upon, and expansion of, the proposition that widely different animals
or plants, and widely different parts of animals or plants, are
constructed upon the same plan. From the rough comparison of the
skeleton of a bird with that of a man by Belon, in the sixteenth
century (to go no farther back), down to the theory of the limbs
and the theory of the skull at the present day; or, from the first
demonstration of the homologies of the parts of a flower by C. F.
Wolff, to the present elaborate analysis of the floral organs,
morphology exhibits a continual advance towards the demonstration of a
fundamental unity among the seeming diversities of living structures.
And this demonstration has been completed by the final establishment
of the cell theory, which involves the admission of a primitive
conformity, not only of all the elementary structures in animals and
plants respectively, but of those in the one of these great divisions
of living things with those in the other. No _à priori_ difficulty can
be said to stand in the way of evolution, when it can be shown that
all animals and all plants proceed by modes of development, which are
similar in principle, from a fundamental protoplasmic material.
5. The innumerable cases of structures, which are rudimentary and
apparently useless, in species, the close allies of which possess
well developed and functionally important homologous structures, are
readily intelligible on the theory of evolution, while it is hard to
conceive their _raison d’être_ on any other hypothesis. However, a
cautious reasoner will probably rather explain such cases deductively
from the doctrine of evolution than endeavour to support the doctrine
of evolution by them. For it is almost impossible to prove that any
structure, however rudimentary, is useless—that is to say, that it
plays no part whatever in the economy; and, if it is in the slightest
degree useful, there is no reason why, on the hypothesis of direct
creation, it should not have been created. Nevertheless, double-edged
as is the argument from rudimentary organs, there is probably none
which has produced a greater effect in promoting the general acceptance
of the theory of evolution.
6. The older advocates of evolution sought for the causes of the
process exclusively in the influence of varying conditions, such
as climate and station, or hybridisation, upon living forms. Even
Treviranus has got no farther than this point. Lamarck introduced
the conception of the action of an animal on itself as a factor in
producing modification. Starting from the well-known fact that the
habitual use of a limb tends to develop the muscles of the limb, and to
produce a greater and greater facility in using it, he made the general
assumption that the effort of an animal to exert an organ in a given
direction tends to develop the organ in that direction. But a little
consideration showed that, though Lamarck had seized what, as far as
it goes, is a true cause of modification, it is a cause the actual
effects of which are wholly inadequate to account for any considerable
modification in animals, and which can have no influence at all in the
vegetable world; and probably nothing contributed so much to discredit
evolution, in the early part of this century, as the floods of easy
ridicule which were poured upon this part of Lamarck’s speculation.
The theory of natural selection, or survival of the fittest, was
suggested by Wells in 1813, and further elaborated by Matthew in 1831.
But the pregnant suggestions of these writers remained practically
unnoticed and forgotten, until the theory was independently devised
and promulgated by Darwin and Wallace in 1858, and the effect of its
publication was immediate and profound.
Those who were unwilling to accept evolution, without better grounds
than such as are offered by Lamarck, or the author of that particularly
unsatisfactory book, the “Vestiges of the Natural History of the
Creation,” and who therefore preferred to suspend their judgment on the
question, found, in the principle of selective breeding, pursued in all
its applications with marvellous knowledge and skill by Mr. Darwin,
a valid explanation of the occurrence of varieties and races; and
they saw clearly that, if the explanation would apply to species, it
would not only solve the problem of their evolution, but that it would
account for the facts of teleology, as well as for those of morphology;
and for the persistence of some forms of life unchanged through long
epochs of time, while others undergo comparatively rapid metamorphosis.
How far “natural selection” suffices for the production of species
remains to be seen. Few can doubt that, if not the whole cause, it is a
very important factor in that operation; and that it must play a great
part in the sorting out of varieties into those which are transitory
and those which are permanent.
But the causes and conditions of variation have yet to be thoroughly
explored; and the importance of natural selection will not be impaired,
even if further inquiries should prove that variability is definite,
and is determined in certain directions rather than in others, by
conditions inherent in that which varies. It is quite conceivable that
every species tends to produce varieties of a limited number and kind,
and that the effect of natural selection is to favour the development
of some of these, while it opposes the development of others along
their predetermined lines of modification.
7. No truths brought to light by biological investigation were better
calculated to inspire distrust of the dogmas intruded upon science
in the name of theology, than those which relate to the distribution
of animals and plants on the surface of the earth. Very skilful
accommodation was needful, if the limitation of sloths to South
America, and of the ornithorhynchus to Australia, was to be reconciled
with the literal interpretation of the history of the deluge; and,
with the establishment of the existence of distinct provinces of
distribution, any serious belief in the peopling of the world by
migration from Mount Ararat came to an end.
Under these circumstances, only one alternative was left for those
who denied the occurrence of evolution—namely, the supposition that
the characteristic animals and plants of each great province were
created, as such, within the limits in which we find them. And as the
hypothesis of “specific centres,” thus formulated, was heterodox from
the theological point of view, and unintelligible under its scientific
aspect, it may be passed over without further notice, as a phase of
transition from the creational to the evolutional hypothesis.
8. In fact, the strongest and most conclusive arguments in favour of
evolution are those which are based upon the facts of geographical,
taken in conjunction with those of geological, distribution.
Both Mr. Darwin and Mr. Wallace lay great stress on the close relation
which obtains between the existing fauna of any region and that of
the immediately antecedent geological epoch in the same region; and
rightly, for it is in truth inconceivable that there should be no
genetic connection between the two. It is possible to put into words
the proposition that all the animals and plants of each geological
epoch were annihilated, and that a new set of very similar forms was
created for the next epoch; but it may be doubted if any one who ever
tried to form a distinct mental image of this process of spontaneous
generation on the grandest scale, ever really succeeded in realising it.
Within the last twenty years, the attention of the best palæontologists
has been withdrawn from the hodman’s work of making “new species”
of fossils, to the scientific task of completing our knowledge of
individual species, and tracing out the succession of the forms
presented by any given type in time.
Those who desire to inform themselves of the nature and extent of
the evidence bearing on these questions may consult the works of
Rütimeyer, Gaudry, Kowalewsky, Marsh, and the writer of the present
article. It must suffice, in this place, to say that the successive
forms of the Equine type have been fully worked out; while those of
nearly all the other existing types of Ungulate mammals and of the
_Carnivora_ have been almost as closely followed through the Tertiary
deposits; the gradations between birds and reptiles have been traced;
and the modifications undergone by the _Crocodilia_, from the Triassic
epoch to the present day, have been demonstrated. On the evidence of
palæontology, the evolution of many existing forms of animal life
from their predecessors is no longer an hypothesis, but an historical
fact; it is only the nature of the physiological factors to which that
evolution is due which is still open to discussion.
XII.
THE COMING OF AGE OF “THE ORIGIN OF SPECIES.”
Many of you will be familiar with the aspect of this small
green-covered book. It is a copy of the first edition of the “Origin of
Species,” and bears the date of its production—the 1st of October 1859.
Only a few months, therefore, are needed to complete the full tale of
twenty-one years since its birthday.
Those whose memories carry them back to this time will remember that
the infant was remarkably lively, and that a great number of excellent
persons mistook its manifestations of a vigorous individuality for
mere naughtiness; in fact there was a very pretty turmoil about its
cradle. My recollections of the period are particularly vivid; for,
having conceived a tender affection for a child of what appeared to me
to be such remarkable promise, I acted for some time in the capacity
of a sort of under-nurse, and thus came in for my share of the storms
which threatened the very life of the young creature. For some years it
was undoubtedly warm work; but considering how exceedingly unpleasant
the apparition of the new-comer must have been to those who did not
fall in love with him at first sight, I think it is to the credit of
our age that the war was not fiercer, and that the more bitter and
unscrupulous forms of opposition died away as soon as they did.
I speak of this period as of something past and gone, possessing
merely an historical, I had almost said an antiquarian interest. For,
during the second decade of the existence of the “Origin of Species,”
opposition, though by no means dead, assumed a different aspect. On the
part of all those who had any reason to respect themselves, it assumed
a thoroughly respectful character. By this time, the dullest began to
perceive that the child was not likely to perish of any congenital
weakness or infantile disorder, but was growing into a stalwart
personage, upon whom mere goody scoldings and threatenings with the
birch-rod were quite thrown away.
In fact, those who have watched the progress of science within the last
ten years will bear me out to the full, when I assert that there is no
field of biological inquiry in which the influence of the “Origin of
Species” is not traceable; the foremost men of science in every country
are either avowed champions of its leading doctrines, or at any rate
abstain from opposing them; a host of young and ardent investigators
seek for and find inspiration and guidance in Mr. Darwin’s great work;
and the general doctrine of evolution, to one side of which it gives
expression, obtains, in the phenomena of biology, a firm base of
operations whence it may conduct its conquest of the whole realm of
nature.
History warns us, however, that it is the customary fate of new truths
to begin as heresies and to end as superstitions; and, as matters now
stand, it is hardly rash to anticipate that, in another twenty years,
the new generation, educated under the influences of the present day,
will be in danger of accepting the main doctrines of the “Origin of
Species,” with as little reflection, and it may be with as little
justification, as so many of our contemporaries, twenty years ago,
rejected them.
Against any such a consummation let us all devoutly pray; for the
scientific spirit is of more value than its products, and irrationally
held truths may be more harmful than reasoned errors. Now the essence
of the scientific spirit is criticism. It tells us that whenever a
doctrine claims our assent we should reply, Take it if you can compel
it. The struggle for existence holds as much in the intellectual as in
the physical world. A theory is a species of thinking, and its right
to exist is coextensive with its power of resisting extinction by its
rivals.
From this point of view, it appears to me that it would be but a poor
way of celebrating the Coming of Age of the “Origin of Species,” were I
merely to dwell upon the facts, undoubted and remarkable as they are,
of its far-reaching influence and of the great following of ardent
disciples who are occupied in spreading and developing its doctrines.
Mere insanities and inanities have before now swollen to portentous
size in the course of twenty years. Let us rather ask this prodigious
change in opinion to justify itself; let us inquire whether anything
has happened since 1859, which will explain, on rational grounds, why
so many are worshipping that which they burned, and burning that which
they worshipped. It is only in this way that we shall acquire the means
of judging whether the movement we have witnessed is a mere eddy of
fashion, or truly one with the irreversible current of intellectual
progress, and, like it, safe from retrogressive reaction.
Every belief is the product of two factors: the first is the state of
the mind to which the evidence in favour of that belief is presented;
and the second is the logical cogency of the evidence itself. In both
these respects, the history of biological science during the last
twenty years appears to me to afford an ample explanation of the change
which has taken place; and a brief consideration of the salient events
of that history will enable us to understand why, if the “Origin of
Species” appeared now, it would meet with a very different reception
from that which greeted it in 1859.
One-and-twenty years ago, in spite of the work commenced by Hutton and
continued with rare skill and patience by Lyell, the dominant view
of the past history of the earth was catastrophic. Great and sudden
physical revolutions, wholesale creations and extinctions of living
beings, were the ordinary machinery of the geological epic brought into
fashion by the misapplied genius of Cuvier. It was gravely maintained
and taught that the end of every geological epoch was signalised by a
cataclysm, by which every living being on the globe was swept away,
to be replaced by a brand-new creation when the world returned to
quiescence. A scheme of nature which appeared to be modelled on the
likeness of a succession of rubbers of whist, at the end of each of
which the players upset the table and called for a new pack, did not
seem to shock anybody.
I may be wrong, but I doubt if, at the present time, there is a single
responsible representative of these opinions left. The progress
of scientific geology has elevated the fundamental principle of
uniformitarianism, that the explanation of the past is to be sought
in the study of the present, into the position of an axiom; and the
wild speculations of the catastrophists, to which we all listened with
respect a quarter of a century ago, would hardly find a single patient
hearer at the present day. No physical geologist now dreams of seeking,
outside the range of known natural causes, for the explanation of
anything that happened millions of years ago, any more than he would be
guilty of the like absurdity in regard to current events.
The effect of this change of opinion upon biological speculation
is obvious. For, if there have been no periodical general physical
catastrophes, what brought about the assumed general extinctions
and re-creations of life which are the corresponding biological
catastrophes? And, if no such interruptions of the ordinary course of
nature have taken place in the organic, any more than in the inorganic,
world, what alternative is there to the admission of evolution?
The doctrine of evolution in biology is the necessary result of the
logical application of the principles of uniformitarianism to the
phenomena of life. Darwin is the natural successor of Hutton and Lyell,
and the “Origin of Species” the logical sequence of the “Principles of
Geology.”
The fundamental doctrine of the “Origin of Species,” as of all forms of
the theory of evolution applied to biology, is “that the innumerable
species, genera, and families of organic beings with which the 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.”[92]
And, in view of the facts of geology, it follows that all living
animals and plants “are the lineal descendants of those which lived
long before the Silurian epoch.”[93]
It is an obvious consequence of this theory of descent with
modification, as it is sometimes called, that all plants and animals,
however different they may now be, must, at one time or other, have
been connected by direct or indirect intermediate gradations, and that
the appearance of isolation presented by various groups of organic
beings must be unreal.
No part of Mr. Darwin’s work ran more directly counter to the
prepossessions of naturalists twenty years ago than this. And such
prepossessions were very excusable, for there was undoubtedly a great
deal to be said, at that time, in favour of the fixity of species
and of the existence of great breaks, which there was no obvious or
probable means of filling up, between various groups of organic beings.
For various reasons, scientific and unscientific, much had been made of
the hiatus between man and the rest of the higher mammalia, and it is
no wonder that issue was first joined on this part of the controversy.
I have no wish to revive past and happily forgotten controversies; but
I must state the simple fact that the distinctions in the cerebral and
other characters, which were so hotly affirmed to separate man from all
other animals in 1860, have all been demonstrated to be non-existent,
and that the contrary doctrine is now universally accepted and taught.
But there were other cases in which the wide structural gaps asserted
to exist between one group of animals and another, were by no means
fictitious; and, when such structural breaks were real, Mr. Darwin
could account for them only by supposing that the intermediate forms
which once existed had become extinct. In a remarkable passage he says—
“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 animal 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.”[94]
Adverse criticism made merry over such suggestions as these. Of course
it was easy to get out of the difficulty by supposing extinction; but
where was the slightest evidence that such intermediate forms between
birds and reptiles as the hypothesis required ever existed? And then
probably followed a tirade upon this terrible forsaking of the paths of
“Baconian induction.”
But the progress of knowledge has justified Mr. Darwin to an extent
which could hardly have been anticipated. In 1862, the specimen of
_Archæopteryx_, which, until the last two or three years, has remained
unique, was discovered; and it is an animal which, in its feathers and
the greater part of its organisation, is a veritable bird, while, in
other parts, it is as distinctly reptilian.
In 1868, I had the honour of bringing under your notice, in this
theatre, the results of investigations made, up to that time, into
the anatomical characters of certain ancient reptiles, which showed
the nature of the modifications in virtue of which the type of the
quadrupedal reptile passed into that of a bipedal bird; and abundant
confirmatory evidence of the justice of the conclusions which I then
laid before you has since come to light.
In 1875, the discovery of the toothed birds of the cretaceous
formation in North America by Professor Marsh completed the series of
transitional forms between birds and reptiles, and removed Mr. Darwin’s
proposition that “many animal 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,” from the
region of hypothesis to that of demonstrable fact.
In 1859, there appeared to be a very sharp and clear hiatus between
vertebrated and invertebrated animals, not only in their structure,
but, what was more important, in their development. I do not think
that we even yet know the precise links of connection between the two;
but the investigations of Kowalewsky and others upon the development
of _Amphioxus_ and of the _Tunicata_ prove, beyond a doubt, that the
differences which were supposed to constitute a barrier between the two
are non-existent. There is no longer any difficulty in understanding
how the vertebrate type may have arisen from the invertebrate, though
the full proof of the manner in which the transition was actually
effected may still be lacking.
Again, in 1859, there appeared to be a no less sharp separation
between the two great groups of flowering and flowerless plants. It
is only subsequently that the series of remarkable investigations
inaugurated by Hofmeister has brought to light the extraordinary and
altogether unexpected modifications of the reproductive apparatus in
the _Lycopodiaceæ_, the _Rhizocarpeæ_, and the _Gymnospermeæ_, by which
the ferns and the mosses are gradually connected with the Phanerogamic
division of the vegetable world.
So, again, it is only since 1859 that we have acquired that wealth of
knowledge of the lowest forms of life which demonstrates the futility
of any attempt to separate the lowest plants from the lowest animals,
and shows that the two kingdoms of living nature have a common
borderland which belongs to both or to neither.
Thus it will be observed that the whole tendency of biological
investigation, since 1859, has been in the direction of removing the
difficulties which the apparent breaks in the series created at that
time; and the recognition of gradation is the first step towards the
acceptance of evolution.
As another great factor in bringing about the change of opinion which
has taken place among naturalists, I count the astonishing progress
which has been made in the study of embryology. Twenty years ago,
not only were we devoid of any accurate knowledge of the mode of
development of many groups of animals and plants, but the methods of
investigation were rude and imperfect. At the present time, there is
no important group of organic beings the development of which has
not been carefully studied; and the modern methods of hardening and
section-making enable the embryologist to determine the nature of the
process, in each case, with a degree of minuteness and accuracy which
is truly astonishing to those whose memories carry them back to the
beginnings of modern histology. And the results of these embryological
investigations are in complete harmony with the requirements of the
doctrine of evolution. The first beginnings of all the higher forms of
animal life are similar, and however diverse their adult conditions,
they start from a common foundation. Moreover, the process of
development of the animal or the plant from its primary egg or germ is
a true process of evolution—a progress from almost formless to more or
less highly organised matter, in virtue of the properties inherent in
that matter.