resemble, the common progenitor of
groups, since become widely divergent. Extinct forms are seldom directly
intermediate between existing forms; but are intermediate only by a long
and circuitous course through many extinct and very different forms. We
can clearly see why the organic remains of closely consecutive formations
are more closely allied to each other, than are those of remote
formations; for the forms are more closely linked together by generation:
we can clearly see why the remains of an intermediate formation are
intermediate in character.
The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palaeontologists, that organisation on
the whole has progressed. If it should hereafter be proved that ancient
animals resemble to a certain extent the embryos of more recent animals of
the same class, the fact will be intelligible. The succession of the same
types of structure within the same areas during the later geological
periods ceases to be mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to be, and
it may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection are
greatly diminished or disappear. On the other hand, all the chief laws of
palaeontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, produced by the laws of variation still acting
round us, and preserved by Natural Selection.
11. GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions. Importance of barriers. Affinity of the productions of the
same continent. Centres of creation. Means of dispersal, by changes of
climate and of the level of the land, and by occasional means. Dispersal
during the Glacial period co-extensive with the world.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be accounted for by their climatal and other physical conditions. Of late,
almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth: for if we exclude the northern parts where the circumpolar land is
almost continuous, all authors agree that one of the most fundamental
divisions in geographical distribution is that between the New and Old
Worlds; yet if we travel over the vast American continent, from the
central parts of the United States to its extreme southern point, we meet
with the most diversified conditions; the most humid districts, arid
deserts, lofty mountains, grassy plains, forests, marshes, lakes, and
great rivers, under almost every temperature. There is hardly a climate or
condition in the Old World which cannot be paralleled in the New—at
least as closely as the same species generally require; for it is a most
rare case to find a group of organisms confined to any small spot, having
conditions peculiar in only a slight degree; for instance, small areas in
the Old World could be pointed out hotter than any in the New World, yet
these are not inhabited by a peculiar fauna or flora. Notwithstanding this
parallelism in the conditions of the Old and New Worlds, how widely
different are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25
deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the productions of
South America south of lat. 35 deg with those north of 25 deg, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to
the productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference of nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude: for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts, and sometimes
even of large rivers, we find different productions; though as mountain
chains, deserts, etc., are not as impassable, or likely to have endured so
long as the oceans separating continents, the differences are very
inferior in degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of
Panama. Westward of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emigrants; here we have
a barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific, with another and totally distinct fauna.
So that here three marine faunas range far northward and southward, in
parallel lines not far from each other, under corresponding climates; but
from being separated from each other by impassable barriers, either of
land or open sea, they are wholly distinct. On the other hand, proceeding
still further westward from the eastern islands of the tropical parts of
the Pacific, we encounter no impassable barriers, and we have innumerable
islands as halting-places, until after travelling over a hemisphere we
come to the shores of Africa; and over this vast space we meet with no
well-defined and distinct marine faunas. Although hardly one shell, crab
or fish is common to the above-named three approximate faunas of Eastern
and Western America and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and many shells are common to the
eastern islands of the Pacific and the eastern shores of Africa, on almost
exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is a
law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance, from
north to south never fails to be struck by the manner in which successive
groups of beings, specifically distinct, yet clearly related, replace each
other. He hears from closely allied, yet distinct kinds of birds, notes
nearly similar, and sees their nests similarly constructed, but not quite
alike, with eggs coloured in nearly the same manner. The plains near the
Straits of Magellan are inhabited by one species of Rhea (American
ostrich), and northward the plains of La Plata by another species of the
same genus; and not by a true ostrich or emeu, like those found in Africa
and Australia under the same latitude. On these same plains of La Plata,
we see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits and belonging to the same order of Rodents, but they
plainly display an American type of structure. We ascend the lofty peaks
of the Cordillera and we find an alpine species of bizcacha; we look to
the waters, and we do not find the beaver or musk-rat, but the coypu and
capybara, rodents of the American type. Innumerable other instances could
be given. If we look to the islands off the American shore, however much
they may differ in geological structure, the inhabitants, though they may
be all peculiar species, are essentially American. We may look back to
past ages, as shown in the last chapter, and we find American types then
prevalent on the American continent and in the American seas. We see in
these facts some deep organic bond, prevailing throughout space and time,
over the same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led to
inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone, as
far as we positively know, produces organisms quite like, or, as we see in
the case of varieties nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct
influence of different physical conditions. The degree of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been effected with more or less ease, at
periods more or less remote;—on the nature and number of the former
immigrants;—and on their action and reaction, in their mutual
struggles for life;—the relation of organism to organism being, as I
have already often remarked, the most important of all relations. Thus the
high importance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes will
have the best chance of seizing on new places, when they spread into new
countries. In their new homes they will be exposed to new conditions, and
will frequently undergo further modification and improvement; and thus
they will become still further victorious, and will produce groups of
modified descendants. On this principle of inheritance with modification,
we can understand how it is that sections of genera, whole genera, and
even families are confined to the same areas, as is so commonly and
notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so far
as it profits the individual in its complex struggle for life, so the
degree of modification in different species will be no uniform quantity.
If, for instance, a number of species, which stand in direct competition
with each other, migrate in a body into a new and afterwards isolated
country, they will be little liable to modification; for neither migration
nor isolation in themselves can do anything. These principles come into
play only by bringing organisms into new relations with each other, and in
a lesser degree with the surrounding physical conditions. As we have seen
in the last chapter that some forms have retained nearly the same
character from an enormously remote geological period, so certain species
have migrated over vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the same genus,
though inhabiting the most distant quarters of the world, must originally
have proceeded from the same source, as they have descended from the same
progenitor. In the case of those species, which have undergone during
whole geological periods but little modification, there is not much
difficulty in believing that they may have migrated from the same region;
for during the vast geographical and climatal changes which will have
supervened since ancient times, almost any amount of migration is
possible. But in many other cases, in which we have reason to believe that
the species of a genus have been produced within comparatively recent
times, there is great difficulty on this head. It is also obvious that the
individuals of the same species, though now inhabiting distant and
isolated regions, must have proceeded from one spot, where their parents
were first produced: for, as explained in the last chapter, it is
incredible that individuals identically the same should ever have been
produced through natural selection from parents specifically distinct.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that each
species was first produced within a single region captivates the mind. He
who rejects it, rejects the vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species
is continuous; and when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space
could not be easily passed over by migration, the fact is given as
something remarkable and exceptional. The capacity of migrating across the
sea is more distinctly limited in terrestrial mammals, than perhaps in any
other organic beings; and, accordingly, we find no inexplicable cases of
the same mammal inhabiting distant points of the world. No geologist will
feel any difficulty in such cases as Great Britain having been formerly
united to Europe, and consequently possessing the same quadrupeds. But if
the same species can be produced at two separate points, why do we not
find a single mammal common to Europe and Australia or South America? The
conditions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Australia; and
some of the aboriginal plants are identically the same at these distant
points of the northern and southern hemispheres? The answer, as I believe,
is, that mammals have not been able to migrate, whereas some plants, from
their varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every kind
have had on distribution, is intelligible only on the view that the great
majority of species have been produced on one side alone, and have not
been able to migrate to the other side. Some few families, many
sub-families, very many genera, and a still greater number of sections of
genera are confined to a single region; and it has been observed by
several naturalists, that the most natural genera, or those genera in
which the species are most closely related to each other, are generally
local, or confined to one area. What a strange anomaly it would be, if,
when coming one step lower in the series, to the individuals of the same
species, a directly opposite rule prevailed; and species were not local,
but had been produced in two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the view
of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous and
of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a moment
pretend that any explanation could be offered of many such cases. But
after some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the summits
of distant mountain-ranges, and at distant points in the arctic and
antarctic regions; and secondly (in the following chapter), the wide
distribution of freshwater productions; and thirdly, the occurrence of the
same terrestrial species on islands and on the mainland, though separated
by hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface, can in many instances
be explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatal and geographical changes and various occasional means of
transport, the belief that this has been the universal law, seems to me
incomparably the safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during some
part of their migration) from the area inhabited by their progenitor. If
it can be shown to be almost invariably the case, that a region, of which
most of its inhabitants are closely related to, or belong to the same
genera with the species of a second region, has probably received at some
former period immigrants from this other region, my theory will be
strengthened; for we can clearly understand, on the principle of
modification, why the inhabitants of a region should be related to those
of another region, whence it has been stocked. A volcanic island, for
instance, upheaved and formed at the distance of a few hundreds of miles
from a continent, would probably receive from it in the course of time a
few colonists, and their descendants, though modified, would still be
plainly related by inheritance to the inhabitants of the continent. Cases
of this nature are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ
much (by substituting the word variety for species) from that lately
advanced in an ingenious paper by Mr. Wallace, in which he concludes, that
"every species has come into existence coincident both in space and time
with a pre-existing closely allied species." And I now know from
correspondence, that this coincidence he attributes to generation with
modification.
The previous remarks on "single and multiple centres of creation" do not
directly bear on another allied question,—namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended
from a succession of improved varieties, which will never have blended
with other individuals or varieties, but will have supplanted each other;
so that, at each successive stage of modification and improvement, all the
individuals of each variety will have descended from a single parent. But
in the majority of cases, namely, with all organisms which habitually
unite for each birth, or which often intercross, I believe that during the
slow process of modification the individuals of the species will have been
kept nearly uniform by intercrossing; so that many individuals will have
gone on simultaneously changing, and the whole amount of modification will
not have been due, at each stage, to descent from a single parent. To
illustrate what I mean: our English racehorses differ slightly from the
horses of every other breed; but they do not owe their difference and
superiority to descent from any single pair, but to continued care in
selecting and training many individuals during many generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration: a region when its
climate was different may have been a high road for migration, but now be
impassable; I shall, however, presently have to discuss this branch of the
subject in some detail. Changes of level in the land must also have been
highly influential: a narrow isthmus now separates two marine faunas;
submerge it, or let it formerly have been submerged, and the two faunas
will now blend or may formerly have blended: where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist will dispute that great mutations of
level have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must recently have been
connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and have united
almost every island to some mainland. If indeed the arguments used by
Forbes are to be trusted, it must be admitted that scarcely a single
island exists which has not recently been united to some continent. This
view cuts the Gordian knot of the dispersal of the same species to the
most distant points, and removes many a difficulty: but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we have
abundant evidence of great oscillations of level in our continents; but
not of such vast changes in their position and extension, as to have
united them within the recent period to each other and to the several
intervening oceanic islands. I freely admit the former existence of many
islands, now buried beneath the sea, which may have served as halting
places for plants and for many animals during their migration. In the
coral-producing oceans such sunken islands are now marked, as I believe,
by rings of coral or atolls standing over them. Whenever it is fully
admitted, as I believe it will some day be, that each species has
proceeded from a single birthplace, and when in the course of time we know
something definite about the means of distribution, we shall be enabled to
speculate with security on the former extension of the land. But I do not
believe that it will ever be proved that within the recent period
continents which are now quite separate, have been continuously, or almost
continuously, united with each other, and with the many existing oceanic
islands. Several facts in distribution,—such as the great difference
in the marine faunas on the opposite sides of almost every continent,—the
close relation of the tertiary inhabitants of several lands and even seas
to their present inhabitants,—a certain degree of relation (as we
shall hereafter see) between the distribution of mammals and the depth of
the sea,—these and other such facts seem to me opposed to the
admission of such prodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by Forbes and admitted by
his many followers. The nature and relative proportions of the inhabitants
of oceanic islands likewise seem to me opposed to the belief of their
former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of
sunken continents;—if they had originally existed as mountain-ranges
on the land, some at least of the islands would have been formed, like
other mountain-summits, of granite, metamorphic schists, old fossiliferous
or other such rocks, instead of consisting of mere piles of volcanic
matter.
I must now say a few words on what are called accidental means, but which
more properly might be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
stated to be ill adapted for wide dissemination; but for transport across
the sea, the greater or less facilities may be said to be almost wholly
unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was
not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of 87 kinds, 64 germinated
after an immersion of 28 days, and a few survived an immersion of 137
days. For convenience sake I chiefly tried small seeds, without the
capsule or fruit; and as all of these sank in a few days, they could not
be floated across wide spaces of the sea, whether or not they were injured
by the salt-water. Afterwards I tried some larger fruits, capsules, etc.,
and some of these floated for a long time. It is well known what a
difference there is in the buoyancy of green and seasoned timber; and it
occurred to me that floods might wash down plants or branches, and that
these might be dried on the banks, and then by a fresh rise in the stream
be washed into the sea. Hence I was led to dry stems and branches of 94
plants with ripe fruit, and to place them on sea water. The majority sank
quickly, but some which whilst green floated for a very short time, when
dried floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried, they floated for 90 days and afterwards when planted they
germinated; an asparagus plant with ripe berries floated for 23 days, when
dried it floated for 85 days, and the seeds afterwards germinated: the
ripe seeds of Helosciadium sank in two days, when dried they floated for
above 90 days, and afterwards germinated. Altogether out of the 94 dried
plants, 18 floated for above 28 days, and some of the 18 floated for a
very much longer period. So that as 64/87 seeds germinated after an
immersion of 28 days; and as 18/94 plants with ripe fruit (but not all the
same species as in the foregoing experiment) floated, after being dried,
for above 28 days, as far as we may infer anything from these scanty
facts, we may conclude that the seeds of 14/100 plants of any country
might be floated by sea-currents during 28 days, and would retain their
power of germination. In Johnston's Physical Atlas, the average rate of
the several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles per diem); on this average, the seeds of 14/100
plants belonging to one country might be floated across 924 miles of sea
to another country; and when stranded, if blown to a favourable spot by an
inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried 98 seeds, mostly different from mine; but he chose many
large fruits and likewise seeds from plants which live near the sea; and
this would have favoured the average length of their flotation and of
their resistance to the injurious action of the salt-water. On the other
hand he did not previously dry the plants or branches with the fruit; and
this, as we have seen, would have caused some of them to have floated much
longer. The result was that 18/98 of his seeds floated for 42 days, and
were then capable of germination. But I do not doubt that plants exposed
to the waves would float for a less time than those protected from violent
movement as in our experiments. Therefore it would perhaps be safer to
assume that the seeds of about 10/100 plants of a flora, after having been
dried, could be floated across a space of sea 900 miles in width, and
would then germinate. The fact of the larger fruits often floating longer
than the small, is interesting; as plants with large seeds or fruit could
hardly be transported by any other means; and Alph. de Candolle has shown
that such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift timber
is thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral-islands in the Pacific, procure
stones for their tools, solely from the roots of drifted trees, these
stones being a valuable royal tax. I find on examination, that when
irregularly shaped stones are embedded in the roots of trees, small
parcels of earth are very frequently enclosed in their interstices and
behind them,—so perfectly that not a particle could be washed away
in the longest transport: out of one small portion of earth thus
COMPLETELY enclosed by wood in an oak about 50 years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcasses of birds, when floating
on the sea, sometimes escape being immediately devoured; and seeds of many
kinds in the crops of floating birds long retain their vitality: peas and
vetches, for instance, are killed by even a few days' immersion in
sea-water; but some taken out of the crop of a pigeon, which had floated
on artificial salt-water for 30 days, to my surprise nearly all
germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may I think safely assume that under such circumstances their rate of
flight would often be 35 miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit will pass
uninjured through even the digestive organs of a turkey. In the course of
two months, I picked up in my garden 12 kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them,
which I tried, germinated. But the following fact is more important: the
crops of birds do not secrete gastric juice, and do not in the least
injure, as I know by trial, the germination of seeds; now after a bird has
found and devoured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for 12 or even 18 hours. A
bird in this interval might easily be blown to the distance of 500 miles,
and hawks are known to look out for tired birds, and the contents of their
torn crops might thus readily get scattered. Mr. Brent informs me that a
friend of his had to give up flying carrier-pigeons from France to
England, as the hawks on the English coast destroyed so many on their
arrival. Some hawks and owls bolt their prey whole, and after an interval
of from twelve to twenty hours, disgorge pellets, which, as I know from
experiments made in the Zoological Gardens, include seeds capable of
germination. Some seeds of the oat, wheat, millet, canary, hemp, clover,
and beet germinated after having been from twelve to twenty-one hours in
the stomachs of different birds of prey; and two seeds of beet grew after
having been thus retained for two days and fourteen hours. Freshwater
fish, I find, eat seeds of many land and water plants: fish are frequently
devoured by birds, and thus the seeds might be transported from place to
place. I forced many kinds of seeds into the stomachs of dead fish, and
then gave their bodies to fishing-eagles, storks, and pelicans; these
birds after an interval of many hours, either rejected the seeds in
pellets or passed them in their excrement; and several of these seeds
retained their power of germination. Certain seeds, however, were always
killed by this process.
Although the beaks and feet of birds are generally quite clean, I can show
that earth sometimes adheres to them: in one instance I removed twenty-two
grains of dry argillaceous earth from one foot of a partridge, and in this
earth there was a pebble quite as large as the seed of a vetch. Thus seeds
might occasionally be transported to great distances; for many facts could
be given showing that soil almost everywhere is charged with seeds.
Reflect for a moment on the millions of quails which annually cross the
Mediterranean; and can we doubt that the earth adhering to their feet
would sometimes include a few minute seeds? But I shall presently have to
recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, I can
hardly doubt that they must occasionally have transported seeds from one
part to another of the arctic and antarctic regions, as suggested by
Lyell; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of the species of
plants common to Europe, in comparison with the plants of other oceanic
islands nearer to the mainland, and (as remarked by Mr. H. C. Watson) from
the somewhat northern character of the flora in comparison with the
latitude, I suspected that these islands had been partly stocked by
ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell
wrote to M. Hartung to inquire whether he had observed erratic boulders on
these islands, and he answered that he had found large fragments of
granite and other rocks, which do not occur in the archipelago. Hence we
may safely infer that icebergs formerly landed their rocky burthens on the
shores of these mid-ocean islands, and it is at least possible that they
may have brought thither the seeds of northern plants.
Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of years, it
would I think be a marvellous fact if many plants had not thus become
widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the sea are
not accidental, nor is the direction of prevalent gales of wind. It should
be observed that scarcely any means of transport would carry seeds for
very great distances; for seeds do not retain their vitality when exposed
for a great length of time to the action of seawater; nor could they be
long carried in the crops or intestines of birds. These means, however,
would suffice for occasional transport across tracts of sea some hundred
miles in breadth, or from island to island, or from a continent to a
neighbouring island, but not from one distant continent to another. The
floras of distant continents would not by such means become mingled in any
great degree; but would remain as distinct as we now see them to be. The
currents, from their course, would never bring seeds from North America to
Britain, though they might and do bring seeds from the West Indies to our
western shores, where, if not killed by so long an immersion in
salt-water, they could not endure our climate. Almost every year, one or
two land-birds are blown across the whole Atlantic Ocean, from North
America to the western shores of Ireland and England; but seeds could be
transported by these wanderers only by one means, namely, in dirt sticking
to their feet, which is in itself a rare accident. Even in this case, how
small would the chance be of a seed falling on favourable soil, and coming
to maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known (and
it would be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from Europe
or any other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar means. I
do not doubt that out of twenty seeds or animals transported to an island,
even if far less well-stocked than Britain, scarcely more than one would
be so well fitted to its new home, as to become naturalised. But this, as
it seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological time,
whilst an island was being upheaved and formed, and before it had become
fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed, which
chanced to arrive, would be sure to germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where the Alpine species
could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility of
their having migrated from one to the other. It is indeed a remarkable
fact to see so many of the same plants living on the snowy regions of the
Alps or Pyrenees, and in the extreme northern parts of Europe; but it is
far more remarkable, that the plants on the White Mountains, in the United
States of America, are all the same with those of Labrador, and nearly all
the same, as we hear from Asa Gray, with those on the loftiest mountains
of Europe. Even as long ago as 1747, such facts led Gmelin to conclude
that the same species must have been independently created at several
distinct points; and we might have remained in this same belief, had not
Agassiz and others called vivid attention to the Glacial period, which, as
we shall immediately see, affords a simple explanation of these facts. We
have evidence of almost every conceivable kind, organic and inorganic,
that within a very recent geological period, central Europe and North
America suffered under an Arctic climate. The ruins of a house burnt by
fire do not tell their tale more plainly, than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were lately
filled. So greatly has the climate of Europe changed, that in Northern
Italy, gigantic moraines, left by old glaciers, are now clothed by the
vine and maize. Throughout a large part of the United States, erratic
boulders, and rocks scored by drifted icebergs and coast-ice, plainly
reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then
pass away, as formerly occurred. As the cold came on, and as each more
southern zone became fitted for arctic beings and ill-fitted for their
former more temperate inhabitants, the latter would be supplanted and
arctic productions would take their places. The inhabitants of the more
temperate regions would at the same time travel southward, unless they
were stopped by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had
reached its maximum, we should have a uniform arctic fauna and flora,
covering the central parts of Europe, as far south as the Alps and
Pyrenees, and even stretching into Spain. The now temperate regions of the
United States would likewise be covered by arctic plants and animals, and
these would be nearly the same with those of Europe; for the present
circumpolar inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world. We may suppose that the
Glacial period came on a little earlier or later in North America than in
Europe, so will the southern migration there have been a little earlier or
later; but this will make no difference in the final result.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the
arctic forms would seize on the cleared and thawed ground, always
ascending higher and higher, as the warmth increased, whilst their
brethren were pursuing their northern journey. Hence, when the warmth had
fully returned, the same arctic species, which had lately lived in a body
together on the lowlands of the Old and New Worlds, would be left isolated
on distant mountain-summits (having been exterminated on all lesser
heights) and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants at points so immensely
remote as on the mountains of the United States and of Europe. We can thus
also understand the fact that the Alpine plants of each mountain-range are
more especially related to the arctic forms living due north or nearly due
north of them: for the migration as the cold came on, and the re-migration
on the returning warmth, will generally have been due south and north. The
Alpine plants, for example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more especially
allied to the plants of northern Scandinavia; those of the United States
to Labrador; those of the mountains of Siberia to the arctic regions of
that country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the Alpine
and Arctic productions of Europe and America, that when in other regions
we find the same species on distant mountain-summits, we may almost
conclude without other evidence, that a colder climate permitted their
former migration across the low intervening tracts, since become too warm
for their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe to
have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very late
period have marched a little further north, and subsequently have
retreated to their present homes; but I have met with no satisfactory
evidence with respect to this intercalated slightly warmer period, since
the Glacial period.
The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed,
and, in accordance with the principles inculcated in this volume, they
will not have been liable to much modification. But with our Alpine
productions, left isolated from the moment of the returning warmth, first
at the bases and ultimately on the summits of the mountains, the case will
have been somewhat different; for it is not likely that all the same
arctic species will have been left on mountain ranges distant from each
other, and have survived there ever since; they will, also, in all
probability have become mingled with ancient Alpine species, which must
have existed on the mountains before the commencement of the Glacial
epoch, and which during its coldest period will have been temporarily
driven down to the plains; they will, also, have been exposed to somewhat
different climatal influences. Their mutual relations will thus have been
in some degree disturbed; consequently they will have been liable to
modification; and this we find has been the case; for if we compare the
present Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the same,
some present varieties, some are ranked as doubtful forms, and some few
are distinct yet closely allied or representative species.
In illustrating what, as I believe, actually took place during the Glacial
period, I assumed that at its commencement the arctic productions were as
uniform round the polar regions as they are at the present day. But the
foregoing remarks on distribution apply not only to strictly arctic forms,
but also to many sub-arctic and to some few northern temperate forms, for
some of these are the same on the lower mountains and on the plains of
North America and Europe; and it may be reasonably asked how I account for
the necessary degree of uniformity of the sub-arctic and northern
temperate forms round the world, at the commencement of the Glacial
period. At the present day, the sub-arctic and northern temperate
productions of the Old and New Worlds are separated from each other by the
Atlantic Ocean and by the extreme northern part of the Pacific. During the
Glacial period, when the inhabitants of the Old and New Worlds lived
further southwards than at present, they must have been still more
completely separated by wider spaces of ocean. I believe the above
difficulty may be surmounted by looking to still earlier changes of
climate of an opposite nature. We have good reason to believe that during
the newer Pliocene period, before the Glacial epoch, and whilst the
majority of the inhabitants of the world were specifically the same as
now, the climate was warmer than at the present day. Hence we may suppose
that the organisms now living under the climate of latitude 60 deg, during
the Pliocene period lived further north under the Polar Circle, in
latitude 66 deg-67 deg; and that the strictly arctic productions then
lived on the broken land still nearer to the pole. Now if we look at a
globe, we shall see that under the Polar Circle there is almost continuous
land from western Europe, through Siberia, to eastern America. And to this
continuity of the circumpolar land, and to the consequent freedom for
intermigration under a more favourable climate, I attribute the necessary
amount of uniformity in the sub-arctic and northern temperate productions
of the Old and New Worlds, at a period anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to large,
but partial oscillations of level, I am strongly inclined to extend the
above view, and to infer that during some earlier and still warmer period,
such as the older Pliocene period, a large number of the same plants and
animals inhabited the almost continuous circumpolar land; and that these
plants and animals, both in the Old and New Worlds, began slowly to
migrate southwards as the climate became less warm, long before the
commencement of the Glacial period. We now see, as I believe, their
descendants, mostly in a modified condition, in the central parts of
Europe and the United States. On this view we can understand the
relationship, with very little identity, between the productions of North
America and Europe,—a relationship which is most remarkable,
considering the distance of the two areas, and their separation by the
Atlantic Ocean. We can further understand the singular fact remarked on by
several observers, that the productions of Europe and America during the
later tertiary stages were more closely related to each other than they
are at the present time; for during these warmer periods the northern
parts of the Old and New Worlds will have been almost continuously united
by land, serving as a bridge, since rendered impassable by cold, for the
inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they must have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and have had to compete with them;
and in the other great region, with those of the Old World. Consequently
we have here everything favourable for much modification,—for far
more modification than with the Alpine productions, left isolated, within
a much more recent period, on the several mountain-ranges and on the
arctic lands of the two Worlds. Hence it has come, that when we compare
the now living productions of the temperate regions of the New and Old
Worlds, we find very few identical species (though Asa Gray has lately
shown that more plants are identical than was formerly supposed), but we
find in every great class many forms, which some naturalists rank as
geographical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of
a marine fauna, which during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary representative
forms on the eastern and western shores of temperate North America; and
the still more striking case of many closely allied crustaceans (as
described in Dana's admirable work), of some fish and other marine
animals, in the Mediterranean and in the seas of Japan,—areas now
separated by a continent and by nearly a hemisphere of equatorial ocean.
These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the
theory of creation. We cannot say that they have been created alike, in
correspondence with the nearly similar physical conditions of the areas;
for if we compare, for instance, certain parts of South America with the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their inhabitants
utterly dissimilar.
But we must return to our more immediate subject, the Glacial period. I am
convinced that Forbes's view may be largely extended. In Europe we have
the plainest evidence of the cold period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may infer,
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. Along the Himalaya, at points 900 miles
apart, glaciers have left the marks of their former low descent; and in
Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. South
of the equator, we have some direct evidence of former glacial action in
New Zealand; and the same plants, found on widely separated mountains in
this island, tell the same story. If one account which has been published
can be trusted, we have direct evidence of glacial action in the
south-eastern corner of Australia.
Looking to America; in the northern half, ice-borne fragments of rock have
been observed on the eastern side as far south as lat. 36 deg-37 deg, and
on the shores of the Pacific, where the climate is now so different, as
far south as lat. 46 deg; erratic boulders have, also, been noticed on the
Rocky Mountains. In the Cordillera of Equatorial South America, glaciers
once extended far below their present level. In central Chile I was
astonished at the structure of a vast mound of detritus, about 800 feet in
height, crossing a valley of the Andes; and this I now feel convinced was
a gigantic moraine, left far below any existing glacier. Further south on
both sides of the continent, from lat. 41 deg to the southernmost
extremity, we have the clearest evidence of former glacial action, in huge
boulders transported far from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have
good evidence in almost every case, that the epoch was included within the
latest geological period. We have, also, excellent evidence, that it
endured for an enormous time, as measured by years, at each point. The
cold may have come on, or have ceased, earlier at one point of the globe
than at another, but seeing that it endured for long at each, and that it
was contemporaneous in a geological sense, it seems to me probable that it
was, during a part at least of the period, actually simultaneous
throughout the world. Without some distinct evidence to the contrary, we
may at least admit as probable that the glacial action was simultaneous on
the eastern and western sides of North America, in the Cordillera under
the equator and under the warmer temperate zones, and on both sides of the
southern extremity of the continent. If this be admitted, it is difficult
to avoid believing that the temperature of the whole world was at this
period simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of
longitude.
On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty and fifty of the
flowering plants of Tierra del Fuego, forming no inconsiderable part of
its scanty flora, are common to Europe, enormously remote as these two
points are; and there are many closely allied species. On the lofty
mountains of equatorial America a host of peculiar species belonging to
European genera occur. On the highest mountains of Brazil, some few
European genera were found by Gardner, which do not exist in the wide
intervening hot countries. So on the Silla of Caraccas the illustrious
Humboldt long ago found species belonging to genera characteristic of the
Cordillera. On the mountains of Abyssinia, several European forms and some
few representatives of the peculiar flora of the Cape of Good Hope occur.
At the Cape of Good Hope a very few European species, believed not to have
been introduced by man, and on the mountains, some few representative
European forms are found, which have not been discovered in the
intertropical parts of Africa. On the Himalaya, and on the isolated
mountain-ranges of the peninsula of India, on the heights of Ceylon, and
on the volcanic cones of Java, many plants occur, either identically the
same or representing each other, and at the same time representing plants
of Europe, not found in the intervening hot lowlands. A list of the genera
collected on the loftier peaks of Java raises a picture of a collection
made on a hill in Europe! Still more striking is the fact that southern
Australian forms are clearly represented by plants growing on the summits
of the mountains of Borneo. Some of these Australian forms, as I hear from
Dr. Hooker, extend along the heights of the peninsula of Malacca, and are
thinly scattered, on the one hand over India and on the other as far north
as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the Flora
of New Zealand,' by Dr. Hooker, analogous and striking facts are given in
regard to the plants of that large island. Hence we see that throughout
the world, the plants growing on the more lofty mountains, and on the
temperate lowlands of the northern and southern hemispheres, are sometimes
identically the same; but they are much oftener specifically distinct,
though related to each other in a most remarkable manner.
This brief abstract applies to plants alone: some strictly analogous facts
could be given on the distribution of terrestrial animals. In marine
productions, similar cases occur; as an example, I may quote a remark by
the highest authority, Professor Dana, that "it is certainly a wonderful
fact that New Zealand should have a closer resemblance in its crustacea to
Great Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of
the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic." Many of the forms living on the
mountains of the warmer regions of the earth and in the southern
hemisphere are of doubtful value, being ranked by some naturalists as
specifically distinct, by others as varieties; but some are certainly
identical, and many, though closely related to northern forms, must be
ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that
the whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. The Glacial period, as
measured by years, must have been very long; and when we remember over
what vast spaces some naturalised plants and animals have spread within a
few centuries, this period will have been ample for any amount of
migration. As the cold came slowly on, all the tropical plants and other
productions will have retreated from both sides towards the equator,
followed in the rear by the temperate productions, and these by the
arctic; but with the latter we are not now concerned. The tropical plants
probably suffered much extinction; how much no one can say; perhaps
formerly the tropics supported as many species as we see at the present
day crowded together at the Cape of Good Hope, and in parts of temperate
Australia. As we know that many tropical plants and animals can withstand
a considerable amount of cold, many might have escaped extermination
during a moderate fall of temperature, more especially by escaping into
the warmest spots. But the great fact to bear in mind is, that all
tropical productions will have suffered to a certain extent. On the other
hand, the temperate productions, after migrating nearer to the equator,
though they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if protected
from the inroads of competitors, can withstand a much warmer climate than
their own. Hence, it seems to me possible, bearing in mind that the
tropical productions were in a suffering state and could not have
presented a firm front against intruders, that a certain number of the
more vigorous and dominant temperate forms might have penetrated the
native ranks and have reached or even crossed the equator. The invasion
would, of course, have been greatly favoured by high land, and perhaps by
a dry climate; for Dr. Falconer informs me that it is the damp with the
heat of the tropics which is so destructive to perennial plants from a
temperate climate. On the other hand, the most humid and hottest districts
will have afforded an asylum to the tropical natives. The mountain-ranges
north-west of the Himalaya, and the long line of the Cordillera, seem to
have afforded two great lines of invasion: and it is a striking fact,
lately communicated to me by Dr. Hooker, that all the flowering plants,
about forty-six in number, common to Tierra del Fuego and to Europe still
exist in North America, which must have lain on the line of march. But I
do not doubt that some temperate productions entered and crossed even the
LOWLANDS of the tropics at the period when the cold was most intense,—when
arctic forms had migrated some twenty-five degrees of latitude from their
native country and covered the land at the foot of the Pyrenees. At this
period of extreme cold, I believe that the climate under the equator at
the level of the sea was about the same with that now felt there at the
height of six or seven thousand feet. During this the coldest period, I
suppose that large spaces of the tropical lowlands were clothed with a
mingled tropical and temperate vegetation, like that now growing with
strange luxuriance at the base of the Himalaya, as graphically described
by Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator,
would re-migrate northward or southward towards their former homes; but
the forms, chiefly northern, which had crossed the equator, would travel
still further from their homes into the more temperate latitudes of the
opposite hemisphere. Although we have reason to believe from geological
evidence that the whole body of arctic shells underwent scarcely any
modification during their long southern migration and re-migration
northward, the case may have been wholly different with those intruding
forms which settled themselves on the intertropical mountains, and in the
southern hemisphere. These being surrounded by strangers will have had to
compete with many new forms of life; and it is probable that selected
modifications in their structure, habits, and constitutions will have
profited them. Thus many of these wanderers, though still plainly related
by inheritance to their brethren of the northern or southern hemispheres,
now exist in their new homes as well-marked varieties or as distinct
species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical plants and allied forms have apparently migrated from the north
to the south, than in a reversed direction. We see, however, a few
southern vegetable forms on the mountains of Borneo and Abyssinia. I
suspect that this preponderant migration from north to south is due to the
greater extent of land in the north, and to the northern forms having
existed in their own homes in greater numbers, and having consequently
been advanced through natural selection and competition to a higher stage
of perfection or dominating power, than the southern forms. And thus, when
they became commingled during the Glacial period, the northern forms were
enabled to beat the less powerful southern forms. Just in the same manner
as we see at the present day, that very many European productions cover
the ground in La Plata, and in a lesser degree in Australia, and have to a
certain extent beaten the natives; whereas extremely few southern forms
have become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata, and during the last
thirty or forty years from Australia. Something of the same kind must have
occurred on the intertropical mountains: no doubt before the Glacial
period they were stocked with endemic Alpine forms; but these have almost
everywhere largely yielded to the more dominant forms, generated in the
larger areas and more efficient workshops of the north. In many islands
the native productions are nearly equalled or even outnumbered by the
naturalised; and if the natives have not been actually exterminated, their
numbers have been greatly reduced, and this is the first stage towards
extinction. A mountain is an island on the land; and the intertropical
mountains before the Glacial period must have been completely isolated;
and I believe that the productions of these islands on the land yielded to
those produced within the larger areas of the north, just in the same way
as the productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.
I am far from supposing that all difficulties are removed on the view here
given in regard to the range and affinities of the allied species which
live in the northern and southern temperate zones and on the mountains of
the intertropical regions. Very many difficulties remain to be solved. I
do not pretend to indicate the exact lines and means of migration, or the
reason why certain species and not others have migrated; why certain
species have been modified and have given rise to new groups of forms, and
others have remained unaltered. We cannot hope to explain such facts,
until we can say why one species and not another becomes naturalised by
man's agency in a foreign land; why one ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own homes.
I have said that many difficulties remain to be solved: some of the most
remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed.
I will only say that as far as regards the occurrence of identical species
at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia,
I believe that towards the close of the Glacial period, icebergs, as
suggested by Lyell, have been largely concerned in their dispersal. But
the existence of several quite distinct species, belonging to genera
exclusively confined to the south, at these and other distant points of
the southern hemisphere, is, on my theory of descent with modification, a
far more remarkable case of difficulty. For some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the Glacial period for their migration, and for their
subsequent modification to the necessary degree. The facts seem to me to
indicate that peculiar and very distinct species have migrated in
radiating lines from some common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer period,
before the commencement of the Glacial period, when the antarctic lands,
now covered with ice, supported a highly peculiar and isolated flora. I
suspect that before this flora was exterminated by the Glacial epoch, a
few forms were widely dispersed to various points of the southern
hemisphere by occasional means of transport, and by the aid, as
halting-places, of existing and now sunken islands, and perhaps at the
commencement of the Glacial period, by icebergs. By these means, as I
believe, the southern shores of America, Australia, New Zealand have
become slightly tinted by the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one
of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the
present distribution both of the same and of allied forms of life can be
explained. The living waters may be said to have flowed during one short
period from the north and from the south, and to have crossed at the
equator; but to have flowed with greater force from the north so as to
have freely inundated the south. As the tide leaves its drift in
horizontal lines, though rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands to a
great height under the equator. The various beings thus left stranded may
be compared with savage races of man, driven up and surviving in the
mountain-fastnesses of almost every land, which serve as a record, full of
interest to us, of the former inhabitants of the surrounding lowlands.
12. GEOGRAPHICAL DISTRIBUTION—continued.
Distribution of fresh-water productions. On the inhabitants of oceanic
islands. Absence of Batrachians and of terrestrial Mammals. On the
relation of the inhabitants of islands to those of the nearest mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have many
fresh-water species, belonging to quite different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. I well remember, when first collecting in the fresh waters of
Brazil, feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to
wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to fish, I
believe that the same species never occur in the fresh waters of distant
continents. But on the same continent the species often range widely and
almost capriciously; for two river-systems will have some fish in common
and some different. A few facts seem to favour the possibility of their
occasional transport by accidental means; like that of the live fish not
rarely dropped by whirlwinds in India, and the vitality of their ova when
removed from the water. But I am inclined to attribute the dispersal of
fresh-water fish mainly to slight changes within the recent period in the
level of the land, having caused rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent geological
period, and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted river-systems
and completely prevented their inosculation, seems to lead to this same
conclusion. With respect to allied fresh-water fish occurring at very
distant points of the world, no doubt there are many cases which cannot at
present be explained: but some fresh-water fish belong to very ancient
forms, and in such cases there will have been ample time for great
geographical changes, and consequently time and means for much migration.
In the second place, salt-water fish can with care be slowly accustomed to
live in fresh water; and, according to Valenciennes, there is hardly a
single group of fishes confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group might travel far along
the shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds, and they are immediately killed by sea water, as are
the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed—and no doubt many others remain to be observed—throw
some light on this subject. When a duck suddenly emerges from a pond
covered with duck-weed, I have twice seen these little plants adhering to
its back; and it has happened to me, in removing a little duck-weed from
one aquarium to another, that I have quite unintentionally stocked the one
with fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended a duck's feet, which might represent those of a
bird sleeping in a natural pond, in an aquarium, where many ova of
fresh-water shells were hatching; and I found that numbers of the
extremely minute and just hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred
off, though at a somewhat more advanced age they would voluntarily drop
off. These just hatched molluscs, though aquatic in their nature, survived
on the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and would be sure to alight on a pool or rivulet, if blown across
sea to an oceanic island or to any other distant point. Sir Charles Lyell
also informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a water-beetle
of the same family, a Colymbetes, once flew on board the 'Beagle,' when
forty-five miles distant from the nearest land: how much farther it might
have flown with a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have only
a very few aquatic members; for these latter seem immediately to acquire,
as if in consequence, a very wide range. I think favourable means of
dispersal explain this fact. I have before mentioned that earth
occasionally, though rarely, adheres in some quantity to the feet and
beaks of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds of
this order I can show are the greatest wanderers, and are occasionally
found on the most remote and barren islands in the open ocean; they would
not be likely to alight on the surface of the sea, so that the dirt would
not be washed off their feet; when making land, they would be sure to fly
to their natural fresh-water haunts. I do not believe that botanists are
aware how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I took
in February three table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond; this mud when dry weighed
only 6 3/4 ounces; I kept it covered up in my study for six months,
pulling up and counting each plant as it grew; the plants were of many
kinds, and were altogether 537 in number; and yet the viscid mud was all
contained in a breakfast cup! Considering these facts, I think it would be
an inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to vast distances, and if consequently the range of
these plants was not very great. The same agency may have come into play
with the eggs of some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish;
they then take flight and go to other waters, or are blown across the sea;
and we have seen that seeds retain their power of germination, when
rejected in pellets or in excrement, many hours afterwards. When I saw the
great size of the seeds of that fine water-lily, the Nelumbium, and
remembered Alph. de Candolle's remarks on this plant, I thought that its
distribution must remain quite inexplicable; but Audubon states that he
found the seeds of the great southern water-lily (probably, according to
Dr. Hooker, the Nelumbium luteum) in a heron's stomach; although I do not
know the fact, yet analogy makes me believe that a heron flying to another
pond and getting a hearty meal of fish, would probably reject from its
stomach a pellet containing the seeds of the Nelumbium undigested; or the
seeds might be dropped by the bird whilst feeding its young, in the same
way as fish are known sometimes to be dropped.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance, on a
rising islet, it will be unoccupied; and a single seed or egg will have a
good chance of succeeding. Although there will always be a struggle for
life between the individuals of the species, however few, already
occupying any pond, yet as the number of kinds is small, compared with
those on the land, the competition will probably be less severe between
aquatic than between terrestrial species; consequently an intruder from
the waters of a foreign country, would have a better chance of seizing on
a place, than in the case of terrestrial colonists. We should, also,
remember that some, perhaps many, fresh-water productions are low in the
scale of nature, and that we have reason to believe that such low beings
change or become modified less quickly than the high; and this will give
longer time than the average for the migration of the same aquatic
species. We should not forget the probability of many species having
formerly ranged as continuously as fresh-water productions ever can range,
over immense areas, and having subsequently become extinct in intermediate
regions. But the wide distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form or in some degree
modified, I believe mainly depends on the wide dispersal of their seeds
and eggs by animals, more especially by fresh-water birds, which have
large powers of flight, and naturally travel from one to another and often
distant piece of water. Nature, like a careful gardener, thus takes her
seeds from a bed of a particular nature, and drops them in another equally
well fitted for them.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty, on the view that
all the individuals both of the same and of allied species have descended
from a single parent; and therefore have all proceeded from a common
birthplace, notwithstanding that in the course of time they have come to
inhabit distant points of the globe. I have already stated that I cannot
honestly admit Forbes's view on continental extensions, which, if
legitimately followed out, would lead to the belief that within the recent
period all existing islands have been nearly or quite joined to some
continent. This view would remove many difficulties, but it would not, I
think, explain all the facts in regard to insular productions. In the
following remarks I shall not confine myself to the mere question of
dispersal; but shall consider some other facts, which bear on the truth of
the two theories of independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large size
and varied stations of New Zealand, extending over 780 miles of latitude,
and compare its flowering plants, only 750 in number, with those on an
equal area at the Cape of Good Hope or in Australia, we must, I think,
admit that something quite independently of any difference in physical
conditions has caused so great a difference in number. Even the uniform
county of Cambridge has 847 plants, and the little island of Anglesea 764,
but a few ferns and a few introduced plants are included in these numbers,
and the comparison in some other respects is not quite fair. We have
evidence that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it, as
they have on New Zealand and on every other oceanic island which can be
named. In St. Helena there is reason to believe that the naturalised
plants and animals have nearly or quite exterminated many native
productions. He who admits the doctrine of the creation of each separate
species, will have to admit, that a sufficient number of the best adapted
plants and animals have not been created on oceanic islands; for man has
unintentionally stocked them from various sources far more fully and
perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is scanty,
the proportion of endemic species (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the number
of the endemic land-shells in Madeira, or of the endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the islands with that of the continent, we shall see
that this is true. This fact might have been expected on my theory, for,
as already explained, species occasionally arriving after long intervals
in a new and isolated district, and having to compete with new associates,
will be eminently liable to modification, and will often produce groups of
modified descendants. But it by no means follows, that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds, are
peculiar; and it is obvious that marine birds could arrive at these
islands more easily than land-birds. Bermuda, on the other hand, which
lies at about the same distance from North America as the Galapagos
Islands do from South America, and which has a very peculiar soil, does
not possess one endemic land bird; and we know from Mr. J. M. Jones's
admirable account of Bermuda, that very many North American birds, during
their great annual migrations, visit either periodically or occasionally
this island. Madeira does not possess one peculiar bird, and many European
and African birds are almost every year blown there, as I am informed by
Mr. E. V. Harcourt. So that these two islands of Bermuda and Madeira have
been stocked by birds, which for long ages have struggled together in
their former homes, and have become mutually adapted to each other; and
when settled in their new homes, each kind will have been kept by the
others to their proper places and habits, and will consequently have been
little liable to modification. Madeira, again, is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is
confined to its shores: now, though we do not know how seashells are
dispersed, yet we can see that their eggs or larvae, perhaps attached to
seaweed or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four hundred
miles of open sea. The different orders of insects in Madeira apparently
present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take the
place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
shown that the proportional numbers of the different orders are very
different from what they are elsewhere. Such cases are generally accounted
for by the physical conditions of the islands; but this explanation seems
to me not a little doubtful. Facility of immigration, I believe, has been
at least as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than the adaptation of hooked
seeds for transportal by the wool and fur of quadrupeds. This case
presents no difficulty on my view, for a hooked seed might be transported
to an island by some other means; and the plant then becoming slightly
modified, but still retaining its hooked seeds, would form an endemic
species, having as useless an appendage as any rudimentary organ,—for
instance, as the shrivelled wings under the soldered elytra of many
insular beetles. Again, islands often possess trees or bushes belonging to
orders which elsewhere include only herbaceous species; now trees, as
Alph. de Candolle has shown, generally have, whatever the cause may be,
confined ranges. Hence trees would be little likely to reach distant
oceanic islands; and an herbaceous plant, though it would have no chance
of successfully competing in stature with a fully developed tree, when
established on an island and having to compete with herbaceous plants
alone, might readily gain an advantage by growing taller and taller and
overtopping the other plants. If so, natural selection would often tend to
add to the stature of herbaceous plants when growing on an island, to
whatever order they belonged, and thus convert them first into bushes and
ultimately into trees.
With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have
never been found on any of the many islands with which the great oceans
are studded. I have taken pains to verify this assertion, and I have found
it strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this
exception (if the information be correct) may be explained through glacial
agency. This general absence of frogs, toads, and newts on so many oceanic
islands cannot be accounted for by their physical conditions; indeed it
seems that islands are peculiarly well fitted for these animals; for frogs
have been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we can
see that there would be great difficulty in their transportal across the
sea, and therefore why they do not exist on any oceanic island. But why,
on the theory of creation, they should not have been created there, it
would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found a
single instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island situated
above 300 miles from a continent or great continental island; and many
islands situated at a much less distance are equally barren. The Falkland
Islands, which are inhabited by a wolf-like fox, come nearest to an
exception; but this group cannot be considered as oceanic, as it lies on a
bank connected with the mainland; moreover, icebergs formerly brought
boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot
be said that small islands will not support small mammals, for they occur
in many parts of the world on very small islands, if close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation
of mammals; many volcanic islands are sufficiently ancient, as shown by
the stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that mammals
appear and disappear at a quicker rate than other and lower animals.
Though terrestrial mammals do not occur on oceanic islands, aerial mammals
do occur on almost every island. New Zealand possesses two bats found
nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all
possess their peculiar bats. Why, it may be asked, has the supposed
creative force produced bats and no other mammals on remote islands? On my
view this question can easily be answered; for no terrestrial mammal can
be transported across a wide space of sea, but bats can fly across. Bats
have been seen wandering by day far over the Atlantic Ocean; and two North
American species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who has
specially studied this family, that many of the same species have enormous
ranges, and are found on continents and on far distant islands. Hence we
have only to suppose that such wandering species have been modified
through natural selection in their new homes in relation to their new
position, and we can understand the presence of endemic bats on islands,
with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the remoteness
of islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near
Celebes by a space of deep ocean; and this space separates two widely
distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied
or identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some
cases owing to the probable naturalisation of certain mammals through
man's agency; but we shall soon have much light thrown on the natural
history of this archipelago by the admirable zeal and researches of Mr.
Wallace. I have not as yet had time to follow up this subject in all other
quarters of the world; but as far as I have gone, the relation generally
holds good. We see Britain separated by a shallow channel from Europe, and
the mammals are the same on both sides; we meet with analogous facts on
many islands separated by similar channels from Australia. The West Indian
Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth,
and here we find American forms, but the species and even the genera are
distinct. As the amount of modification in all cases depends to a certain
degree on the lapse of time, and as during changes of level it is obvious
that islands separated by shallow channels are more likely to have been
continuously united within a recent period to the mainland than islands
separated by deeper channels, we can understand the frequent relation
between the depth of the sea and the degree of affinity of the mammalian
inhabitants of islands with those of a neighbouring continent,—an
inexplicable relation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic islands,—namely,
the scarcity of kinds—the richness in endemic forms in particular
classes or sections of classes,—the absence of whole groups, as of
batrachians, and of terrestrial mammals notwithstanding the presence of
aerial bats,—the singular proportions of certain orders of plants,—herbaceous
forms having been developed into trees, etc.,—seem to me to accord
better with the view of occasional means of transport having been largely
efficient in the long course of time, than with the view of all our
oceanic islands having been formerly connected by continuous land with the
nearest continent; for on this latter view the migration would probably
have been more complete; and if modification be admitted, all the forms of
life would have been more equally modified, in accordance with the
paramount importance of the relation of organism to organism.
I do not deny that there are many and grave difficulties in understanding
how several of the inhabitants of the more remote islands, whether still
retaining the same specific form or modified since their arrival, could
have reached their present homes. But the probability of many islands
having existed as halting-places, of which not a wreck now remains, must
not be overlooked. I will here give a single instance of one of the cases
of difficulty. Almost all oceanic islands, even the most isolated and
smallest, are inhabited by land-shells, generally by endemic species, but
sometimes by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and are
killed by it. Yet there must be, on my view, some unknown, but highly
efficient means for their transportal. Would the just-hatched young
occasionally crawl on and adhere to the feet of birds roosting on the
ground, and thus get transported? It occurred to me that land-shells, when
hybernating and having a membranous diaphragm over the mouth of the shell,
might be floated in chinks of drifted timber across moderately wide arms
of the sea. And I found that several species did in this state withstand
uninjured an immersion in sea-water during seven days: one of these shells
was the Helix pomatia, and after it had again hybernated I put it in
sea-water for twenty days, and it perfectly recovered. As this species has
a thick calcareous operculum, I removed it, and when it had formed a new
membranous one, I immersed it for fourteen days in sea-water, and it
recovered and crawled away: but more experiments are wanted on this head.
The most striking and important fact for us in regard to the inhabitants
of islands, is their affinity to those of the nearest mainland, without
being actually the same species. Numerous instances could be given of this
fact. I will give only one, that of the Galapagos Archipelago, situated
under the equator, between 500 and 600 miles from the shores of South
America. Here almost every product of the land and water bears the
unmistakeable stamp of the American continent. There are twenty-six land
birds, and twenty-five of these are ranked by Mr. Gould as distinct
species, supposed to have been created here; yet the close affinity of
most of these birds to American species in every character, in their
habits, gestures, and tones of voice, was manifest. So it is with the
other animals, and with nearly all the plants, as shown by Dr. Hooker in
his admirable memoir on the Flora of this archipelago. The naturalist,
looking at the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, yet feels that he is
standing on American land. Why should this be so? why should the species
which are supposed to have been created in the Galapagos Archipelago, and
nowhere else, bear so plain a stamp of affinity to those created in
America? There is nothing in the conditions of life, in the geological
nature of the islands, in their height or climate, or in the proportions
in which the several classes are associated together, which resembles
closely the conditions of the South American coast: in fact there is a
considerable dissimilarity in all these respects. On the other hand, there
is a considerable degree of resemblance in the volcanic nature of the
soil, in climate, height, and size of the islands, between the Galapagos
and Cape de Verde Archipelagos: but what an entire and absolute difference
in their inhabitants! The inhabitants of the Cape de Verde Islands are
related to those of Africa, like those of the Galapagos to America. I
believe this grand fact can receive no sort of explanation on the ordinary
view of independent creation; whereas on the view here maintained, it is
obvious that the Galapagos Islands would be likely to receive colonists,
whether by occasional means of transport or by formerly continuous land,
from America; and the Cape de Verde Islands from Africa; and that such
colonists would be liable to modification;—the principle of
inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the
nearest continent, or of other near islands. The exceptions are few, and
most of them can be explained. Thus the plants of Kerguelen Land, though
standing nearer to Africa than to America, are related, and that very
closely, as we know from Dr. Hooker's account, to those of America: but on
the view that this island has been mainly stocked by seeds brought with
earth and stones on icebergs, drifted by the prevailing currents, this
anomaly disappears. New Zealand in its endemic plants is much more closely
related to Australia, the nearest mainland, than to any other region: and
this is what might have been expected; but it is also plainly related to
South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this difficulty
almost disappears on the view that both New Zealand, South America, and
other southern lands were long ago partially stocked from a nearly
intermediate though distant point, namely from the antarctic islands, when
they were clothed with vegetation, before the commencement of the Glacial
period. The affinity, which, though feeble, I am assured by Dr. Hooker is
real, between the flora of the south-western corner of Australia and of
the Cape of Good Hope, is a far more remarkable case, and is at present
inexplicable: but this affinity is confined to the plants, and will, I do
not doubt, be some day explained.
The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus the
several islands of the Galapagos Archipelago are tenanted, as I have
elsewhere shown, in a quite marvellous manner, by very closely related
species; so that the inhabitants of each separate island, though mostly
distinct, are related in an incomparably closer degree to each other than
to the inhabitants of any other part of the world. And this is just what
might have been expected on my view, for the islands are situated so near
each other that they would almost certainly receive immigrants from the
same original source, or from each other. But this dissimilarity between
the endemic inhabitants of the islands may be used as an argument against
my views; for it may be asked, how has it happened in the several islands
situated within sight of each other, having the same geological nature,
the same height, climate, etc., that many of the immigrants should have
been differently modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises in chief part from the
deeply-seated error of considering the physical conditions of a country as
the most important for its inhabitants; whereas it cannot, I think, be
disputed that the nature of the other inhabitants, with which each has to
compete, is at least as important, and generally a far more important
element of success. Now if we look to those inhabitants of the Galapagos
Archipelago which are found in other parts of the world (laying on one
side for the moment the endemic species, which cannot be here fairly
included, as we are considering how they have come to be modified since
their arrival), we find a considerable amount of difference in the several
islands. This difference might indeed have been expected on the view of
the islands having been stocked by occasional means of transport—a
seed, for instance, of one plant having been brought to one island, and
that of another plant to another island. Hence when in former times an
immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly be
exposed to different conditions of life in the different islands, for it
would have to compete with different sets of organisms: a plant, for
instance, would find the best-fitted ground more perfectly occupied by
distinct plants in one island than in another, and it would be exposed to
the attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species
spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous instances, is that the new species
formed in the separate islands have not quickly spread to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and sweep across
the archipelago, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they
appear to be on a map. Nevertheless a good many species, both those found
in other parts of the world and those confined to the archipelago, are
common to the several islands, and we may infer from certain facts that
these have probably spread from some one island to the others. But we
often take, I think, an erroneous view of the probability of closely
allied species invading each other's territory, when put into free
intercommunication. Undoubtedly if one species has any advantage whatever
over another, it will in a very brief time wholly or in part supplant it;
but if both are equally well fitted for their own places in nature, both
probably will hold their own places and keep separate for almost any
length of time. Being familiar with the fact that many species,
naturalised through man's agency, have spread with astonishing rapidity
over new countries, we are apt to infer that most species would thus
spread; but we should remember that the forms which become naturalised in
new countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct species, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to distinct genera. In
the Galapagos Archipelago, many even of the birds, though so well adapted
for flying from island to island, are distinct on each; thus there are
three closely-allied species of mocking-thrush, each confined to its own
island. Now let us suppose the mocking-thrush of Chatham Island to be
blown to Charles Island, which has its own mocking-thrush: why should it
succeed in establishing itself there? We may safely infer that Charles
Island is well stocked with its own species, for annually more eggs are
laid there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted for
its home as is the species peculiar to Chatham Island. Sir C. Lyell and
Mr. Wollaston have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of Porto Santo
possess many distinct but representative land-shells, some of which live
in crevices of stone; and although large quantities of stone are annually
transported from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species: nevertheless both islands
have been colonised by some European land-shells, which no doubt had some
advantage over the indigenous species. From these considerations I think
we need not greatly marvel at the endemic and representative species,
which inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as in
the several districts of the same continent, pre-occupation has probably
played an important part in checking the commingling of species under the
same conditions of life. Thus, the south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of that
region whence colonists could most readily have been derived,—the
colonists having been subsequently modified and better fitted to their new
homes,—is of the widest application throughout nature. We see this
on every mountain, in every lake and marsh. For Alpine species, excepting
in so far as the same forms, chiefly of plants, have spread widely
throughout the world during the recent Glacial epoch, are related to those
of the surrounding lowlands;—thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, etc., all of strictly
American forms, and it is obvious that a mountain, as it became slowly
upheaved, would naturally be colonised from the surrounding lowlands. So
it is with the inhabitants of lakes and marshes, excepting in so far as
great facility of transport has given the same general forms to the whole
world. We see this same principle in the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. And
it will, I believe, be universally found to be true, that wherever in two
regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some identical
species, showing, in accordance with the foregoing view, that at some
former period there has been intercommunication or migration between the
two regions. And wherever many closely-allied species occur, there will be
found many forms which some naturalists rank as distinct species, and some
as varieties; these doubtful forms showing us the steps in the process of
modification.
This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way. Mr.
Gould remarked to me long ago, that in those genera of birds which range
over the world, many of the species have very wide ranges. I can hardly
doubt that this rule is generally true, though it would be difficult to
prove it. Amongst mammals, we see it strikingly displayed in Bats, and in
a lesser degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many
individual species have enormous ranges. It is not meant that in
world-ranging genera all the species have a wide range, or even that they
have on an AVERAGE a wide range; but only that some of the species range
very widely; for the facility with which widely-ranging species vary and
give rise to new forms will largely determine their average range. For
instance, two varieties of the same species inhabit America and Europe,
and the species thus has an immense range; but, if the variation had been
a little greater, the two varieties would have been ranked as distinct
species, and the common range would have been greatly reduced. Still less
is it meant, that a species which apparently has the capacity of crossing
barriers and ranging widely, as in the case of certain powerfully-winged
birds, will necessarily range widely; for we should never forget that to
range widely implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the struggle for
life with foreign associates. But on the view of all the species of a
genus having descended from a single parent, though now distributed to the
most remote points of the world, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very
widely; for it is necessary that the unmodified parent should range
widely, undergoing modification during its diffusion, and should place
itself under diverse conditions favourable for the conversion of its
offspring, firstly into new varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have
been ample time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration of some of the
species into all quarters of the world, where they may have become
slightly modified in relation to their new conditions. There is, also,
some reason to believe from geological evidence that organisms low in the
scale within each great class, generally change at a slower rate than the
higher forms; and consequently the lower forms will have had a better
chance of ranging widely and of still retaining the same specific
character. This fact, together with the seeds and eggs of many low forms
being very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately been
admirably discussed by Alph. de Candolle in regard to plants, namely, that
the lower any group of organisms is, the more widely it is apt to range.
The relations just discussed,—namely, low and slowly-changing
organisms ranging more widely than the high,—some of the species of
widely-ranging genera themselves ranging widely,—such facts, as
alpine, lacustrine, and marsh productions being related (with the
exceptions before specified) to those on the surrounding low lands and dry
lands, though these stations are so different—the very close
relation of the distinct species which inhabit the islets of the same
archipelago,—and especially the striking relation of the inhabitants
of each whole archipelago or island to those of the nearest mainland,—are,
I think, utterly inexplicable on the ordinary view of the independent
creation of each species, but are explicable on the view of colonisation
from the nearest and readiest source, together with the subsequent
modification and better adaptation of the colonists to their new homes.
SUMMARY OF LAST AND PRESENT CHAPTERS.
In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of all the changes of
climate and of the level of the land, which have certainly occurred within
the recent period, and of other similar changes which may have occurred
within the same period; if we remember how profoundly ignorant we are with
respect to the many and curious means of occasional transport,—a
subject which has hardly ever been properly experimentised on; if we bear
in mind how often a species may have ranged continuously over a wide area,
and then have become extinct in the intermediate tracts, I think the
difficulties in believing that all the individuals of the same species,
wherever located, have descended from the same parents, are not
insuperable. And we are led to this conclusion, which has been arrived at
by many naturalists under the designation of single centres of creation,
by some general considerations, more especially from the importance of
barriers and from the analogical distribution of sub-genera, genera, and
families.
With respect to the distinct species of the same genus, which on my theory
must have spread from one parent-source; if we make the same allowances as
before for our ignorance, and remember that some forms of life change most
slowly, enormous periods of time being thus granted for their migration, I
do not think that the difficulties are insuperable; though they often are
in this case, and in that of the individuals of the same species,
extremely grave.
As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at
some little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of allied
species, have proceeded from some one source; then I think all the grand
leading facts of geographical distribution are explicable on the theory of
migration (generally of the more dominant forms of life), together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water,
which separate our several zoological and botanical provinces. We can thus
understand the localisation of sub-genera, genera, and families; and how
it is that under different latitudes, for instance in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and
deserts, are in so mysterious a manner linked together by affinity, and
are likewise linked to the extinct beings which formerly inhabited the
same continent. Bearing in mind that the mutual relations of organism to
organism are of the highest importance, we can see why two areas having
nearly the same physical conditions should often be inhabited by very
different forms of life; for according to the length of time which has
elapsed since new inhabitants entered one region; according to the nature
of the communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each
other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, infinitely
diversified conditions of life,—there would be an almost endless
amount of organic action and reaction,—and we should find, as we do
find, some groups of beings greatly, and some only slightly modified,—some
developed in great force, some existing in scanty numbers—in the
different great geographical provinces of the world.
On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all its
species common to other quarters of the world. We can see why whole groups
of organisms, as batrachians and terrestrial mammals, should be absent
from oceanic islands, whilst the most isolated islands possess their own
peculiar species of aerial mammals or bats. We can see why there should be
some relation between the presence of mammals, in a more or less modified
condition, and the depth of the sea between an island and the mainland. We
can clearly see why all the inhabitants of an archipelago, though
specifically distinct on the several islets, should be closely related to
each other, and likewise be related, but less closely, to those of the
nearest continent or other source whence immigrants were probably derived.
We can see why in two areas, however distant from each other, there should
be a correlation, in the presence of identical species, of varieties, of
doubtful species, and of distinct but representative species.
As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the exceptions to the rule are so few, that they
may fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period
under different conditions or by occasional means of transport, and by the
species having become extinct in the intermediate tracts. Both in time and
space, species and groups of species have their points of maximum
development. Groups of species, belonging either to a certain period of
time, or to a certain area, are often characterised by trifling characters
in common, as of sculpture or colour. In looking to the long succession of
ages, as in now looking to distant provinces throughout the world, we find
that some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of the
same order, differ greatly. In both time and space the lower members of
each class generally change less than the higher; but there are in both
cases marked exceptions to the rule. On my theory these several relations
throughout time and space are intelligible; for whether we look to the
forms of life which have changed during successive ages within the same
quarter of the world, or to those which have changed after having migrated
into distant quarters, in both cases the forms within each class have been
connected by the same bond of ordinary generation; and the more nearly any
two forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have
been the same, and modifications have been accumulated by the same power
of natural selection.
13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.
CLASSIFICATION, groups subordinate to groups. Natural system. Rules and
difficulties in classification, explained on the theory of descent with
modification. Classification of varieties. Descent always used in
classification. Analogical or adaptive characters. Affinities, general,
complex and radiating. Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the same
individual. EMBRYOLOGY, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age. RUDIMENTARY
ORGANS; their origin explained. Summary.
From the first dawn of life, all organic beings are found to resemble each
other in descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the grouping
of the stars in constellations. The existence of groups would have been of
simple signification, if one group had been exclusively fitted to inhabit
the land, and another the water; one to feed on flesh, another on
vegetable matter, and so on; but the case is widely different in nature;
for it is notorious how commonly members of even the same subgroup have
different habits. In our second and fourth chapters, on Variation and on
Natural Selection, I have attempted to show that it is the widely ranging,
the much diffused and common, that is the dominant species belonging to
the larger genera, which vary most. The varieties, or incipient species,
thus produced ultimately become converted, as I believe, into new and
distinct species; and these, on the principle of inheritance, tend to
produce other new and dominant species. Consequently the groups which are
now large, and which generally include many dominant species, tend to go
on increasing indefinitely in size. I further attempted to show that from
the varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, there is a constant
tendency in their characters to diverge. This conclusion was supported by
looking at the great diversity of the forms of life which, in any small
area, come into the closest competition, and by looking to certain facts
in naturalisation.
I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the less divergent, the less improved, and preceding forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and all the genera on this line form together one class,
for all have descended from one ancient but unseen parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
sub-family, distinct from that including the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they form a
family distinct from that including the three genera still further to the
right hand, which diverged at a still earlier period. And all these
genera, descended from (A), form an order distinct from the genera
descended from (I). So that we here have many species descended from a
single progenitor grouped into genera; and the genera are included in, or
subordinate to, sub-families, families, and orders, all united into one
class. Thus, the grand fact in natural history of the subordination of
group under group, which, from its familiarity, does not always
sufficiently strike us, is in my judgment fully explained.
Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging together
those living objects which are most alike, and for separating those which
are most unlike; or as an artificial means for enunciating, as briefly as
possible, general propositions,—that is, by one sentence to give the
characters common, for instance, to all mammals, by another those common
to all carnivora, by another those common to the dog-genus, and then by
adding a single sentence, a full description is given of each kind of dog.
The ingenuity and utility of this system are indisputable. But many
naturalists think that something more is meant by the Natural System; they
believe that it reveals the plan of the Creator; but unless it be
specified whether order in time or space, or what else is meant by the
plan of the Creator, it seems to me that nothing is thus added to our
knowledge. Such expressions as that famous one of Linnaeus, and which we
often meet with in a more or less concealed form, that the characters do
not make the genus, but that the genus gives the characters, seem to imply
that something more is included in our classification, than mere
resemblance. I believe that something more is included; and that
propinquity of descent,—the only known cause of the similarity of
organic beings,—is the bond, hidden as it is by various degrees of
modification, which is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that
those parts of the structure which determined the habits of life, and the
general place of each being in the economy of nature, would be of very
high importance in classification. Nothing can be more false. No one
regards the external similarity of a mouse to a shrew, of a dugong to a
whale, of a whale to a fish, as of any importance. These resemblances,
though so intimately connected with the whole life of the being, are
ranked as merely "adaptive or analogical characters;" but to the
consideration of these resemblances we shall have to recur. It may even be
given as a general rule, that the less any part of the organisation is
concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says,
"The generative organs being those which are most remotely related to the
habits and food of an animal, I have always regarded as affording very
clear indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character." So with plants, how remarkable it is that the organs
of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the organs
of reproduction, with their product the seed, are of paramount importance!
We must not, therefore, in classifying, trust to resemblances in parts of
the organisation, however important they may be for the welfare of the
being in relation to the outer world. Perhaps from this cause it has
partly arisen, that almost all naturalists lay the greatest stress on
resemblances in organs of high vital or physiological importance. No doubt
this view of the classificatory importance of organs which are important
is generally, but by no means always, true. But their importance for
classification, I believe, depends on their greater constancy throughout
large groups of species; and this constancy depends on such organs having
generally been subjected to less change in the adaptation of the species
to their conditions of life. That the mere physiological importance of an
organ does not determine its classificatory value, is almost shown by the
one fact, that in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have worked at
any group without being struck with this fact; and it has been most fully
acknowledged in the writings of almost every author. It will suffice to
quote the highest authority, Robert Brown, who in speaking of certain
organs in the Proteaceae, says their generic importance, "like that of all
their parts, not only in this but, as I apprehend, in every natural
family, is very unequal, and in some cases seems to be entirely lost."
Again in another work he says, the genera of the Connaraceae "differ in
having one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular aestivation. Any one of these characters singly is
frequently of more than generic importance, though here even when all
taken together they appear insufficient to separate Cnestis from
Connarus." To give an example amongst insects, in one great division of
the Hymenoptera, the antennae, as Westwood has remarked, are most constant
in structure; in another division they differ much, and the differences
are of quite subordinate value in classification; yet no one probably will
say that the antennae in these two divisions of the same order are of
unequal physiological importance. Any number of instances could be given
of the varying importance for classification of the same important organ
within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the rudimentary florets are of the
highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which
are universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles—the inflection of the
angle of the jaws in Marsupials—the manner in which the wings of
insects are folded—mere colour in certain Algae—mere
pubescence on parts of the flower in grasses—the nature of the
dermal covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of hair, this
external and trifling character would, I think, have been considered by
naturalists as important an aid in determining the degree of affinity of
this strange creature to birds and reptiles, as an approach in structure
in any one internal and important organ.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with several other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may
depart from its allies in several characters, both of high physiological
importance and of almost universal prevalence, and yet leave us in no
doubt where it should be ranked. Hence, also, it has been found, that a
classification founded on any single character, however important that may
be, has always failed; for no part of the organisation is universally
constant. The importance of an aggregate of characters, even when none are
important, alone explains, I think, that saying of Linnaeus, that the
characters do not give the genus, but the genus gives the characters; for
this saying seems founded on an appreciation of many trifling points of
resemblance, too slight to be defined. Certain plants, belonging to the
Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, "the greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus
laugh at our classification." But when Aspicarpa produced in France,
during several years, only degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type of
the order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained amongst the Malpighiaceae. This case seems
to me well to illustrate the spirit with which our classifications are
sometimes necessarily founded.
Practically when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining
a group, or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If certain characters
are always found correlated with others, though no apparent bond of
connexion can be discovered between them, especial value is set on them.
As in most groups of animals, important organs, such as those for
propelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable
in classification; but in some groups of animals all these, the most
important vital organs, are found to offer characters of quite subordinate
value.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications of
course include all ages of each species. But it is by no means obvious, on
the ordinary view, why the structure of the embryo should be more
important for this purpose than that of the adult, which alone plays its
full part in the economy of nature. Yet it has been strongly urged by
those great naturalists, Milne Edwards and Agassiz, that embryonic
characters are the most important of any in the classification of animals;
and this doctrine has very generally been admitted as true. The same fact
holds good with flowering plants, of which the two main divisions have
been founded on characters derived from the embryo,—on the number
and position of the embryonic leaves or cotyledons, and on the mode of
development of the plumule and radicle. In our discussion on embryology,
we shall see why such characters are so valuable, on the view of
classification tacitly including the idea of descent.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but in the case of crustaceans, such definition has hitherto been
found impossible. There are crustaceans at the opposite ends of the
series, which have hardly a character in common; yet the species at both
ends, from being plainly allied to others, and these to others, and so
onwards, can be recognised as unequivocally belonging to this, and to no
other class of the Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted on
their arbitrary value. Instances could be given amongst plants and
insects, of a group of forms, first ranked by practised naturalists as
only a genus, and then raised to the rank of a sub-family or family; and
this has been done, not because further research has detected important
structural differences, at first overlooked, but because numerous allied
species, with slightly different grades of difference, have been
subsequently discovered.
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between any
two or more species, are those which have been inherited from a common
parent, and, in so far, all true classification is genealogical; that
community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.
But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to the
other groups, must be strictly genealogical in order to be natural; but
that the AMOUNT of difference in the several branches or groups, though
allied in the same degree in blood to their common progenitor, may differ
greatly, being due to the different degrees of modification which they
have undergone; and this is expressed by the forms being ranked under
different genera, families, sections, or orders. The reader will best
understand what is meant, if he will take the trouble of referring to the
diagram in the fourth chapter. We will suppose the letters A to L to
represent allied genera, which lived during the Silurian epoch, and these
have descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted modified
descendants to the present day, represented by the fifteen genera (a14 to
z14) on the uppermost horizontal line. Now all these modified descendants
from a single species, are represented as related in blood or descent to
the same degree; they may metaphorically be called cousins to the same
millionth degree; yet they differ widely and in different degrees from
each other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species, descended from
A, be ranked in the same genus with the parent A; or those from I, with
the parent I. But the existing genus F14 may be supposed to have been but
slightly modified; and it will then rank with the parent-genus F; just as
some few still living organic beings belong to Silurian genera. So that
the amount or value of the differences between organic beings all related
to each other in the same degree in blood, has come to be widely
different. Nevertheless their genealogical ARRANGEMENT remains strictly
true, not only at the present time, but at each successive period of
descent. All the modified descendants from A will have inherited something
in common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants, at each successive
period. If, however, we choose to suppose that any of the descendants of A
or of I have been so much modified as to have more or less completely lost
traces of their parentage, in this case, their places in a natural
classification will have been more or less completely lost,—as
sometimes seems to have occurred with existing organisms. All the
descendants of the genus F, along its whole line of descent, are supposed
to have been but little modified, and they yet form a single genus. But
this genus, though much isolated, will still occupy its proper
intermediate position; for F originally was intermediate in character
between A and I, and the several genera descended from these two genera
will have inherited to a certain extent their characters. This natural
arrangement is shown, as far as is possible on paper, in the diagram, but
in much too simple a manner. If a branching diagram had not been used, and
only the names of the groups had been written in a linear series, it would
have been still less possible to have given a natural arrangement; and it
is notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature amongst the beings of the same
group. Thus, on the view which I hold, the natural system is genealogical
in its arrangement, like a pedigree; but the degrees of modification which
the different groups have undergone, have to be expressed by ranking them
under different so-called genera, sub-families, families, sections,
orders, and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, had to be included, such an arrangement would, I think, be the
only possible one. Yet it might be that some very ancient language had
altered little, and had given rise to few new languages, whilst others
(owing to the spreading and subsequent isolation and states of
civilisation of the several races, descended from a common race) had
altered much, and had given rise to many new languages and dialects. The
various degrees of difference in the languages from the same stock, would
have to be expressed by groups subordinate to groups; but the proper or
even only possible arrangement would still be genealogical; and this would
be strictly natural, as it would connect together all languages, extinct
and modern, by the closest affinities, and would give the filiation and
origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one species.
These are grouped under species, with sub-varieties under varieties; and
with our domestic productions, several other grades of difference are
requisite, as we have seen with pigeons. The origin of the existence of
groups subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of modification. Nearly
the same rules are followed in classifying varieties, as with species.
Authors have insisted on the necessity of classing varieties on a natural
instead of an artificial system; we are cautioned, for instance, not to
class two varieties of the pine-apple together, merely because their
fruit, though the most important part, happens to be nearly identical; no
one puts the swedish and common turnips together, though the esculent and
thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the body, etc.;
whereas with sheep the horns are much less serviceable, because less
constant. In classing varieties, I apprehend if we had a real pedigree, a
genealogical classification would be universally preferred; and it has
been attempted by some authors. For we might feel sure, whether there had
been more or less modification, the principle of inheritance would keep
the forms together which were allied in the greatest number of points. In
tumbler pigeons, though some sub-varieties differ from the others in the
important character of having a longer beak, yet all are kept together
from having the common habit of tumbling; but the short-faced breed has
nearly or quite lost this habit; nevertheless, without any reasoning or
thinking on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects. If it could be
proved that the Hottentot had descended from the Negro, I think he would
be classed under the Negro group, however much he might differ in colour
and other important characters from negroes.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, or
that of a species, the two sexes; and how enormously these sometimes
differ in the most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the males and
hermaphrodites of certain cirripedes, when adult, and yet no one dreams of
separating them. The naturalist includes as one species the several larval
stages of the same individual, however much they may differ from each
other and from the adult; as he likewise includes the so-called alternate
generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters; he includes
varieties, not solely because they closely resemble the parent-form, but
because they are descended from it. He who believes that the cowslip is
descended from the primrose, or conversely, ranks them together as a
single species, and gives a single definition. As soon as three Orchidean
forms (Monochanthus, Myanthus, and Catasetum), which had previously been
ranked as three distinct genera, were known to be sometimes produced on
the same spike, they were immediately included as a single species. But it
may be asked, what ought we to do, if it could be proved that one species
of kangaroo had been produced, by a long course of modification, from a
bear? Ought we to rank this one species with bears, and what should we do
with the other species? The supposition is of course preposterous; and I
might answer by the argumentum ad hominem, and ask what should be done if
a perfect kangaroo were seen to come out of the womb of a bear? According
to all analogy, it would be ranked with bears; but then assuredly all the
other species of the kangaroo family would have to be classed under the
bear genus. The whole case is preposterous; for where there has been close
descent in common, there will certainly be close resemblance or affinity.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups,
though in these cases the modification has been greater in degree, and has
taken a longer time to complete? I believe it has thus been unconsciously
used; and only thus can I understand the several rules and guides which
have been followed by our best systematists. We have no written pedigrees;
we have to make out community of descent by resemblances of any kind.
Therefore we choose those characters which, as far as we can judge, are
the least likely to have been modified in relation to the conditions of
life to which each species has been recently exposed. Rudimentary
structures on this view are as good as, or even sometimes better than,
other parts of the organisation. We care not how trifling a character may
be—let it be the mere inflection of the angle of the jaw, the manner
in which an insect's wing is folded, whether the skin be covered by hair
or feathers—if it prevail throughout many and different species,
especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with such
different habits, only by its inheritance from a common parent. We may err
in this respect in regard to single points of structure, but when several
characters, let them be ever so trifling, occur together throughout a
large group of beings having different habits, we may feel almost sure, on
the theory of descent, that these characters have been inherited from a
common ancestor. And we know that such correlated or aggregated characters
have especial value in classification.
We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and yet be
safely classed with them. This may be safely done, and is often done, as
long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet if these extreme forms
are connected together by a chain of intermediate groups, we may at once
infer their community of descent, and we put them all into the same class.
As we find organs of high physiological importance—those which serve
to preserve life under the most diverse conditions of existence—are
generally the most constant, we attach especial value to them; but if
these same organs, in another group or section of a group, are found to
differ much, we at once value them less in our classification. We shall
hereafter, I think, clearly see why embryological characters are of such
high classificatory importance. Geographical distribution may sometimes be
brought usefully into play in classing large and widely-distributed
genera, because all the species of the same genus, inhabiting any distinct
and isolated region, have in all probability descended from the same
parents.
We can understand, on these views, the very important distinction between
real affinities and analogical or adaptive resemblances. Lamarck first
called attention to this distinction, and he has been ably followed by
Macleay and others. The resemblance, in the shape of the body and in the
fin-like anterior limbs, between the dugong, which is a pachydermatous
animal, and the whale, and between both these mammals and fishes, is
analogical. Amongst insects there are innumerable instances: thus
Linnaeus, misled by external appearances, actually classed an homopterous
insect as a moth. We see something of the same kind even in our domestic
varieties, as in the thickened stems of the common and swedish turnip. The
resemblance of the greyhound and racehorse is hardly more fanciful than
the analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for
classification, only in so far as they reveal descent, we can clearly
understand why analogical or adaptive character, although of the utmost
importance to the welfare of the being, are almost valueless to the
systematist. For animals, belonging to two most distinct lines of descent,
may readily become adapted to similar conditions, and thus assume a close
external resemblance; but such resemblances will not reveal—will
rather tend to conceal their blood-relationship to their proper lines of
descent. We can also understand the apparent paradox, that the very same
characters are analogical when one class or order is compared with
another, but give true affinities when the members of the same class or
order are compared one with another: thus the shape of the body and
fin-like limbs are only analogical when whales are compared with fishes,
being adaptations in both classes for swimming through the water; but the
shape of the body and fin-like limbs serve as characters exhibiting true
affinity between the several members of the whale family; for these
cetaceans agree in so many characters, great and small, that we cannot
doubt that they have inherited their general shape of body and structure
of limbs from a common ancestor. So it is with fishes.
As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,—to
inhabit for instance the three elements of land, air, and water,—we
can perhaps understand how it is that a numerical parallelism has
sometimes been observed between the sub-groups in distinct classes. A
naturalist, struck by a parallelism of this nature in any one class, by
arbitrarily raising or sinking the value of the groups in other classes
(and all our experience shows that this valuation has hitherto been
arbitrary), could easily extend the parallelism over a wide range; and
thus the septenary, quinary, quaternary, and ternary classifications have
probably arisen.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which
they belong large and their parents dominant, they are almost sure to
spread widely, and to seize on more and more places in the economy of
nature. The larger and more dominant groups thus tend to go on increasing
in size; and they consequently supplant many smaller and feebler groups.
Thus we can account for the fact that all organisms, recent and extinct,
are included under a few great orders, under still fewer classes, and all
in one great natural system. As showing how few the higher groups are in
number, and how widely spread they are throughout the world, the fact is
striking, that the discovery of Australia has not added a single insect
belonging to a new order; and that in the vegetable kingdom, as I learn
from Dr. Hooker, it has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more
ancient forms of life often present characters in some slight degree
intermediate between existing groups. A few old and intermediate
parent-forms having occasionally transmitted to the present day
descendants but little modified, will give to us our so-called osculant or
aberrant groups. The more aberrant any form is, the greater must be the
number of connecting forms which on my theory have been exterminated and
utterly lost. And we have some evidence of aberrant forms having suffered
severely from extinction, for they are generally represented by extremely
few species; and such species as do occur are generally very distinct from
each other, which again implies extinction. The genera Ornithorhynchus and
Lepidosiren, for example, would not have been less aberrant had each been
represented by a dozen species instead of by a single one; but such
richness in species, as I find after some investigation, does not commonly
fall to the lot of aberrant genera. We can, I think, account for this fact
only by looking at aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by some unusual
coincidence of favourable circumstances.
Mr. Waterhouse has remarked that, when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
and not to any one marsupial species more than to another. As the points
of affinity of the bizcacha to Marsupials are believed to be real and not
merely adaptive, they are due on my theory to inheritance in common.
Therefore we must suppose either that all Rodents, including the bizcacha,
branched off from some very ancient Marsupial, which will have had a
character in some degree intermediate with respect to all existing
Marsupials; or that both Rodents and Marsupials branched off from a common
progenitor, and that both groups have since undergone much modification in
divergent directions. On either view we may suppose that the bizcacha has
retained, by inheritance, more of the character of its ancient progenitor
than have other Rodents; and therefore it will not be specially related to
any one existing Marsupial, but indirectly to all or nearly all
Marsupials, from having partially retained the character of their common
progenitor, or of an early member of the group. On the other hand, of all
Marsupials, as Mr. Waterhouse has remarked, the phascolomys resembles most
nearly, not any one species, but the general order of Rodents. In this
case, however, it may be strongly suspected that the resemblance is only
analogical, owing to the phascolomys having become adapted to habits like
those of a Rodent. The elder De Candolle has made nearly similar
observations on the general nature of the affinities of distinct orders of
plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common parent, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
parent of a whole family of species, now broken up by extinction into
distinct groups and sub-groups, will have transmitted some of its
characters, modified in various ways and degrees, to all; and the several
species will consequently be related to each other by circuitous lines of
affinity of various lengths (as may be seen in the diagram so often
referred to), mounting up through many predecessors. As it is difficult to
show the blood-relationship between the numerous kindred of any ancient
and noble family, even by the aid of a genealogical tree, and almost
impossible to do this without this aid, we can understand the
extraordinary difficulty which naturalists have experienced in describing,
without the aid of a diagram, the various affinities which they perceive
between the many living and extinct members of the same great natural
class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account even for the distinctness of whole classes
from each other—for instance, of birds from all other vertebrate
animals—by the belief that many ancient forms of life have been
utterly lost, through which the early progenitors of birds were formerly
connected with the early progenitors of the other vertebrate classes.
There has been less entire extinction of the forms of life which once
connected fishes with batrachians. There has been still less in some other
classes, as in that of the Crustacea, for here the most wonderfully
diverse forms are still tied together by a long, but broken, chain of
affinities. Extinction has only separated groups: it has by no means made
them; for if every form which has ever lived on this earth were suddenly
to reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all would
blend together by steps as fine as those between the finest existing
varieties, nevertheless a natural classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the
diagram: the letters, A to L, may represent eleven Silurian genera, some
of which have produced large groups of modified descendants. Every
intermediate link between these eleven genera and their primordial parent,
and every intermediate link in each branch and sub-branch of their
descendants, may be supposed to be still alive; and the links to be as
fine as those between the finest varieties. In this case it would be quite
impossible to give any definition by which the several members of the
several groups could be distinguished from their more immediate parents;
or these parents from their ancient and unknown progenitor. Yet the
natural arrangement in the diagram would still hold good; and, on the
principle of inheritance, all the forms descended from A, or from I, would
have something in common. In a tree we can specify this or that branch,
though at the actual fork the two unite and blend together. We could not,
as I have said, define the several groups; but we could pick out types, or
forms, representing most of the characters of each group, whether large or
small, and thus give a general idea of the value of the differences
between them. This is what we should be driven to, if we were ever to
succeed in collecting all the forms in any class which have lived
throughout all time and space. We shall certainly never succeed in making
so perfect a collection: nevertheless, in certain classes, we are tending
in this direction; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction and
divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals of
both sexes and of all ages, although having few characters in common,
under one species; we use descent in classing acknowledged varieties,
however different they may be from their parent; and I believe this
element of descent is the hidden bond of connexion which naturalists have
sought under the term of the Natural System. On this idea of the natural
system being, in so far as it has been perfected, genealogical in its
arrangement, with the grades of difference between the descendants from a
common parent, expressed by the terms genera, families, orders, etc., we
can understand the rules which we are compelled to follow in our
classification. We can understand why we value certain resemblances far
more than others; why we are permitted to use rudimentary and useless
organs, or others of trifling physiological importance; why, in comparing
one group with a distinct group, we summarily reject analogical or
adaptive characters, and yet use these same characters within the limits
of the same group. We can clearly see how it is that all living and
extinct forms can be grouped together in one great system; and how the
several members of each class are connected together by the most complex
and radiating lines of affinities. We shall never, probably, disentangle
the inextricable web of affinities between the members of any one class;
but when we have a distinct object in view, and do not look to some
unknown plan of creation, we may hope to make sure but slow progress.
MORPHOLOGY.
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general name of Morphology. This is the most interesting department of
natural history, and may be said to be its very soul. What can be more
curious than that the hand of a man, formed for grasping, that of a mole
for digging, the leg of the horse, the paddle of the porpoise, and the
wing of the bat, should all be constructed on the same pattern, and should
include the same bones, in the same relative positions? Geoffroy St.
Hilaire has insisted strongly on the high importance of relative connexion
in homologous organs: the parts may change to almost any extent in form
and size, and yet they always remain connected together in the same order.
We never find, for instance, the bones of the arm and forearm, or of the
thigh and leg, transposed. Hence the same names can be given to the
homologous bones in widely different animals. We see the same great law in
the construction of the mouths of insects: what can be more different than
the immensely long spiral proboscis of a sphinx-moth, the curious folded
one of a bee or bug, and the great jaws of a beetle?—yet all these
organs, serving for such different purposes, are formed by infinitely
numerous modifications of an upper lip, mandibles, and two pairs of
maxillae. Analogous laws govern the construction of the mouths and limbs
of crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the 'Nature of Limbs.' On the
ordinary view of the independent creation of each being, we can only say
that so it is;—that it has so pleased the Creator to construct each
animal and plant.
The explanation is manifest on the theory of the natural selection of
successive slight modifications,—each modification being profitable
in some way to the modified form, but often affecting by correlation of
growth other parts of the organisation. In changes of this nature, there
will be little or no tendency to modify the original pattern, or to
transpose parts. The bones of a limb might be shortened and widened to any
extent, and become gradually enveloped in thick membrane, so as to serve
as a fin; or a webbed foot might have all its bones, or certain bones,
lengthened to any extent, and the membrane connecting them increased to
any extent, so as to serve as a wing: yet in all this great amount of
modification there will be no tendency to alter the framework of bones or
the relative connexion of the several parts. If we suppose that the
ancient progenitor, the archetype as it may be called, of all mammals, had
its limbs constructed on the existing general pattern, for whatever
purpose they served, we can at once perceive the plain signification of
the homologous construction of the limbs throughout the whole class. So
with the mouths of insects, we have only to suppose that their common
progenitor had an upper lip, mandibles, and two pair of maxillae, these
parts being perhaps very simple in form; and then natural selection will
account for the infinite diversity in structure and function of the mouths
of insects. Nevertheless, it is conceivable that the general pattern of an
organ might become so much obscured as to be finally lost, by the atrophy
and ultimately by the complete abortion of certain parts, by the soldering
together of other parts, and by the doubling or multiplication of others,—variations
which we know to be within the limits of possibility. In the paddles of
the extinct gigantic sea-lizards, and in the mouths of certain suctorial
crustaceans, the general pattern seems to have been thus to a certain
extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual. Most
physiologists believe that the bones of the skull are homologous with—that
is correspond in number and in relative connexion with—the elemental
parts of a certain number of vertebrae. The anterior and posterior limbs
in each member of the vertebrate and articulate classes are plainly
homologous. We see the same law in comparing the wonderfully complex jaws
and legs in crustaceans. It is familiar to almost every one, that in a
flower the relative position of the sepals, petals, stamens, and pistils,
as well as their intimate structure, are intelligible on the view that
they consist of metamorphosed leaves, arranged in a spire. In monstrous
plants, we often get direct evidence of the possibility of one organ being
transformed into another; and we can actually see in embryonic crustaceans
and in many other animals, and in flowers, that organs, which when mature
become extremely different, are at an early stage of growth exactly alike.
How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the benefit
derived from the yielding of the separate pieces in the act of parturition
of mammals, will by no means explain the same construction in the skulls
of birds. Why should similar bones have been created in the formation of
the wing and leg of a bat, used as they are for such totally different
purposes? Why should one crustacean, which has an extremely complex mouth
formed of many parts, consequently always have fewer legs; or conversely,
those with many legs have simpler mouths? Why should the sepals, petals,
stamens, and pistils in any individual flower, though fitted for such
widely different purposes, be all constructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages; and
in flowering plants, we see a series of successive spiral whorls of
leaves. An indefinite repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or little-modified forms;
therefore we may readily believe that the unknown progenitor of the
vertebrata possessed many vertebrae; the unknown progenitor of the
articulata, many segments; and the unknown progenitor of flowering plants,
many spiral whorls of leaves. We have formerly seen that parts many times
repeated are eminently liable to vary in number and structure;
consequently it is quite probable that natural selection, during a
long-continued course of modification, should have seized on a certain
number of the primordially similar elements, many times repeated, and have
adapted them to the most diverse purposes. And as the whole amount of
modification will have been effected by slight successive steps, we need
not wonder at discovering in such parts or organs, a certain degree of
fundamental resemblance, retained by the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts of one
species with those of another and distinct species, we can indicate but
few serial homologies; that is, we are seldom enabled to say that one part
or organ is homologous with another in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one
part, as we find in the other great classes of the animal and vegetable
kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebrae, both jaws and legs, etc.,—as having been
metamorphosed, not one from the other, but from some common element.
Naturalists, however, use such language only in a metaphorical sense: they
are far from meaning that during a long course of descent, primordial
organs of any kind—vertebrae in the one case and legs in the other—have
actually been modified into skulls or jaws. Yet so strong is the
appearance of a modification of this nature having occurred, that
naturalists can hardly avoid employing language having this plain
signification. On my view these terms may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters, which they would probably have retained through inheritance,
if they had really been metamorphosed during a long course of descent from
true legs, or from some simple appendage, is explained.
EMBRYOLOGY.
It has already been casually remarked that certain organs in the
individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos, also, of
distinct animals within the same class are often strikingly similar: a
better proof of this cannot be given, than a circumstance mentioned by
Agassiz, namely, that having forgotten to ticket the embryo of some
vertebrate animal, he cannot now tell whether it be that of a mammal,
bird, or reptile. The vermiform larvae of moths, flies, beetles, etc.,
resemble each other much more closely than do the mature insects; but in
the case of larvae, the embryos are active, and have been adapted for
special lines of life. A trace of the law of embryonic resemblance,
sometimes lasts till a rather late age: thus birds of the same genus, and
of closely allied genera, often resemble each other in their first and
second plumage; as we see in the spotted feathers in the thrush group. In
the cat tribe, most of the species are striped or spotted in lines; and
stripes can be plainly distinguished in the whelp of the lion. We
occasionally though rarely see something of this kind in plants: thus the
embryonic leaves of the ulex or furze, and the first leaves of the
phyllodineous acaceas, are pinnate or divided like the ordinary leaves of
the leguminosae.
The points of structure, in which the embryos of widely different animals
of the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like course of the
arteries near the branchial slits are related to similar conditions,—in
the young mammal which is nourished in the womb of its mother, in the egg
of the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation, than we have
to believe that the same bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one will
suppose that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals, or are related to the
conditions to which they are exposed.
The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. From such special adaptations,
the similarity of the larvae or active embryos of allied animals is
sometimes much obscured; and cases could be given of the larvae of two
species, or of two groups of species, differing quite as much, or even
more, from each other than do their adult parents. In most cases, however,
the larvae, though active, still obey more or less closely the law of
common embryonic resemblance. Cirripedes afford a good instance of this:
even the illustrious Cuvier did not perceive that a barnacle was, as it
certainly is, a crustacean; but a glance at the larva shows this to be the
case in an unmistakeable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several stages barely
distinguishable.
The embryo in the course of development generally rises in organisation: I
use this expression, though I am aware that it is hardly possible to
define clearly what is meant by the organisation being higher or lower.
But no one probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal is generally
considered as lower in the scale than the larva, as with certain parasitic
crustaceans. To refer once again to cirripedes: the larvae in the first
stage have three pairs of legs, a very simple single eye, and a
probosciformed mouth, with which they feed largely, for they increase much
in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory legs,
a pair of magnificent compound eyes, and extremely complex antennae; but
they have a closed and imperfect mouth, and cannot feed: their function at
this stage is, to search by their well-developed organs of sense, and to
reach by their active powers of swimming, a proper place on which to
become attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennae, and their two eyes are now reconverted into a minute,
single, and very simple eye-spot. In this last and complete state,
cirripedes may be considered as either more highly or more lowly organised
than they were in the larval condition. But in some genera the larvae
become developed either into hermaphrodites having the ordinary structure,
or into what I have called complemental males: and in the latter, the
development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time, and is destitute of mouth, stomach, or other
organ of importance, excepting for reproduction.
We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or
the fin of a porpoise, should not have been sketched out with all the
parts in proper proportion, as soon as any structure became visible in the
embryo. And in some whole groups of animals and in certain members of
other groups, the embryo does not at any period differ widely from the
adult: thus Owen has remarked in regard to cuttle-fish, "there is no
metamorphosis; the cephalopodic character is manifested long before the
parts of the embryo are completed;" and again in spiders, "there is
nothing worthy to be called a metamorphosis." The larvae of insects,
whether adapted to the most diverse and active habits, or quite inactive,
being fed by their parents or placed in the midst of proper nutriment, yet
nearly all pass through a similar worm-like stage of development; but in
some few cases, as in that of Aphis, if we look to the admirable drawings
by Professor Huxley of the development of this insect, we see no trace of
the vermiform stage.
How, then, can we explain these several facts in embryology,—namely
the very general, but not universal difference in structure between the
embryo and the adult;—of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being at
this early period of growth alike;—of embryos of different species
within the same class, generally, but not universally, resembling each
other;—of the structure of the embryo not being closely related to
its conditions of existence, except when the embryo becomes at any period
of life active and has to provide for itself;—of the embryo
apparently having sometimes a higher organisation than the mature animal,
into which it is developed. I believe that all these facts can be
explained, as follows, on the view of descent with modification.
It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily appear
at an equally early period. But we have little evidence on this head—indeed
the evidence rather points the other way; for it is notorious that
breeders of cattle, horses, and various fancy animals, cannot positively
tell, until some time after the animal has been born, what its merits or
form will ultimately turn out. We see this plainly in our own children; we
cannot always tell whether the child will be tall or short, or what its
precise features will be. The question is not, at what period of life any
variation has been caused, but at what period it is fully displayed. The
cause may have acted, and I believe generally has acted, even before the
embryo is formed; and the variation may be due to the male and female
sexual elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect thus
caused at a very early period, even before the formation of the embryo,
may appear late in life; as when an hereditary disease, which appears in
old age alone, has been communicated to the offspring from the
reproductive element of one parent. Or again, as when the horns of
cross-bred cattle have been affected by the shape of the horns of either
parent. For the welfare of a very young animal, as long as it remains in
its mother's womb, or in the egg, or as long as it is nourished and
protected by its parent, it must be quite unimportant whether most of its
characters are fully acquired a little earlier or later in life. It would
not signify, for instance, to a bird which obtained its food best by
having a long beak, whether or not it assumed a beak of this particular
length, as long as it was fed by its parents. Hence, I conclude, that it
is quite possible, that each of the many successive modifications, by
which each species has acquired its present structure, may have supervened
at a not very early period of life; and some direct evidence from our
domestic animals supports this view. But in other cases it is quite
possible that each successive modification, or most of them, may have
appeared at an extremely early period.
I have stated in the first chapter, that there is some evidence to render
it probable, that at whatever age any variation first appears in the
parent, it tends to reappear at a corresponding age in the offspring.
Certain variations can only appear at corresponding ages, for instance,
peculiarities in the caterpillar, cocoon, or imago states of the
silk-moth; or, again, in the horns of almost full-grown cattle. But
further than this, variations which, for all that we can see, might have
appeared earlier or later in life, tend to appear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably
the case; and I could give a good many cases of variations (taking the
word in the largest sense) which have supervened at an earlier age in the
child than in the parent.
These two principles, if their truth be admitted, will, I believe, explain
all the above specified leading facts in embryology. But first let us look
at a few analogous cases in domestic varieties. Some authors who have
written on Dogs, maintain that the greyhound and bulldog, though appearing
so different, are really varieties most closely allied, and have probably
descended from the same wild stock; hence I was curious to see how far
their puppies differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging by the eye,
seemed almost to be the case; but on actually measuring the old dogs and
their six-days old puppies, I found that the puppies had not nearly
acquired their full amount of proportional difference. So, again, I was
told that the foals of cart and race-horses differed as much as the
full-grown animals; and this surprised me greatly, as I think it probable
that the difference between these two breeds has been wholly caused by
selection under domestication; but having had careful measurements made of
the dam and of a three-days old colt of a race and heavy cart-horse, I
find that the colts have by no means acquired their full amount of
proportional difference.
As the evidence appears to me conclusive, that the several domestic breeds
of Pigeon have descended from one wild species, I compared young pigeons
of various breeds, within twelve hours after being hatched; I carefully
measured the proportions (but will not here give details) of the beak,
width of mouth, length of nostril and of eyelid, size of feet and length
of leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,
carriers, and tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they would, I cannot
doubt, be ranked in distinct genera, had they been natural productions.
But when the nestling birds of these several breeds were placed in a row,
though most of them could be distinguished from each other, yet their
proportional differences in the above specified several points were
incomparably less than in the full-grown birds. Some characteristic points
of difference—for instance, that of the width of mouth—could
hardly be detected in the young. But there was one remarkable exception to
this rule, for the young of the short-faced tumbler differed from the
young of the wild rock-pigeon and of the other breeds, in all its
proportions, almost exactly as much as in the adult state.
The two principles above given seem to me to explain these facts in regard
to the later embryonic stages of our domestic varieties. Fanciers select
their horses, dogs, and pigeons, for breeding, when they are nearly grown
up: they are indifferent whether the desired qualities and structures have
been acquired earlier or later in life, if the full-grown animal possesses
them. And the cases just given, more especially that of pigeons, seem to
show that the characteristic differences which give value to each breed,
and which have been accumulated by man's selection, have not generally
first appeared at an early period of life, and have been inherited by the
offspring at a corresponding not early period. But the case of the
short-faced tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier period
than usual, or, if not so, the differences must have been inherited, not
at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles—which
latter, though not proved true, can be shown to be in some degree probable—to
species in a state of nature. Let us take a genus of birds, descended on
my theory from some one parent-species, and of which the several new
species have become modified through natural selection in accordance with
their diverse habits. Then, from the many slight successive steps of
variation having supervened at a rather late age, and having been
inherited at a corresponding age, the young of the new species of our
supposed genus will manifestly tend to resemble each other much more
closely than do the adults, just as we have seen in the case of pigeons.
We may extend this view to whole families or even classes. The fore-limbs,
for instance, which served as legs in the parent-species, may become, by a
long course of modification, adapted in one descendant to act as hands, in
another as paddles, in another as wings; and on the above two principles—namely
of each successive modification supervening at a rather late age, and
being inherited at a corresponding late age—the fore-limbs in the
embryos of the several descendants of the parent-species will still
resemble each other closely, for they will not have been modified. But in
each individual new species, the embryonic fore-limbs will differ greatly
from the fore-limbs in the mature animal; the limbs in the latter having
undergone much modification at a rather late period of life, and having
thus been converted into hands, or paddles, or wings. Whatever influence
long-continued exercise or use on the one hand, and disuse on the other,
may have in modifying an organ, such influence will mainly affect the
mature animal, which has come to its full powers of activity and has to
gain its own living; and the effects thus produced will be inherited at a
corresponding mature age. Whereas the young will remain unmodified, or be
modified in a lesser degree, by the effects of use and disuse.
In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life, or
each step might be inherited at an earlier period than that at which it
first appeared. In either case (as with the short-faced tumbler) the young
or embryo would closely resemble the mature parent-form. We have seen that
this is the rule of development in certain whole groups of animals, as
with cuttle-fish and spiders, and with a few members of the great class of
insects, as with Aphis. With respect to the final cause of the young in
these cases not undergoing any metamorphosis, or closely resembling their
parents from their earliest age, we can see that this would result from
the two following contingencies; firstly, from the young, during a course
of modification carried on for many generations, having to provide for
their own wants at a very early stage of development, and secondly, from
their following exactly the same habits of life with their parents; for in
this case, it would be indispensable for the existence of the species,
that the child should be modified at a very early age in the same manner
with its parents, in accordance with their similar habits. Some further
explanation, however, of the embryo not undergoing any metamorphosis is
perhaps requisite. If, on the other hand, it profited the young to follow
habits of life in any degree different from those of their parent, and
consequently to be constructed in a slightly different manner, then, on
the principle of inheritance at corresponding ages, the active young or
larvae might easily be rendered by natural selection different to any
conceivable extent from their parents. Such differences might, also,
become correlated with successive stages of development; so that the
larvae, in the first stage, might differ greatly from the larvae in the
second stage, as we have seen to be the case with cirripedes. The adult
might become fitted for sites or habits, in which organs of locomotion or
of the senses, etc., would be useless; and in this case the final
metamorphosis would be said to be retrograde.
As all the organic beings, extinct and recent, which have ever lived on
this earth have to be classed together, and as all have been connected by
the finest gradations, the best, or indeed, if our collections were nearly
perfect, the only possible arrangement, would be genealogical. Descent
being on my view the hidden bond of connexion which naturalists have been
seeking under the term of the natural system. On this view we can
understand how it is that, in the eyes of most naturalists, the structure
of the embryo is even more important for classification than that of the
adult. For the embryo is the animal in its less modified state; and in so
far it reveals the structure of its progenitor. In two groups of animal,
however much they may at present differ from each other in structure and
habits, if they pass through the same or similar embryonic stages, we may
feel assured that they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related. Thus, community
in embryonic structure reveals community of descent. It will reveal this
community of descent, however much the structure of the adult may have
been modified and obscured; we have seen, for instance, that cirripedes
can at once be recognised by their larvae as belonging to the great class
of crustaceans. As the embryonic state of each species and group of
species partially shows us the structure of their less modified ancient
progenitors, we can clearly see why ancient and extinct forms of life
should resemble the embryos of their descendants,—our existing
species. Agassiz believes this to be a law of nature; but I am bound to
confess that I only hope to see the law hereafter proved true. It can be
proved true in those cases alone in which the ancient state, now supposed
to be represented in many embryos, has not been obliterated, either by the
successive variations in a long course of modification having supervened
at a very early age, or by the variations having been inherited at an
earlier period than that at which they first appeared. It should also be
borne in mind, that the supposed law of resemblance of ancient forms of
life to the embryonic stages of recent forms, may be true, but yet, owing
to the geological record not extending far enough back in time, may remain
for a long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are second
in importance to none in natural history, are explained on the principle
of slight modifications not appearing, in the many descendants from some
one ancient progenitor, at a very early period in the life of each, though
perhaps caused at the earliest, and being inherited at a corresponding not
early period. Embryology rises greatly in interest, when we thus look at
the embryo as a picture, more or less obscured, of the common parent-form
of each great class of animals.
RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.
Organs or parts in this strange condition, bearing the stamp of inutility,
are extremely common throughout nature. For instance, rudimentary mammae
are very general in the males of mammals: I presume that the
"bastard-wing" in birds may be safely considered as a digit in a
rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and hind
limbs. Some of the cases of rudimentary organs are extremely curious; for
instance, the presence of teeth in foetal whales, which when grown up have
not a tooth in their heads; and the presence of teeth, which never cut
through the gums, in the upper jaws of our unborn calves. It has even been
stated on good authority that rudiments of teeth can be detected in the
beaks of certain embryonic birds. Nothing can be plainer than that wings
are formed for flight, yet in how many insects do we see wings so reduced
in size as to be utterly incapable of flight, and not rarely lying under
wing-cases, firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one of which
will have full-sized wings, and another mere rudiments of membrane; and
here it is impossible to doubt, that the rudiments represent wings.
Rudimentary organs sometimes retain their potentiality, and are merely not
developed: this seems to be the case with the mammae of male mammals, for
many instances are on record of these organs having become well developed
in full-grown males, and having secreted milk. So again there are normally
four developed and two rudimentary teats in the udders of the genus Bos,
but in our domestic cows the two sometimes become developed and give milk.
In individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil; and
Kolreuter found that by crossing such male plants with an hermaphrodite
species, the rudiment of the pistil in the hybrid offspring was much
increased in size; and this shows that the rudiment and the perfect pistil
are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules protected in the ovarium at its
base. The pistil consists of a stigma supported on the style; but in some
Compositae, the male florets, which of course cannot be fecundated, have a
pistil, which is in a rudimentary state, for it is not crowned with a
stigma; but the style remains well developed, and is clothed with hairs as
in other compositae, for the purpose of brushing the pollen out of the
surrounding anthers. Again, an organ may become rudimentary for its proper
purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or lung.
Other similar instances could be given.
Rudimentary organs in the individuals of the same species are very liable
to vary in degree of development and in other respects. Moreover, in
closely allied species, the degree to which the same organ has been
rendered rudimentary occasionally differs much. This latter fact is well
exemplified in the state of the wings of the female moths in certain
groups. Rudimentary organs may be utterly aborted; and this implies, that
we find in an animal or plant no trace of an organ, which analogy would
lead us to expect to find, and which is occasionally found in monstrous
individuals of the species. Thus in the snapdragon (antirrhinum) we
generally do not find a rudiment of a fifth stamen; but this may sometimes
be seen. In tracing the homologies of the same part in different members
of a class, nothing is more common, or more necessary, than the use and
discovery of rudiments. This is well shown in the drawings given by Owen
of the bones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the embryo,
but afterwards wholly disappear. It is also, I believe, a universal rule,
that a rudimentary part or organ is of greater size relatively to the
adjoining parts in the embryo, than in the adult; so that the organ at
this early age is less rudimentary, or even cannot be said to be in any
degree rudimentary. Hence, also, a rudimentary organ in the adult, is
often said to have retained its embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment: for the
same reasoning power which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs, are imperfect and useless. In
works on natural history rudimentary organs are generally said to have
been created "for the sake of symmetry," or in order "to complete the
scheme of nature;" but this seems to me no explanation, merely a
restatement of the fact. Would it be thought sufficient to say that
because planets revolve in elliptic courses round the sun, satellites
follow the same course round the planets, for the sake of symmetry, and to
complete the scheme of nature? An eminent physiologist accounts for the
presence of rudimentary organs, by supposing that they serve to excrete
matter in excess, or injurious to the system; but can we suppose that the
minute papilla, which often represents the pistil in male flowers, and
which is formed merely of cellular tissue, can thus act? Can we suppose
that the formation of rudimentary teeth which are subsequently absorbed,
can be of any service to the rapidly growing embryonic calf by the
excretion of precious phosphate of lime? When a man's fingers have been
amputated, imperfect nails sometimes appear on the stumps: I could as soon
believe that these vestiges of nails have appeared, not from unknown laws
of growth, but in order to excrete horny matter, as that the rudimentary
nails on the fin of the manatee were formed for this purpose.
On my view of descent with modification, the origin of rudimentary organs
is simple. We have plenty of cases of rudimentary organs in our domestic
productions,—as the stump of a tail in tailless breeds,—the
vestige of an ear in earless breeds,—the reappearance of minute
dangling horns in hornless breeds of cattle, more especially, according to
Youatt, in young animals,—and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in monsters. But I
doubt whether any of these cases throw light on the origin of rudimentary
organs in a state of nature, further than by showing that rudiments can be
produced; for I doubt whether species under nature ever undergo abrupt
changes. I believe that disuse has been the main agency; that it has led
in successive generations to the gradual reduction of various organs,
until they have become rudimentary,—as in the case of the eyes of
animals inhabiting dark caverns, and of the wings of birds inhabiting
oceanic islands, which have seldom been forced to take flight, and have
ultimately lost the power of flying. Again, an organ useful under certain
conditions, might become injurious under others, as with the wings of
beetles living on small and exposed islands; and in this case natural
selection would continue slowly to reduce the organ, until it was rendered
harmless and rudimentary.
Any change in function, which can be effected by insensibly small steps,
is within the power of natural selection; so that an organ rendered,
during changed habits of life, useless or injurious for one purpose, might
easily be modified and used for another purpose. Or an organ might be
retained for one alone of its former functions. An organ, when rendered
useless, may well be variable, for its variations cannot be checked by
natural selection. At whatever period of life disuse or selection reduces
an organ, and this will generally be when the being has come to maturity
and to its full powers of action, the principle of inheritance at
corresponding ages will reproduce the organ in its reduced state at the
same age, and consequently will seldom affect or reduce it in the embryo.
Thus we can understand the greater relative size of rudimentary organs in
the embryo, and their lesser relative size in the adult. But if each step
of the process of reduction were to be inherited, not at the corresponding
age, but at an extremely early period of life (as we have good reason to
believe to be possible) the rudimentary part would tend to be wholly lost,
and we should have a case of complete abortion. The principle, also, of
economy, explained in a former chapter, by which the materials forming any
part or structure, if not useful to the possessor, will be saved as far as
is possible, will probably often come into play; and this will tend to
cause the entire obliteration of a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency in every
part of the organisation, which has long existed, to be inherited—we
can understand, on the genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary
organs may be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which serve as a
clue in seeking for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary
doctrine of creation, might even have been anticipated, and can be
accounted for by the laws of inheritance.
SUMMARY.
In this chapter I have attempted to show, that the subordination of group
to group in all organisms throughout all time; that the nature of the
relationship, by which all living and extinct beings are united by
complex, radiating, and circuitous lines of affinities into one grand
system; the rules followed and the difficulties encountered by naturalists
in their classifications; the value set upon characters, if constant and
prevalent, whether of high vital importance, or of the most trifling
importance, or, as in rudimentary organs, of no importance; the wide
opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules;—all naturally
follow on the view of the common parentage of those forms which are
considered by naturalists as allied, together with their modification
through natural selection, with its contingencies of extinction and
divergence of character. In considering this view of classification, it
should be borne in mind that the element of descent has been universally
used in ranking together the sexes, ages, and acknowledged varieties of
the same species, however different they may be in structure. If we extend
the use of this element of descent,—the only certainly known cause
of similarity in organic beings,—we shall understand what is meant
by the natural system: it is genealogical in its attempted arrangement,
with the grades of acquired difference marked by the terms varieties,
species, genera, families, orders, and classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible,—whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on
the same pattern in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from
each other in structure and function; and the resemblance in different
species of a class of the homologous parts or organs, though fitted in the
adult members for purposes as different as possible. Larvae are active
embryos, which have become specially modified in relation to their habits
of life, through the principle of modifications being inherited at
corresponding ages. On this same principle—and bearing in mind, that
when organs are reduced in size, either from disuse or selection, it will
generally be at that period of life when the being has to provide for its
own wants, and bearing in mind how strong is the principle of inheritance—the
occurrence of rudimentary organs and their final abortion, present to us
no inexplicable difficulties; on the contrary, their presence might have
been even anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view that an
arrangement is only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled,
have all descended, each within its own class or group, from common
parents, and have all been modified in the course of descent, that I
should without hesitation adopt this view, even if it were unsupported by
other facts or arguments.
14. RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species. How far the
theory of natural selection may be extended. Effects of its adoption on
the study of Natural history. Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I have
endeavoured to give to them their full force. Nothing at first can appear
more difficult to believe than that the more complex organs and instincts
should have been perfected, not by means superior to, though analogous
with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot
be considered real if we admit the following propositions, namely,—that
gradations in the perfection of any organ or instinct, which we may
consider, either do now exist or could have existed, each good of its
kind,—that all organs and instincts are, in ever so slight a degree,
variable,—and, lastly, that there is a struggle for existence
leading to the preservation of each profitable deviation of structure or
instinct. The truth of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially amongst broken and
failing groups of organic beings; but we see so many strange gradations in
nature, as is proclaimed by the canon, "Natura non facit saltum," that we
ought to be extremely cautious in saying that any organ or instinct, or
any whole being, could not have arrived at its present state by many
graduated steps. There are, it must be admitted, cases of special
difficulty on the theory of natural selection; and one of the most curious
of these is the existence of two or three defined castes of workers or
sterile females in the same community of ants; but I have attempted to
show how this difficulty can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted together, but
that it is incidental on constitutional differences in the reproductive
systems of the intercrossed species. We see the truth of this conclusion
in the vast difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first used as the
father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds are
rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need not
feel surprise at hybrids being in some degree sterile, for their
constitutions can hardly fail to have been disturbed from being compounded
of two distinct organisations. This parallelism is supported by another
parallel, but directly opposite, class of facts; namely, that the vigour
and fertility of all organic beings are increased by slight changes in
their conditions of life, and that the offspring of slightly modified
forms or varieties acquire from being crossed increased vigour and
fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of life
and crosses between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are grave enough. All the individuals
of the same species, and all the species of the same genus, or even higher
group, must have descended from common parents; and therefore, in however
distant and isolated parts of the world they are now found, they must in
the course of successive generations have passed from some one part to the
others. We are often wholly unable even to conjecture how this could have
been effected. Yet, as we have reason to believe that some species have
retained the same specific form for very long periods, enormously long as
measured by years, too much stress ought not to be laid on the occasional
wide diffusion of the same species; for during very long periods of time
there will always be a good chance for wide migration by many means. A
broken or interrupted range may often be accounted for by the extinction
of the species in the intermediate regions. It cannot be denied that we
are as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern periods;
and such changes will obviously have greatly facilitated migration. As an
example, I have attempted to show how potent has been the influence of the
Glacial period on the distribution both of the same and of representative
species throughout the world. We are as yet profoundly ignorant of the
many occasional means of transport. With respect to distinct species of
the same genus inhabiting very distant and isolated regions, as the
process of modification has necessarily been slow, all the means of
migration will have been possible during a very long period; and
consequently the difficulty of the wide diffusion of species of the same
genus is in some degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our present varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With respect
to existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover DIRECTLY connecting links between
them, but only between each and some extinct and supplanted form. Even on
a wide area, which has during a long period remained continuous, and of
which the climate and other conditions of life change insensibly in going
from a district occupied by one species into another district occupied by
a closely allied species, we have no just right to expect often to find
intermediate varieties in the intermediate zone. For we have reason to
believe that only a few species are undergoing change at any one period;
and all changes are slowly effected. I have also shown that the
intermediate varieties which will at first probably exist in the
intermediate zones, will be liable to be supplanted by the allied forms on
either hand; and the latter, from existing in greater numbers, will
generally be modified and improved at a quicker rate than the intermediate
varieties, which exist in lesser numbers; so that the intermediate
varieties will, in the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why is
not every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? We meet with no such evidence, and this is
the most obvious and forcible of the many objections which may be urged
against my theory. Why, again, do whole groups of allied species appear,
though certainly they often falsely appear, to have come in suddenly on
the several geological stages? Why do we not find great piles of strata
beneath the Silurian system, stored with the remains of the progenitors of
the Silurian groups of fossils? For certainly on my theory such strata
must somewhere have been deposited at these ancient and utterly unknown
epochs in the world's history.
I can answer these questions and grave objections only on the supposition
that the geological record is far more imperfect than most geologists
believe. It cannot be objected that there has not been time sufficient for
any amount of organic change; for the lapse of time has been so great as
to be utterly inappreciable by the human intellect. The number of
specimens in all our museums is absolutely as nothing compared with the
countless generations of countless species which certainly have existed.
We should not be able to recognise a species as the parent of any one or
more species if we were to examine them ever so closely, unless we
likewise possessed many of the intermediate links between their past or
parent and present states; and these many links we could hardly ever
expect to discover, owing to the imperfection of the geological record.
Numerous existing doubtful forms could be named which are probably
varieties; but who will pretend that in future ages so many fossil links
will be discovered, that naturalists will be able to decide, on the common
view, whether or not these doubtful forms are varieties? As long as most
of the links between any two species are unknown, if any one link or
intermediate variety be discovered, it will simply be classed as another
and distinct species. Only a small portion of the world has been
geologically explored. Only organic beings of certain classes can be
preserved in a fossil condition, at least in any great number. Widely
ranging species vary most, and varieties are often at first local,—both
causes rendering the discovery of intermediate links less likely. Local
varieties will not spread into other and distant regions until they are
considerably modified and improved; and when they do spread, if discovered
in a geological formation, they will appear as if suddenly created there,
and will be simply classed as new species. Most formations have been
intermittent in their accumulation; and their duration, I am inclined to
believe, has been shorter than the average duration of specific forms.
Successive formations are separated from each other by enormous blank
intervals of time; for fossiliferous formations, thick enough to resist
future degradation, can be accumulated only where much sediment is
deposited on the subsiding bed of the sea. During the alternate periods of
elevation and of stationary level the record will be blank. During these
latter periods there will probably be more variability in the forms of
life; during periods of subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath the lowest
Silurian strata, I can only recur to the hypothesis given in the ninth
chapter. That the geological record is imperfect all will admit; but that
it is imperfect to the degree which I require, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that all species have changed; and they have changed in the
manner which my theory requires, for they have changed slowly and in a
graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to each
other, than are the fossils from formations distant from each other in
time.
Such is the sum of the several chief objections and difficulties which may
justly be urged against my theory; and I have now briefly recapitulated
the answers and explanations which can be given to them. I have felt these
difficulties far too heavily during many years to doubt their weight. But
it deserves especial notice that the more important objections relate to
questions on which we are confessedly ignorant; nor do we know how
ignorant we are. We do not know all the possible transitional gradations
between the simplest and the most perfect organs; it cannot be pretended
that we know all the varied means of Distribution during the long lapse of
years, or that we know how imperfect the Geological Record is. Grave as
these several difficulties are, in my judgment they do not overthrow the
theory of descent with modification.
Now let us turn to the other side of the argument. Under domestication we
see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life;
so that this system, when not rendered impotent, fails to reproduce
offspring exactly like the parent-form. Variability is governed by many
complex laws,—by correlation of growth, by use and disuse, and by
the direct action of the physical conditions of life. There is much
difficulty in ascertaining how much modification our domestic productions
have undergone; but we may safely infer that the amount has been large,
and that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On
the other hand we have evidence that variability, when it has once come
into play, does not wholly cease; for new varieties are still occasionally
produced by our most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally exposes
organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man can and does select the
variations given to him by nature, and thus accumulate them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful to him at the time, without any thought of
altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation,
individual differences so slight as to be quite inappreciable by an
uneducated eye. This process of selection has been the great agency in the
production of the most distinct and useful domestic breeds. That many of
the breeds produced by man have to a large extent the character of natural
species, is shown by the inextricable doubts whether very many of them are
varieties or aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In the
preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful and
ever-acting means of selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common to all
organic beings. This high rate of increase is proved by calculation, by
the effects of a succession of peculiar seasons, and by the results of
naturalisation, as explained in the third chapter. More individuals are
born than can possibly survive. A grain in the balance will determine
which individual shall live and which shall die,—which variety or
species shall increase in number, and which shall decrease, or finally
become extinct. As the individuals of the same species come in all
respects into the closest competition with each other, the struggle will
generally be most severe between them; it will be almost equally severe
between the varieties of the same species, and next in severity between
the species of the same genus. But the struggle will often be very severe
between beings most remote in the scale of nature. The slightest advantage
in one being, at any age or during any season, over those with which it
comes into competition, or better adaptation in however slight a degree to
the surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a struggle
between the males for possession of the females. The most vigorous
individuals, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will
often depend on having special weapons or means of defence, or on the
charms of the males; and the slightest advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied
under nature, in the same way as they generally have varied under the
changed conditions of domestication. And if there be any variability under
nature, it would be an unaccountable fact if natural selection had not
come into play. It has often been asserted, but the assertion is quite
incapable of proof, that the amount of variation under nature is a
strictly limited quantity. Man, though acting on external characters alone
and often capriciously, can produce within a short period a great result
by adding up mere individual differences in his domestic productions; and
every one admits that there are at least individual differences in species
under nature. But, besides such differences, all naturalists have admitted
the existence of varieties, which they think sufficiently distinct to be
worthy of record in systematic works. No one can draw any clear
distinction between individual differences and slight varieties; or
between more plainly marked varieties and sub-species, and species. Let it
be observed how naturalists differ in the rank which they assign to the
many representative forms in Europe and North America.
If then we have under nature variability and a powerful agent always ready
to act and select, why should we doubt that variations in any way useful
to beings, under their excessively complex relations of life, would be
preserved, accumulated, and inherited? Why, if man can by patience select
variations most useful to himself, should nature fail in selecting
variations useful, under changing conditions of life, to her living
products? What limit can be put to this power, acting during long ages and
rigidly scrutinising the whole constitution, structure, and habits of each
creature,—favouring the good and rejecting the bad? I can see no
limit to this power, in slowly and beautifully adapting each form to the
most complex relations of life. The theory of natural selection, even if
we looked no further than this, seems to me to be in itself probable. I
have already recapitulated, as fairly as I could, the opposed difficulties
and objections: now let us turn to the special facts and arguments in
favour of the theory.
On the view that species are only strongly marked and permanent varieties,
and that each species first existed as a variety, we can see why it is
that no line of demarcation can be drawn between species, commonly
supposed to have been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary laws. On this
same view we can understand how it is that in each region where many
species of a genus have been produced, and where they now flourish, these
same species should present many varieties; for where the manufactory of
species has been active, we might expect, as a general rule, to find it
still in action; and this is the case if varieties be incipient species.
Moreover, the species of the larger genera, which afford the greater
number of varieties or incipient species, retain to a certain degree the
character of varieties; for they differ from each other by a less amount
of difference than do the species of smaller genera. The closely allied
species also of the larger genera apparently have restricted ranges, and
they are clustered in little groups round other species—in which
respects they resemble varieties. These are strange relations on the view
of each species having been independently created, but are intelligible if
all species first existed as varieties.
As each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by so much the more as they become more
diversified in habits and structure, so as to be enabled to seize on many
and widely different places in the economy of nature, there will be a
constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence during a long-continued course of
modification, the slight differences, characteristic of varieties of the
same species, tend to be augmented into the greater differences
characteristic of species of the same genus. New and improved varieties
will inevitably supplant and exterminate the older, less improved and
intermediate varieties; and thus species are rendered to a large extent
defined and distinct objects. Dominant species belonging to the larger
groups tend to give birth to new and dominant forms; so that each large
group tends to become still larger, and at the same time more divergent in
character. But as all groups cannot thus succeed in increasing in size,
for the world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in size
and diverging in character, together with the almost inevitable
contingency of much extinction, explains the arrangement of all the forms
of life, in groups subordinate to groups, all within a few great classes,
which we now see everywhere around us, and which has prevailed throughout
all time. This grand fact of the grouping of all organic beings seems to
me utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification; it
can act only by very short and slow steps. Hence the canon of "Natura non
facit saltum," which every fresh addition to our knowledge tends to make
more strictly correct, is on this theory simply intelligible. We can
plainly see why nature is prodigal in variety, though niggard in
innovation. But why this should be a law of nature if each species has
been independently created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have been
created to prey on insects on the ground; that upland geese, which never
or rarely swim, should have been created with webbed feet; that a thrush
should have been created to dive and feed on sub-aquatic insects; and that
a petrel should have been created with habits and structure fitting it for
the life of an auk or grebe! and so on in endless other cases. But on the
view of each species constantly trying to increase in number, with natural
selection always ready to adapt the slowly varying descendants of each to
any unoccupied or ill-occupied place in nature, these facts cease to be
strange, or perhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants of
each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any one
country, although on the ordinary view supposed to have been specially
created and adapted for that country, being beaten and supplanted by the
naturalised productions from another land. Nor ought we to marvel if all
the contrivances in nature be not, as far as we can judge, absolutely
perfect; and if some of them be abhorrent to our ideas of fitness. We need
not marvel at the sting of the bee causing the bee's own death; at drones
being produced in such vast numbers for one single act, and being then
slaughtered by their sterile sisters; at the astonishing waste of pollen
by our fir-trees; at the instinctive hatred of the queen bee for her own
fertile daughters; at ichneumonidae feeding within the live bodies of
caterpillars; and at other such cases. The wonder indeed is, on the theory
of natural selection, that more cases of the want of absolute perfection
have not been observed.
The complex and little known laws governing variation are the same, as far
as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone, they
occasionally assume some of the characters of the species proper to that
zone. In both varieties and species, use and disuse seem to have produced
some effect; for it is difficult to resist this conclusion when we look,
for instance, at the logger-headed duck, which has wings incapable of
flight, in nearly the same condition as in the domestic duck; or when we
look at the burrowing tucutucu, which is occasionally blind, and then at
certain moles, which are habitually blind and have their eyes covered with
skin; or when we look at the blind animals inhabiting the dark caves of
America and Europe. In both varieties and species correlation of growth
seems to have played a most important part, so that when one part has been
modified other parts are necessarily modified. In both varieties and
species reversions to long-lost characters occur. How inexplicable on the
theory of creation is the occasional appearance of stripes on the shoulder
and legs of the several species of the horse-genus and in their hybrids!
How simply is this fact explained if we believe that these species have
descended from a striped progenitor, in the same manner as the several
domestic breeds of pigeon have descended from the blue and barred
rock-pigeon!
On the ordinary view of each species having been independently created,
why should the specific characters, or those by which the species of the
same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the colour
of a flower be more likely to vary in any one species of a genus, if the
other species, supposed to have been created independently, have
differently coloured flowers, than if all the species of the genus have
the same coloured flowers? If species are only well-marked varieties, of
which the characters have become in a high degree permanent, we can
understand this fact; for they have already varied since they branched off
from a common progenitor in certain characters, by which they have come to
be specifically distinct from each other; and therefore these same
characters would be more likely still to be variable than the generic
characters which have been inherited without change for an enormous
period. It is inexplicable on the theory of creation why a part developed
in a very unusual manner in any one species of a genus, and therefore, as
we may naturally infer, of great importance to the species, should be
eminently liable to variation; but, on my view, this part has undergone,
since the several species branched off from a common progenitor, an
unusual amount of variability and modification, and therefore we might
expect this part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and yet not
be more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long
period; for in this case it will have been rendered constant by
long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can thus
understand why nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt sometimes comes into
play in modifying instincts; but it certainly is not indispensable, as we
see, in the case of neuter insects, which leave no progeny to inherit the
effects of long-continued habit. On the view of all the species of the
same genus having descended from a common parent, and having inherited
much in common, we can understand how it is that allied species, when
placed under considerably different conditions of life, yet should follow
nearly the same instincts; why the thrush of South America, for instance,
lines her nest with mud like our British species. On the view of instincts
having been slowly acquired through natural selection we need not marvel
at some instincts being apparently not perfect and liable to mistakes, and
at many instincts causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents,—in being absorbed
into each other by successive crosses, and in other such points,—as
do the crossed offspring of acknowledged varieties. On the other hand,
these would be strange facts if species have been independently created,
and varieties have been produced by secondary laws.
If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, support the theory of descent with
modification. New species have come on the stage slowly and at successive
intervals; and the amount of change, after equal intervals of time, is
widely different in different groups. The extinction of species and of
whole groups of species, which has played so conspicuous a part in the
history of the organic world, almost inevitably follows on the principle
of natural selection; for old forms will be supplanted by new and improved
forms. Neither single species nor groups of species reappear when the
chain of ordinary generation has once been broken. The gradual diffusion
of dominant forms, with the slow modification of their descendants, causes
the forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate in character
between the fossils in the formations above and below, is simply explained
by their intermediate position in the chain of descent. The grand fact
that all extinct organic beings belong to the same system with recent
beings, falling either into the same or into intermediate groups, follows
from the living and the extinct being the offspring of common parents. As
the groups which have descended from an ancient progenitor have generally
diverged in character, the progenitor with its early descendants will
often be intermediate in character in comparison with its later
descendants; and thus we can see why the more ancient a fossil is, the
oftener it stands in some degree intermediate between existing and allied
groups. Recent forms are generally looked at as being, in some vague
sense, higher than ancient and extinct forms; and they are in so far
higher as the later and more improved forms have conquered the older and
less improved organic beings in the struggle for life. Lastly, the law of
the long endurance of allied forms on the same continent,—of
marsupials in Australia, of edentata in America, and other such cases,—is
intelligible, for within a confined country, the recent and the extinct
will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand, on
the theory of descent with modification, most of the great leading facts
in Distribution. We can see why there should be so striking a parallelism
in the distribution of organic beings throughout space, and in their
geological succession throughout time; for in both cases the beings have
been connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which must have struck every traveller, namely, that on the same
continent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabitants
within each great class are plainly related; for they will generally be
descendants of the same progenitors and early colonists. On this same
principle of former migration, combined in most cases with modification,
we can understand, by the aid of the Glacial period, the identity of some
few plants, and the close alliance of many others, on the most distant
mountains, under the most different climates; and likewise the close
alliance of some of the inhabitants of the sea in the northern and
southern temperate zones, though separated by the whole intertropical
ocean. Although two areas may present the same physical conditions of
life, we need feel no surprise at their inhabitants being widely
different, if they have been for a long period completely separated from
each other; for as the relation of organism to organism is the most
important of all relations, and as the two areas will have received
colonists from some third source or from each other, at various periods
and in different proportions, the course of modification in the two areas
will inevitably be different.
On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that
many should be peculiar. We can clearly see why those animals which cannot
cross wide spaces of ocean, as frogs and terrestrial mammals, should not
inhabit oceanic islands; and why, on the other hand, new and peculiar
species of bats, which can traverse the ocean, should so often be found on
islands far distant from any continent. Such facts as the presence of
peculiar species of bats, and the absence of all other mammals, on oceanic
islands, are utterly inexplicable on the theory of independent acts of
creation.
The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the same
parents formerly inhabited both areas; and we almost invariably find that
wherever many closely allied species inhabit two areas, some identical
species common to both still exist. Wherever many closely allied yet
distinct species occur, many doubtful forms and varieties of the same
species likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in nearly
all the plants and animals of the Galapagos archipelago, of Juan
Fernandez, and of the other American islands being related in the most
striking manner to the plants and animals of the neighbouring American
mainland; and those of the Cape de Verde archipelago and other African
islands to the African mainland. It must be admitted that these facts
receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group, and
with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see
how it is, that the mutual affinities of the species and genera within
each class are so complex and circuitous. We see why certain characters
are far more serviceable than others for classification;—why
adaptive characters, though of paramount importance to the being, are of
hardly any importance in classification; why characters derived from
rudimentary parts, though of no service to the being, are often of high
classificatory value; and why embryological characters are the most
valuable of all. The real affinities of all organic beings are due to
inheritance or community of descent. The natural system is a genealogical
arrangement, in which we have to discover the lines of descent by the most
permanent characters, however slight their vital importance may be.
The framework of bones being the same in the hand of a man, wing of a bat,
fin of the porpoise, and leg of the horse,—the same number of
vertebrae forming the neck of the giraffe and of the elephant,—and
innumerable other such facts, at once explain themselves on the theory of
descent with slow and slight successive modifications. The similarity of
pattern in the wing and leg of a bat, though used for such different
purpose,—in the jaws and legs of a crab,—in the petals,
stamens, and pistils of a flower, is likewise intelligible on the view of
the gradual modification of parts or organs, which were alike in the early
progenitor of each class. On the principle of successive variations not
always supervening at an early age, and being inherited at a corresponding
not early period of life, we can clearly see why the embryos of mammals,
birds, reptiles, and fishes should be so closely alike, and should be so
unlike the adult forms. We may cease marvelling at the embryo of an
air-breathing mammal or bird having branchial slits and arteries running
in loops, like those in a fish which has to breathe the air dissolved in
water, by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning
of rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in
the struggle for existence, and will thus have little power of acting on
an organ during early life; hence the organ will not be much reduced or
rendered rudimentary at this early age. The calf, for instance, has
inherited teeth, which never cut through the gums of the upper jaw, from
an early progenitor having well-developed teeth; and we may believe, that
the teeth in the mature animal were reduced, during successive
generations, by disuse or by the tongue and palate having been fitted by
natural selection to browse without their aid; whereas in the calf, the
teeth have been left untouched by selection or disuse, and on the
principle of inheritance at corresponding ages have been inherited from a
remote period to the present day. On the view of each organic being and
each separate organ having been specially created, how utterly
inexplicable it is that parts, like the teeth in the embryonic calf or
like the shrivelled wings under the soldered wing-covers of some beetles,
should thus so frequently bear the plain stamp of inutility! Nature may be
said to have taken pains to reveal, by rudimentary organs and by
homologous structures, her scheme of modification, which it seems that we
wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability of
species? It cannot be asserted that organic beings in a state of nature
are subject to no variation; it cannot be proved that the amount of
variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility is
a special endowment and sign of creation. The belief that species were
immutable productions was almost unavoidable as long as the history of the
world was thought to be of short duration; and now that we have acquired
some idea of the lapse of time, we are too apt to assume, without proof,
that the geological record is so perfect that it would have afforded us
plain evidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow
in admitting any great change of which we do not see the intermediate
steps. The difficulty is the same as that felt by so many geologists, when
Lyell first insisted that long lines of inland cliffs had been formed, and
great valleys excavated, by the slow action of the coast-waves. The mind
cannot possibly grasp the full meaning of the term of a hundred million
years; it cannot add up and perceive the full effects of many slight
variations, accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such
expressions as the "plan of creation," "unity of design," etc., and to
think that we give an explanation when we only restate a fact. Any one
whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject my theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt on the
immutability of species, may be influenced by this volume; but I look with
confidence to the future, to young and rising naturalists, who will be
able to view both sides of the question with impartiality. Whoever is led
to believe that species are mutable will do good service by
conscientiously expressing his conviction; for only thus can the load of
prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This
seems to me a strange conclusion to arrive at. They admit that a multitude
of forms, which till lately they themselves thought were special
creations, and which are still thus looked at by the majority of
naturalists, and which consequently have every external characteristic
feature of true species,—they admit that these have been produced by
variation, but they refuse to extend the same view to other and very
slightly different forms. Nevertheless they do not pretend that they can
define, or even conjecture, which are the created forms of life, and which
are those produced by secondary laws. They admit variation as a vera causa
in one case, they arbitrarily reject it in another, without assigning any
distinction in the two cases. The day will come when this will be given as
a curious illustration of the blindness of preconceived opinion. These
authors seem no more startled at a miraculous act of creation than at an
ordinary birth. But do they really believe that at innumerable periods in
the earth's history certain elemental atoms have been commanded suddenly
to flash into living tissues? Do they believe that at each supposed act of
creation one individual or many were produced? Were all the infinitely
numerous kinds of animals and plants created as eggs or seed, or as full
grown? and in the case of mammals, were they created bearing the false
marks of nourishment from the mother's womb? Although naturalists very
properly demand a full explanation of every difficulty from those who
believe in the mutability of species, on their own side they ignore the
whole subject of the first appearance of species in what they consider
reverent silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away in
force. But some arguments of the greatest weight extend very far. All the
members of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very wide
intervals between existing orders. Organs in a rudimentary condition
plainly show that an early progenitor had the organ in a fully developed
state; and this in some instances necessarily implies an enormous amount
of modification in the descendants. Throughout whole classes various
structures are formed on the same pattern, and at an embryonic age the
species closely resemble each other. Therefore I cannot doubt that the
theory of descent with modification embraces all the members of the same
class. I believe that animals have descended from at most only four or
five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy may
be a deceitful guide. Nevertheless all living things have much in common,
in their chemical composition, their germinal vesicles, their cellular
structure, and their laws of growth and reproduction. We see this even in
so trifling a circumstance as that the same poison often similarly affects
plants and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. Therefore I should infer
from analogy that probably all the organic beings which have ever lived on
this earth have descended from some one primordial form, into which life
was first breathed. When the views entertained in this volume on the
origin of species, or when analogous views are generally admitted, we can
dimly foresee that there will be a considerable revolution in natural
history. Systematists will be able to pursue their labours as at present;
but they will not be incessantly haunted by the shadowy doubt whether this
or that form be in essence a species. This I feel sure, and I speak after
experience, will be no slight relief. The endless disputes whether or not
some fifty species of British brambles are true species will cease.
Systematists will have only to decide (not that this will be easy) whether
any form be sufficiently constant and distinct from other forms, to be
capable of definition; and if definable, whether the differences be
sufficiently important to deserve a specific name. This latter point will
become a far more essential consideration than it is at present; for
differences, however slight, between any two forms, if not blended by
intermediate gradations, are looked at by most naturalists as sufficient
to raise both forms to the rank of species. Hereafter we shall be
compelled to acknowledge that the only distinction between species and
well-marked varieties is, that the latter are known, or believed, to be
connected at the present day by intermediate gradations, whereas species
were formerly thus connected. Hence, without quite rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names, as with the primrose and cowslip; and in
this case scientific and common language will come into accordance. In
short, we shall have to treat species in the same manner as those
naturalists treat genera, who admit that genera are merely artificial
combinations made for convenience. This may not be a cheering prospect;
but we shall at least be freed from the vain search for the undiscovered
and undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer look
at an organic being as a savage looks at a ship, as at something wholly
beyond his comprehension; when we regard every production of nature as one
which has had a history; when we contemplate every complex structure and
instinct as the summing up of many contrivances, each useful to the
possessor, nearly in the same way as when we look at any great mechanical
invention as the summing up of the labour, the experience, the reason, and
even the blunders of numerous workmen; when we thus view each organic
being, how far more interesting, I speak from experience, will the study
of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects of
use and disuse, on the direct action of external conditions, and so forth.
The study of domestic productions will rise immensely in value. A new
variety raised by man will be a far more important and interesting subject
for study than one more species added to the infinitude of already
recorded species. Our classifications will come to be, as far as they can
be so made, genealogies; and will then truly give what may be called the
plan of creation. The rules for classifying will no doubt become simpler
when we have a definite object in view. We possess no pedigrees or
armorial bearings; and we have to discover and trace the many diverging
lines of descent in our natural genealogies, by characters of any kind
which have long been inherited. Rudimentary organs will speak infallibly
with respect to the nature of long-lost structures. Species and groups of
species, which are called aberrant, and which may fancifully be called
living fossils, will aid us in forming a picture of the ancient forms of
life. Embryology will reveal to us the structure, in some degree obscured,
of the prototypes of each great class.
When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have within a not very
remote period descended from one parent, and have migrated from some one
birthplace; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the
differences of the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent in
relation to their apparent means of immigration, some light can be thrown
on ancient geography.
The noble science of Geology loses glory from the extreme imperfection of
the record. The crust of the earth with its embedded remains must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual concurrence
of circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some
security the duration of these intervals by a comparison of the preceding
and succeeding organic forms. We must be cautious in attempting to
correlate as strictly contemporaneous two formations, which include few
identical species, by the general succession of their forms of life. As
species are produced and exterminated by slowly acting and still existing
causes, and not by miraculous acts of creation and by catastrophes; and as
the most important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,—the improvement
of one being entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual time.
A number of species, however, keeping in a body might remain for a long
period unchanged, whilst within this same period, several of these
species, by migrating into new countries and coming into competition with
foreign associates, might become modified; so that we must not overrate
the accuracy of organic change as a measure of time. During early periods
of the earth's history, when the forms of life were probably fewer and
simpler, the rate of change was probably slower; and at the first dawn of
life, when very few forms of the simplest structure existed, the rate of
change may have been slow in an extreme degree. The whole history of the
world, as at present known, although of a length quite incomprehensible by
us, will hereafter be recognised as a mere fragment of time, compared with
the ages which have elapsed since the first creature, the progenitor of
innumerable extinct and living descendants, was created.
In the distant future I see open fields for far more important researches.
Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be
thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Silurian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distant futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species of
each genus, and all the species of many genera, have left no descendants,
but have become utterly extinct. We can so far take a prophetic glance
into futurity as to foretel that it will be the common and widely-spread
species, belonging to the larger and dominant groups, which will
ultimately prevail and procreate new and dominant species. As all the
living forms of life are the lineal descendants of those which lived long
before the Silurian epoch, we may feel certain that the ordinary
succession by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence we may look with some confidence to a
secure future of equally inappreciable length. And as natural selection
works solely by and for the good of each being, all corporeal and mental
endowments will tend to progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different from
each other, and dependent on each other in so complex a manner, have all
been produced by laws acting around us. These laws, taken in the largest
sense, being Growth with Reproduction; Inheritance which is almost implied
by reproduction; Variability from the indirect and direct action of the
external conditions of life, and from use and disuse; a Ratio of Increase
so high as to lead to a Struggle for Life, and as a consequence to Natural
Selection, entailing Divergence of Character and the Extinction of
less-improved forms. Thus, from the war of nature, from famine and death,
the most exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur in
this view of life, with its several powers, having been originally
breathed into a few forms or into one; and that, whilst this planet has
gone cycling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have been, and
are being, evolved.
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