LECTURES ON EVOLUTION

ESSAY #3 FROM “SCIENCE AND HEBREW TRADITION”

By Thomas Henry Huxley

Contents

I. THE THREE HYPOTHESES RESPECTING THE HISTORY OF NATURE

We live in and form part of a system of things of immense diversity and
perplexity, which we call Nature; and it is a matter of the deepest
interest to all of us that we should form just conceptions of the
constitution of that system and of its past history. With relation to this
universe, man is, in extent, little more than a mathematical point; in
duration but a fleeting shadow; he is a mere reed shaken in the winds of
force. But as Pascal long ago remarked, although a mere reed, he is a
thinking reed; and in virtue of that wonderful capacity of thought, he has
the power of framing for himself a symbolic conception of the universe,
which, although doubtless highly imperfect and inadequate as a picture of
the great whole, is yet sufficient to serve him as a chart for the
guidance of his practical affairs. It has taken long ages of toilsome and
often fruitless labour to enable man to look steadily at the shifting
scenes of the phantasmagoria of Nature, to notice what is fixed among her
fluctuations, and what is regular among her apparent irregularities; and
it is only comparatively lately, within the last few centuries, that the
conception of a universal order and of a definite course of things, which
we term the course of Nature, has emerged.

But, once originated, the conception of the constancy of the order of
Nature has become the dominant idea of modern thought. To any person who
is familiar with the facts upon which that conception is based, and is
competent to estimate their significance, it has ceased to be conceivable
that chance should have any place in the universe, or that events should
depend upon any but the natural sequence of cause and effect. We have come
to look upon the present as the child of the past and as the parent of the
future; and, as we have excluded chance from a place in the universe, so
we ignore, even as a possibility, the notion of any interference with the
order of Nature. Whatever may be men’s speculative doctrines, it is quite
certain that every intelligent person guides his life and risks his
fortune upon the belief that the order of Nature is constant, and that the
chain of natural causation is never broken.

In fact, no belief which we entertain has so complete a logical basis as
that to which I have just referred. It tacitly underlies every process of
reasoning; it is the foundation of every act of the will. It is based upon
the broadest induction, and it is verified by the most constant, regular,
and universal of deductive processes. But we must recollect that any human
belief, however broad its basis, however defensible it may seem, is, after
all, only a probable belief, and that our widest and safest
generalisations are simply statements of the highest degree of
probability. Though we are quite clear about the constancy of the order of
Nature, at the present time, and in the present state of things, it by no
means necessarily follows that we are justified in expanding this
generalisation into the infinite past, and in denying, absolutely, that
there may have been a time when Nature did not follow a fixed order, when
the relations of cause and effect were not definite, and when
extra-natural agencies interfered with the general course of Nature.
Cautious men will allow that a universe so different from that which we
know may have existed; just as a very candid thinker may admit that a
world in which two and two do not make four, and in which two straight
lines do inclose a space, may exist. But the same caution which forces the
admission of such possibilities demands a great deal of evidence before it
recognises them to be anything more substantial. And when it is asserted
that, so many thousand years ago, events occurred in a manner utterly
foreign to and inconsistent with the existing laws of Nature, men, who
without being particularly cautious, are simply honest thinkers, unwilling
to deceive themselves or delude others, ask for trustworthy evidence of
the fact.

Did things so happen or did they not? This is a historical question, and
one the answer to which must be sought in the same way as the solution of
any other historical problem.

So far as I know, there are only three hypotheses which ever have been
entertained, or which well can be entertained, respecting the past history
of Nature. I will, in the first place, state the hypotheses, and then I
will consider what evidence bearing upon them is in our possession, and by
what light of criticism that evidence is to be interpreted.

Upon the first hypothesis, the assumption is, that phenomena of Nature
similar to those exhibited by the present world have always existed; in
other words, that the universe has existed, from all eternity, in what may
be broadly termed its present condition.

The second hypothesis is that the present state of things has had only a
limited duration; and that, at some period in the past, a condition of the
world, essentially similar to that which we now know, came into existence,
without any precedent condition from which it could have naturally
proceeded. The assumption that successive states of Nature have arisen,
each without any relation of natural causation to an antecedent state, is
a mere modification of this second hypothesis.

The third hypothesis also assumes that the present state of things has had
but a limited duration; but it supposes that this state has been evolved
by a natural process from an antecedent state, and that from another, and
so on; and, on this hypothesis, the attempt to assign any limit to the
series of past changes is, usually, given up.

It is so needful to form clear and distinct notions of what is really
meant by each of these hypotheses that I will ask you to imagine what,
according to each, would have been visible to a spectator of the events
which constitute the history of the earth. On the first hypothesis,
however far back in time that spectator might be placed, he would see a
world essentially, though perhaps not in all its details, similar to that
which now exists. The animals which existed would be the ancestors of
those which now live, and similar to them; the plants, in like manner,
would be such as we know; and the mountains, plains, and waters would
foreshadow the salient features of our present land and water. This view
was held more or less distinctly, sometimes combined with the notion of
recurrent cycles of change, in ancient times; and its influence has been
felt down to the present day. It is worthy of remark that it is a
hypothesis which is not inconsistent with the doctrine of
Uniformitarianism, with which geologists are familiar. That doctrine was
held by Hutton, and in his earlier days by Lyell. Hutton was struck by the
demonstration of astronomers that the perturbations of the planetary
bodies, however great they may be, yet sooner or later right themselves;
and that the solar system possesses a self-adjusting power by which these
aberrations are all brought back to a mean condition. Hutton imagined that
the like might be true of terrestrial changes; although no one recognised
more clearly than he the fact that the dry land is being constantly washed
down by rain and rivers and deposited in the sea; and that thus, in a
longer or shorter time, the inequalities of the earth’s surface must be
levelled, and its high lands brought down to the ocean. But, taking into
account the internal forces of the earth, which, upheaving the sea-bottom
give rise to new land, he thought that these operations of degradation and
elevation might compensate each other; and that thus, for any assignable
time, the general features of our planet might remain what they are. And
inasmuch as, under these circumstances, there need be no limit to the
propagation of animals and plants, it is clear that the consistent working
out of the uniformitarian idea might lead to the conception of the
eternity of the world. Not that I mean to say that either Hutton or Lyell
held this conception—assuredly not; they would have been the first
to repudiate it. Nevertheless, the logical development of some of their
arguments tends directly towards this hypothesis.

The second hypothesis supposes that the present order of things, at some
no very remote time, had a sudden origin, and that the world, such as it
now is, had chaos for its phenomenal antecedent. That is the doctrine
which you will find stated most fully and clearly in the immortal poem of
John Milton—the English Divina Commedia— “Paradise
Lost.” I believe it is largely to the influence of that remarkable work,
combined with the daily teachings to which we have all listened in our
childhood, that this hypothesis owes its general wide diffusion as one of
the current beliefs of English-speaking people. If you turn to the seventh
book of “Paradise Lost,” you will find there stated the hypothesis to
which I refer, which is briefly this: That this visible universe of ours
came into existence at no great distance of time from the present; and
that the parts of which it is composed made their appearance, in a certain
definite order, in the space of six natural days, in such a manner that,
on the first of these days, light appeared; that, on the second, the
firmament, or sky, separated the waters above, from the waters beneath the
firmament; that, on the third day, the waters drew away from the dry land,
and upon it a varied vegetable life, similar to that which now exists,
made its appearance; that the fourth day was signalised by the apparition
of the sun, the stars, the moon, and the planets; that, on the fifth day,
aquatic animals originated within the waters; that, on the sixth day, the
earth gave rise to our four-footed terrestrial creatures, and to all
varieties of terrestrial animals except birds, which had appeared on the
preceding day; and, finally, that man appeared upon the earth, and the
emergence of the universe from chaos was finished. Milton tells us,
without the least ambiguity, what a spectator of these marvellous
occurrences would have witnessed. I doubt not that his poem is familiar to
all of you, but I should like to recall one passage to your minds, in
order that I may be justified in what I have said regarding the perfectly
concrete, definite, picture of the origin of the animal world which Milton
draws. He says:—

There is no doubt as to the meaning of this statement, nor as to what a
man of Milton’s genius expected would have been actually visible to an
eye-witness of this mode of origination of living things.

The third hypothesis, or the hypothesis of evolution, supposes that, at
any comparatively late period of past time, our imaginary spectator would
meet with a state of things very similar to that which now obtains; but
that the likeness of the past to the present would gradually become less
and less, in proportion to the remoteness of his period of observation
from the present day; that the existing distribution of mountains and
plains, of rivers and seas, would show itself to be the product of a slow
process of natural change operating upon more and more widely different
antecedent conditions of the mineral frame-work of the earth; until, at
length, in place of that frame-work, he would behold only a vast nebulous
mass, representing the constituents of the sun and of the planetary
bodies. Preceding the forms of life which now exist, our observer would
see animals and plants, not identical with them, but like them, increasing
their differences with their antiquity and, at the same time, becoming
simpler and simpler; until, finally, the world of life would present
nothing but that undifferentiated protoplasmic matter which, so far as our
present knowledge goes, is the common foundation of all vital activity.

The hypothesis of evolution supposes that in all this vast progression
there would be no breach of continuity, no point at which we could say
“This is a natural process,” and “This is not a natural process;” but that
the whole might be compared to that wonderful operation of development
which may be seen going on every day under our eyes, in virtue of which
there arises, out of the semi-fluid comparatively homogeneous substance
which we call an egg, the complicated organisation of one of the higher
animals. That, in a few words, is what is meant by the hypothesis of
evolution.

I have already suggested that, in dealing with these three hypotheses, in
endeavouring to form a judgment as to which of them is the more worthy of
belief, or whether none is worthy of belief—in which case our
condition of mind should be that suspension of judgment which is so
difficult to all but trained intellects—we should be indifferent to
all a priori considerations. The question is a question of
historical fact. The universe has come into existence somehow or other,
and the problem is, whether it came into existence in one fashion, or
whether it came into existence in another; and, as an essential
preliminary to further discussion, permit me to say two or three words as
to the nature and the kinds of historical evidence.

The evidence as to the occurrence of any event in past time may be ranged
under two heads which, for convenience’ sake, I will speak of as
testimonial evidence and as circumstantial evidence. By testimonial
evidence I mean human testimony; and by circumstantial evidence I mean
evidence which is not human testimony. Let me illustrate by a familiar
example what I understand by these two kinds of evidence, and what is to
be said respecting their value.

Suppose that a man tells you that he saw a person strike another and kill
him; that is testimonial evidence of the fact of murder. But it is
possible to have circumstantial evidence of the fact of murder; that is to
say, you may find a man dying with a wound upon his head having exactly
the form and character of the wound which is made by an axe, and, with due
care in taking surrounding circumstances into account, you may conclude
with the utmost certainty that the man has been murdered; that his death
is the consequence of a blow inflicted by another man with that implement.
We are very much in the habit of considering circumstantial evidence as of
less value than testimonial evidence, and it may be that, where the
circumstances are not perfectly clear and intelligible, it is a dangerous
and unsafe kind of evidence; but it must not be forgotten that, in many
cases, circumstantial is quite as conclusive as testimonial evidence, and
that, not unfrequently, it is a great deal weightier than testimonial
evidence. For example, take the case to which I referred just now. The
circumstantial evidence may be better and more convincing than the
testimonial evidence; for it may be impossible, under the conditions that
I have defined, to suppose that the man met his death from any cause but
the violent blow of an axe wielded by another man. The circumstantial
evidence in favour of a murder having been committed, in that case, is as
complete and as convincing as evidence can be. It is evidence which is
open to no doubt and to no falsification. But the testimony of a witness
is open to multitudinous doubts. He may have been mistaken. He may have
been actuated by malice. It has constantly happened that even an accurate
man has declared that a thing has happened in this, that, or the other
way, when a careful analysis of the circumstantial evidence has shown that
it did not happen in that way, but in some other way.

We may now consider the evidence in favour of or against the three
hypotheses. Let me first direct your attention to what is to be said about
the hypothesis of the eternity of the state of things in which we now
live. What will first strike you is, that it is a hypothesis which,
whether true or false, is not capable of verification by any evidence.
For, in order to obtain either circumstantial or testimonial evidence
sufficient to prove the eternity of duration of the present state of
nature, you must have an eternity of witnesses or an infinity of
circumstances, and neither of these is attainable. It is utterly
impossible that such evidence should be carried beyond a certain point of
time; and all that could be said, at most, would be, that so far as the
evidence could be traced, there was nothing to contradict the hypothesis.
But when you look, not to the testimonial evidence—which,
considering the relative insignificance of the antiquity of human records,
might not be good for much in this case—but to the circumstantial
evidence, then you find that this hypothesis is absolutely incompatible
with such evidence as we have; which is of so plain and so simple a
character that it is impossible in any way to escape from the conclusions
which it forces upon us.

You are, doubtless, all aware that the outer substance of the earth, which
alone is accessible to direct observation, is not of a homogeneous
character, but that it is made up of a number of layers or strata, the
titles of the principal groups of which are placed upon the accompanying
diagram. Each of these groups represents a number of beds of sand, of
stone, of clay, of slate, and of various other materials.

On careful examination, it is found that the materials of which each of
these layers of more or less hard rock are composed are, for the most
part, of the same nature as those which are at present being formed under
known conditions on the surface of the earth. For example, the chalk,
which constitutes a great part of the Cretaceous formation in some parts
of the world, is practically identical in its physical and chemical
characters with a substance which is now being formed at the bottom of the
Atlantic Ocean, and covers an enormous area; other beds of rock are
comparable with the sands which are being formed upon sea-shores, packed
together, and so on. Thus, omitting rocks of igneous origin, it is
demonstrable that all these beds of stone, of which a total of not less
than seventy thousand feet is known, have been formed by natural agencies,
either out of the waste and washing of the dry land, or else by the
accumulation of the exuviae of plants and animals. Many of these strata
are full of such exuviae—the so-called “fossils.” Remains of
thousands of species of animals and plants, as perfectly recognisable as
those of existing forms of life which you meet with in museums, or as the
shells which you pick up upon the sea-beach, have been imbedded in the
ancient sands, or muds, or limestones, just as they are being imbedded
now, in sandy, or clayey, or calcareous subaqueous deposits. They furnish
us with a record, the general nature of which cannot be misinterpreted, of
the kinds of things that have lived upon the surface of the earth during
the time that is registered by this great thickness of stratified rocks.
But even a superficial study of these fossils shows us that the animals
and plants which live at the present time have had only a temporary
duration; for the remains of such modern forms of life are met with, for
the most part, only in the uppermost or latest tertiaries, and their
number rapidly diminishes in the lower deposits of that epoch. In the
older tertiaries, the places of existing animals and plants are taken by
other forms, as numerous and diversified as those which live now in the
same localities, but more or less different from them; in the mesozoic
rocks, these are replaced by others yet more divergent from modern types;
and, in the paleozoic formations, the contrast is still more marked. Thus
the circumstantial evidence absolutely negatives the conception of the
eternity of the present condition of things. We can say, with certainty,
that the present condition of things has existed for a comparatively short
period; and that, so far as animal and vegetable nature are concerned, it
has been preceded by a different condition. We can pursue this evidence
until we reach the lowest of the stratified rocks, in which we lose the
indications of life altogether. The hypothesis of the eternity of the
present state of nature may therefore be put out of court.

We now come to what I will term Milton’s hypothesis—the hypothesis
that the present condition of things has endured for a comparatively short
time; and, at the commencement of that time, came into existence within
the course of six days. I doubt not that it may have excited some surprise
in your minds that I should have spoken of this as Milton’s hypothesis,
rather than that I should have chosen the terms which are more customary,
such as “the doctrine of creation,” or “the Biblical doctrine,” or “the
doctrine of Moses,” all of which denominations, as applied to the
hypothesis to which I have just referred, are certainly much more familiar
to you than the title of the Miltonic hypothesis. But I have had what I
cannot but think are very weighty reasons for taking the course which I
have pursued. In the first place, I have discarded the title of the
“doctrine of creation,” because my present business is not with the
question why the objects which constitute Nature came into existence, but
when they came into existence, and in what order. This is as strictly a
historical question as the question when the Angles and the Jutes invaded
England, and whether they preceded or followed the Romans. But the
question about creation is a philosophical problem, and one which cannot
be solved, or even approached, by the historical method. What we want to
learn is, whether the facts, so far as they are known, afford evidence
that things arose in the way described by Milton, or whether they do not;
and, when that question is settled it will be time enough to inquire into
the causes of their origination.

In the second place, I have not spoken of this doctrine as the Biblical
doctrine. It is quite true that persons as diverse in their general views
as Milton the Protestant and the celebrated Jesuit Father Suarez, each put
upon the first chapter of Genesis the interpretation embodied in Milton’s
poem. It is quite true that this interpretation is that which has been
instilled into every one of us in our childhood; but I do not for one
moment venture to say that it can properly be called the Biblical
doctrine. It is not my business, and does not lie within my competency, to
say what the Hebrew text does, and what it does not signify; moreover,
were I to affirm that this is the Biblical doctrine, I should be met by
the authority of many eminent scholars, to say nothing of men of science,
who, at various times, have absolutely denied that any such doctrine is to
be found in Genesis. If we are to listen to many expositors of no mean
authority, we must believe that what seems so clearly defined in Genesis—as
if very great pains had been taken that there should be no possibility of
mistake—is not the meaning of the text at all. The account is
divided into periods that we may make just as long or as short as
convenience requires. We are also to understand that it is consistent with
the original text to believe that the most complex plants and animals may
have been evolved by natural processes, lasting for millions of years, out
of structureless rudiments. A person who is not a Hebrew scholar can only
stand aside and admire the marvellous flexibility of a language which
admits of such diverse interpretations. But assuredly, in the face of such
contradictions of authority upon matters respecting which he is
incompetent to form any judgment, he will abstain, as I do, from giving
any opinion.

In the third place, I have carefully abstained from speaking of this as
the Mosaic doctrine, because we are now assured upon the authority of the
highest critics and even of dignitaries of the Church, that there is no
evidence that Moses wrote the Book of Genesis, or knew anything about it.
You will understand that I give no judgment—it would be an
impertinence upon my part to volunteer even a suggestion—upon such a
subject. But, that being the state of opinion among the scholars and the
clergy, it is well for the unlearned in Hebrew lore, and for the laity, to
avoid entangling themselves in such a vexed question. Happily, Milton
leaves us no excuse for doubting what he means, and I shall therefore be
safe in speaking of the opinion in question as the Miltonic hypothesis.

Now we have to test that hypothesis. For my part, I have no prejudice one
way or the other. If there is evidence in favour of this view, I am
burdened by no theoretical difficulties in the way of accepting it; but
there must be evidence. Scientific men get an awkward habit—no, I
won’t call it that, for it is a valuable habit—of believing nothing
unless there is evidence for it; and they have a way of looking upon
belief which is not based upon evidence, not only as illogical, but as
immoral. We will, if you please, test this view by the circumstantial
evidence alone; for, from what I have said, you will understand that I do
not propose to discuss the question of what testimonial evidence is to be
adduced in favour of it. If those whose business it is to judge are not at
one as to the authenticity of the only evidence of that kind which is
offered, nor as to the facts to which it bears witness, the discussion of
such evidence is superfluous.

But I may be permitted to regret this necessity of rejecting the
testimonial evidence the less, because the examination of the
circumstantial evidence leads to the conclusion, not only that it is
incompetent to justify the hypothesis, but that, so far as it goes, it is
contrary to the hypothesis.

The considerations upon which I base this conclusion are of the simplest
possible character. The Miltonic hypothesis contains assertions of a very
definite character relating to the succession of living forms. It is
stated that plants, for example, made their appearance upon the third day,
and not before. And you will understand that what the poet means by plants
are such plants as now live, the ancestors, in the ordinary way of
propagation of like by like, of the trees and shrubs which flourish in the
present world. It must needs be so; for, if they were different, either
the existing plants have been the result of a separate origination since
that described by Milton, of which we have no record, nor any ground for
supposition that such an occurrence has taken place; or else they have
arisen by a process of evolution from the original stocks.

In the second place, it is clear that there was no animal life before the
fifth day, and that, on the fifth day, aquatic animals and birds appeared.
And it is further clear that terrestrial living things, other than birds,
made their appearance upon the sixth day and not before. Hence, it follows
that, if, in the large mass of circumstantial evidence as to what really
has happened in the past history of the globe we find indications of the
existence of terrestrial animals, other than birds, at a certain period,
it is perfectly certain that all that has taken place, since that time,
must be referred to the sixth day.

In the great Carboniferous formation, whence America derives so vast a
proportion of her actual and potential wealth, in the beds of coal which
have been formed from the vegetation of that period, we find abundant
evidence of the existence of terrestrial animals. They have been
described, not only by European but by your own naturalists. There are to
be found numerous insects allied to our cockroaches. There are to be found
spiders and scorpions of large size, the latter so similar to existing
scorpions that it requires the practised eye of the naturalist to
distinguish them. Inasmuch as these animals can be proved to have been
alive in the Carboniferous epoch, it is perfectly clear that, if the
Miltonic account is to be accepted, the huge mass of rocks extending from
the middle of the Palaeozoic formations to the uppermost members of the
series, must belong to the day which is termed by Milton the sixth. But,
further, it is expressly stated that aquatic animals took their origin on
the fifth day, and not before; hence, all formations in which remains of
aquatic animals can be proved to exist, and which therefore testify that
such animals lived at the time when these formations were in course of
deposition, must have been deposited during or since the period which
Milton speaks of as the fifth day. But there is absolutely no
fossiliferous formation in which the remains of aquatic animals are
absent. The oldest fossils in the Silurian rocks are exuviae of marine
animals; and if the view which is entertained by Principal Dawson and Dr.
Carpenter respecting the nature of the Eozoon be well-founded,
aquatic animals existed at a period as far antecedent to the deposition of
the coal as the coal is from us; inasmuch as the Eozoon is met with
in those Laurentian strata which lie at the bottom of the series of
stratified rocks. Hence it follows, plainly enough, that the whole series
of stratified rocks, if they are to be brought into harmony with Milton,
must be referred to the fifth and sixth days, and that we cannot hope to
find the slightest trace of the products of the earlier days in the
geological record. When we consider these simple facts, we see how
absolutely futile are the attempts that have been made to draw a parallel
between the story told by so much of the crust of the earth as is known to
us and the story which Milton tells. The whole series of fossiliferous
stratified rocks must be referred to the last two days; and neither the
Carboniferous, nor any other, formation can afford evidence of the work of
the third day.

Not only is there this objection to any attempt to establish a harmony
between the Miltonic account and the facts recorded in the fossiliferous
rocks, but there is a further difficulty. According to the Miltonic
account, the order in which animals should have made their appearance in
the stratified rocks would be thus: Fishes, including the great whales,
and birds; after them, all varieties of terrestrial animals except birds.
Nothing could be further from the facts as we find them; we know of not
the slightest evidence of the existence of birds before the Jurassic, or
perhaps the Triassic, formation; while terrestrial animals, as we have
just seen, occur in the Carboniferous rocks.

If there were any harmony between the Miltonic account and the
circumstantial evidence, we ought to have abundant evidence of the
existence of birds in the Carboniferous, the Devonian, and the Silurian
rocks. I need hardly say that this is not the case, and that not a trace
of birds makes its appearance until the far later period which I have
mentioned.

And again, if it be true that all varieties of fishes and the great
whales, and the like, made their appearance on the fifth day, we ought to
find the remains of these animals in the older rocks—in those which
were deposited before the Carboniferous epoch. Fishes we do find, in
considerable number and variety; but the great whales are absent, and the
fishes are not such as now live. Not one solitary species of fish now in
existence is to be found in the Devonian or Silurian formations. Hence we
are introduced afresh to the dilemma which I have already placed before
you: either the animals which came into existence on the fifth day were
not such as those which are found at present, are not the direct and
immediate ancestors of those which now exist; in which case, either fresh
creations of which nothing is said, or a process of evolution, must have
occurred; or else the whole story must be given up, as not only devoid of
any circumstantial evidence, but contrary to such evidence as exists.

I placed before you in a few words, some little time ago, a statement of
the sum and substance of Milton’s hypothesis. Let me now try to state as
briefly, the effect of the circumstantial evidence bearing upon the past
history of the earth which is furnished, without the possibility of
mistake, with no chance of error as to its chief features, by the
stratified rocks. What we find is, that the great series of formations
represents a period of time of which our human chronologies hardly afford
us a unit of measure. I will not pretend to say how we ought to estimate
this time, in millions or in billions of years. For my purpose, the
determination of its absolute duration is wholly unessential. But that the
time was enormous there can be no question.

It results from the simplest methods of interpretation, that leaving out
of view certain patches of metamorphosed rocks, and certain volcanic
products, all that is now dry land has once been at the bottom of the
waters. It is perfectly certain that, at a comparatively recent period of
the world’s history—the Cretaceous epoch—none of the great
physical features which at present mark the surface of the globe existed.
It is certain that the Rocky Mountains were not. It is certain that the
Himalaya Mountains were not. It is certain that the Alps and the Pyrenees
had no existence. The evidence is of the plainest possible character and
is simply this:—We find raised up on the flanks of these mountains,
elevated by the forces of upheaval which have given rise to them, masses
of Cretaceous rock which formed the bottom of the sea before those
mountains existed. It is therefore clear that the elevatory forces which
gave rise to the mountains operated subsequently to the Cretaceous epoch;
and that the mountains themselves are largely made up of the materials
deposited in the sea which once occupied their place. As we go back in
time, we meet with constant alternations of sea and land, of estuary and
open ocean; and, in correspondence with these alternations, we observe the
changes in the fauna and flora to which I have referred.

But the inspection of these changes gives us no right to believe that
there has been any discontinuity in natural processes. There is no trace
of general cataclysms, of universal deluges, or sudden destructions of a
whole fauna or flora. The appearances which were formerly interpreted in
that way have all been shown to be delusive, as our knowledge has
increased and as the blanks which formerly appeared to exist between the
different formations have been filled up. That there is no absolute break
between formation and formation, that there has been no sudden
disappearance of all the forms of life and replacement of them by others,
but that changes have gone on slowly and gradually, that one type has died
out and another has taken its place, and that thus, by insensible degrees,
one fauna has been replaced by another, are conclusions strengthened by
constantly increasing evidence. So that within the whole of the immense
period indicated by the fossiliferous stratified rocks, there is assuredly
not the slightest proof of any break in the uniformity of Nature’s
operations, no indication that events have followed other than a clear and
orderly sequence.

That, I say, is the natural and obvious teaching of the circumstantial
evidence contained in the stratified rocks. I leave you to consider how
far, by any ingenuity of interpretation, by any stretching of the meaning
of language, it can be brought into harmony with the Miltonic hypothesis.

There remains the third hypothesis, that of which I have spoken as the
hypothesis of evolution; and I purpose that, in lectures to come, we
should discuss it as carefully as we have considered the other two
hypotheses. I need not say that it is quite hopeless to look for
testimonial evidence of evolution. The very nature of the case precludes
the possibility of such evidence, for the human race can no more be
expected to testify to its own origin, than a child can be tendered as a
witness of its own birth. Our sole inquiry is, what foundation
circumstantial evidence lends to the hypothesis, or whether it lends none,
or whether it controverts the hypothesis. I shall deal with the matter
entirely as a question of history. I shall not indulge in the discussion
of any speculative probabilities. I shall not attempt to show that Nature
is unintelligible unless we adopt some such hypothesis. For anything I
know about the matter, it may be the way of Nature to be unintelligible;
she is often puzzling, and I have no reason to suppose that she is bound
to fit herself to our notions.

I shall place before you three kinds of evidence entirely based upon what
is known of the forms of animal life which are contained in the series of
stratified rocks. I shall endeavour to show you that there is one kind of
evidence which is neutral, which neither helps evolution nor is
inconsistent with it. I shall then bring forward a second kind of evidence
which indicates a strong probability in favour of evolution, but does not
prove it; and, lastly, I shall adduce a third kind of evidence which,
being as complete as any evidence which we can hope to obtain upon such a
subject, and being wholly and strikingly in favour of evolution, may
fairly be called demonstrative evidence of its occurrence.

II. THE HYPOTHESIS OF EVOLUTION. THE NEUTRAL AND THE FAVOURABLE

EVIDENCE.

In the preceding lecture I pointed out that there are three hypotheses
which may be entertained, and which have been entertained, respecting the
past history of life upon the globe. According to the first of these
hypotheses, living beings, such as now exist, have existed from all
eternity upon this earth. We tested that hypothesis by the circumstantial
evidence, as I called it, which is furnished by the fossil remains
contained in the earth’s crust, and we found that it was obviously
untenable. I then proceeded to consider the second hypothesis, which I
termed the Miltonic hypothesis, not because it is of any particular
consequence whether John Milton seriously entertained it or not, but
because it is stated in a clear and unmistakable manner in his great poem.
I pointed out to you that the evidence at our command as completely and
fully negatives that hypothesis as it did the preceding one. And I confess
that I had too much respect for your intelligence to think it necessary to
add that the negation was equally clear and equally valid, whatever the
source from which that hypothesis might be derived, or whatever the
authority by which it might be supported. I further stated that, according
to the third hypothesis, or that of evolution, the existing state of
things is the last term of a long series of states, which, when traced
back, would be found to show no interruption and no breach in the
continuity of natural causation. I propose, in the present and the
following lecture, to test this hypothesis rigorously by the evidence at
command, and to inquire how far that evidence can be said to be
indifferent to it, how far it can be said to be favourable to it, and,
finally, how far it can be said to be demonstrative.

From almost the origin of the discussions about the existing condition of
the animal and vegetable worlds and the causes which have determined that
condition, an argument has been put forward as an objection to evolution,
which we shall have to consider very seriously. It is an argument which
was first clearly stated by Cuvier in his criticism of the doctrines
propounded by his great contemporary, Lamarck. The French expedition to
Egypt had called the attention of learned men to the wonderful store of
antiquities in that country, and there had been brought back to France
numerous mummified corpses of the animals which the ancient Egyptians
revered and preserved, and which, at a reasonable computation, must have
lived not less than three or four thousand years before the time at which
they were thus brought to light. Cuvier endeavoured to test the hypothesis
that animals have undergone gradual and progressive modifications of
structure, by comparing the skeletons and such other parts of the mummies
as were in a fitting state of preservation, with the corresponding parts
of the representatives of the same species now living in Egypt. He arrived
at the conviction that no appreciable change had taken place in these
animals in the course of this considerable lapse of time, and the justice
of his conclusion is not disputed.

It is obvious that, if it can be proved that animals have endured, without
undergoing any demonstrable change of structure, for so long a period as
four thousand years, no form of the hypothesis of evolution which assumes
that animals undergo a constant and necessary progressive change can be
tenable; unless, indeed, it be further assumed that four thousand years is
too short a time for the production of a change sufficiently great to be
detected.

But it is no less plain that if the process of evolution of animals is not
independent of surrounding conditions; if it may be indefinitely hastened
or retarded by variations in these conditions; or if evolution is simply a
process of accommodation to varying conditions; the argument against the
hypothesis of evolution based on the unchanged character of the Egyptian
fauna is worthless. For the monuments which are coeval with the mummies
testify as strongly to the absence of change in the physical geography and
the general conditions of the land of Egypt, for the time in question, as
the mummies do to the unvarying characters of its living population.

The progress of research since Cuvier’s time has supplied far more
striking examples of the long duration of specific forms of life than
those which are furnished by the mummified Ibises and Crocodiles of Egypt.
A remarkable case is to be found in your own country, in the neighbourhood
of the falls of Niagara. In the immediate vicinity of the whirlpool, and
again upon Goat Island, in the superficial deposits which cover the
surface of the rocky subsoil in those regions, there are found remains of
animals in perfect preservation, and among them, shells belonging to
exactly the same species as those which at present inhabit the still
waters of Lake Erie. It is evident, from the structure of the country,
that these animal remains were deposited in the beds in which they occur
at a time when the lake extended over the region in which they are found.
This involves the conclusion that they lived and died before the falls had
cut their way back through the gorge of Niagara; and, indeed, it has been
determined that, when these animals lived, the falls of Niagara must have
been at least six miles further down the river than they are at present.
Many computations have been made of the rate at which the falls are thus
cutting their way back. Those computations have varied greatly, but I
believe I am speaking within the bounds of prudence, if I assume that the
falls of Niagara have not retreated at a greater pace than about a foot a
year. Six miles, speaking roughly, are 30,000 feet; 30,000 feet, at a foot
a year, gives 30,000 years; and thus we are fairly justified in concluding
that no less a period than this has passed since the shell-fish, whose
remains are left in the beds to which I have referred, were living
creatures.

But there is still stronger evidence of the long duration of certain
types. I have already stated that, as we work our way through the great
series of the Tertiary formations, we find many species of animals
identical with those which live at the present day, diminishing in
numbers, it is true, but still existing, in a certain proportion, in the
oldest of the Tertiary rocks. Furthermore, when we examine the rocks of
the Cretaceous epoch, we find the remains of some animals which the
closest scrutiny cannot show to be, in any important respect, different
from those which live at the present time. That is the case with one of
the cretaceous lamp-shells (Terebratula), which has continued to
exist unchanged, or with insignificant variations, down to the present
day. Such is the case with the Globigerinæ, the skeletons of which,
aggregated together, form a large proportion of our English chalk. Those
Globigerinae can be traced down to the Globigerinae which
live at the surface of the present great oceans, and the remains of which,
falling to the bottom of the sea, give rise to a chalky mud. Hence it must
be admitted that certain existing species of animals show no distinct sign
of modification, or transformation, in the course of a lapse of time as
great as that which carries us back to the Cretaceous period; and which,
whatever its absolute measure, is certainly vastly greater than thirty
thousand years.

There are groups of species so closely allied together, that it needs the
eye of a naturalist to distinguish them one from another. If we disregard
the small differences which separate these forms, and consider all the
species of such groups as modifications of one type, we shall find that,
even among the higher animals, some types have had a marvellous duration.
In the chalk, for example, there is found a fish belonging to the highest
and the most differentiated group of osseous fishes, which goes by the
name of Beryx. The remains of that fish are among the most
beautiful and well-preserved of the fossils found in our English chalk. It
can be studied anatomically, so far as the hard parts are concerned,
almost as well as if it were a recent fish. But the genus Beryx is
represented, at the present day, by very closely allied species which are
living in the Pacific and Atlantic Oceans. We may go still farther back. I
have already referred to the fact that the Carboniferous formations, in
Europe and in America, contain the remains of scorpions in an admirable
state of preservation, and that those scorpions are hardly distinguishable
from such as now live. I do not mean to say that they are not different,
but close scrutiny is needed in order to distinguish them from modern
scorpions.

More than this. At the very bottom of the Silurian series, in beds which
are by some authorities referred to the Cambrian formation, where the
signs of life begin to fail us—even there, among the few and scanty
animal remains which are discoverable, we find species of molluscous
animals which are so closely allied to existing forms that, at one time,
they were grouped under the same generic name. I refer to the well-known
Lingula of the Lingula flags, lately, in consequence of some
slight differences, placed in the new genus Lingulella.
Practically, it belongs to the same great generic group as the Lingula,
which is to be found at the present day upon your own shores and those of
many other parts of the world.

The same truth is exemplified if we turn to certain great periods of the
earth’s history—as, for example, the Mesozoic epoch. There are
groups of reptiles, such as the Ichthyosauria and the Plesiosauria,
which appear shortly after the commencement of this epoch, and they occur
in vast numbers. They disappear with the chalk and, throughout the whole
of the great series of Mesozoic rocks, they present no such modifications
as can safely be considered evidence of progressive modification.

Facts of this kind are undoubtedly fatal to any form of the doctrine of
evolution which postulates the supposition that there is an intrinsic
necessity, on the part of animal forms which have once come into
existence, to undergo continual modification; and they are as distinctly
opposed to any view which involves the belief, that such modification may
occur, must take place, at the same rate, in all the different types of
animal or vegetable life. The facts, as I have placed them before you,
obviously directly contradict any form of the hypothesis of evolution
which stands in need of these two postulates.

But, one great service that has been rendered by Mr. Darwin to the
doctrine of evolution in general is this: he has shown that there are two
chief factors in the process of evolution: one of them is the tendency to
vary, the existence of which in all living forms may be proved by
observation; the other is the influence of surrounding conditions upon
what I may call the parent form and the variations which are thus evolved
from it. The cause of the production of variations is a matter not at all
properly understood at present. Whether variation depends upon some
intricate machinery—if I may use the phrase—of the living
organism itself, or whether it arises through the influence of conditions
upon that form, is not certain, and the question may, for the present, be
left open. But the important point is that, granting the existence of the
tendency to the production of variations; then, whether the variations
which are produced shall survive and supplant the parent, or whether the
parent form shall survive and supplant the variations, is a matter which
depends entirely on those conditions which give rise to the struggle for
existence. If the surrounding conditions are such that the parent form is
more competent to deal with them, and flourish in them than the derived
forms, then, in the struggle for existence, the parent form will maintain
itself and the derived forms will be exterminated. But if, on the
contrary, the conditions are such as to be more favourable to a derived
than to the parent form, the parent form will be extirpated and the
derived form will take its place. In the first case, there will be no
progression, no change of structure, through any imaginable series of
ages; in the second place there will be modification of change and form.

Thus the existence of these persistent types, as I have termed them, is no
real obstacle in the way of the theory of evolution. Take the case of the
scorpions to which I have just referred. No doubt, since the Carboniferous
epoch, conditions have always obtained, such as existed when the scorpions
of that epoch flourished; conditions in which scorpions find themselves
better off, more competent to deal with the difficulties in their way,
than any variation from the scorpion type which they may have produced;
and, for that reason, the scorpion type has persisted, and has not been
supplanted by any other form. And there is no reason, in the nature of
things, why, as long as this world exists, if there be conditions more
favourable to scorpions than to any variation which may arise from them,
these forms of life should not persist.

Therefore, the stock objection to the hypothesis of evolution, based on
the long duration of certain animal and vegetable types, is no objection
at all. The facts of this character—and they are numerous—belong
to that class of evidence which I have called indifferent. That is to say,
they may afford no direct support to the doctrine of evolution, but they
are capable of being interpreted in perfect consistency with it.

There is another order of facts belonging to the class of negative or
indifferent evidence. The great group of Lizards, which abound in the
present world, extends through the whole series of formations as far back
as the Permian, or latest Palaeozoic, epoch. These Permian lizards differ
astonishingly little from the lizards which exist at the present day.
Comparing the amount of the differences between them and modern lizards,
with the prodigious lapse of time between the Permian epoch and the
present day, it may be said that the amount of change is insignificant.
But, when we carry our researches farther back in time, we find no trace
of lizards, nor of any true reptile whatever, in the whole mass of
formations beneath the Permian.

Now, it is perfectly clear that if our palaeontological collections are to
be taken, even approximately, as an adequate representation of all the
forms of animals and plants that have ever lived; and if the record
furnished by the known series of beds of stratified rock covers the whole
series of events which constitute the history of life on the globe, such a
fact as this directly contravenes the hypothesis of evolution; because
this hypothesis postulates that the existence of every form must have been
preceded by that of some form little different from it. Here, however, we
have to take into consideration that important truth so well insisted upon
by Lyell and by Darwin—the imperfection of the geological record. It
can be demonstrated that the geological record must be incomplete, that it
can only preserve remains found in certain favourable localities and under
particular conditions; that it must be destroyed by processes of
denudation, and obliterated by processes of metamorphosis. Beds of rock of
any thickness crammed full of organic remains, may yet, either by the
percolation of water through them, or by the influence of subterranean
heat, lose all trace of these remains, and present the appearance of beds
of rock formed under conditions in which living forms were absent. Such
metamorphic rocks occur in formations of all ages; and, in various cases,
there are very good grounds for the belief that they have contained
organic remains, and that those remains have been absolutely obliterated.

I insist upon the defects of the geological record the more because those
who have not attended to these matters are apt to say, “It is all very
well, but, when you get into a difficulty with your theory of evolution,
you appeal to the incompleteness and the imperfection of the geological
record;” and I want to make it perfectly clear to you that this
imperfection is a great fact, which must be taken into account in all our
speculations, or we shall constantly be going wrong.

You see the singular series of footmarks, drawn of its natural size in the
large diagram hanging up here (Fig. 2), which I owe to the kindness of my
friend Professor Marsh, with whom I had the opportunity recently of
visiting the precise locality in Massachusetts in which these tracks
occur. I am, therefore, able to give you my own testimony, if needed, that
the diagram accurately represents what we saw. The valley of the
Connecticut is classical ground for the geologist. It contains great beds
of sandstone, covering many square miles, which have evidently formed a
part of an ancient sea-shore, or, it may be, lake-shore. For a certain
period of time after their deposition, these beds have remained
sufficiently soft to receive the impressions of the feet of whatever
animals walked over them, and to preserve them afterwards, in exactly the
same way as such impressions are at this hour preserved on the shores of
the Bay of Fundy and elsewhere. The diagram represents the track of some
gigantic animal, which walked on its hind legs. You see the series of
marks made alternately by the right and by the left foot; so that, from
one impression to the other of the three-toed foot on the same side, is
one stride, and that stride, as we measured it, is six feet nine inches. I
leave you, therefore, to form an impression of the magnitude of the
creature which, as it walked along the ancient shore, made these
impressions.

Of such impressions there are untold thousands upon these sandstones.
Fifty or sixty different kinds have been discovered, and they cover vast
areas. But, up to this present time, not a bone, not a fragment, of any
one of the animals which left these great footmarks has been found; in
fact, the only animal remains which have been met with in all these
deposits, from the time of their discovery to the present day—though
they have been carefully hunted over—is a fragmentary skeleton of
one of the smaller forms. What has become of the bones of all these
animals? You see we are not dealing with little creatures, but with
animals that make a step of six feet nine inches; and their remains must
have been left somewhere. The probability is, that they have been
dissolved away, and completely lost.

I have had occasion to work out the nature of fossil remains, of which
there was nothing left except casts of the bones, the solid material of
the skeleton having been dissolved out by percolating water. It was a
chance, in this case, that the sandstone happened to be of such a
constitution as to set, and to allow the bones to be afterward dissolved
out, leaving cavities of the exact shape of the bones. Had that
constitution been other than what it was, the bones would have been
dissolved, the layers of sandstone would have fallen together into one
mass, and not the slightest indication that the animal had existed would
have been discoverable.

I know of no more striking evidence than these facts afford, of the
caution which should be used in drawing the conclusion, from the absence
of organic remains in a deposit, that animals or plants did not exist at
the time it was formed. I believe that, with a right understanding of the
doctrine of evolution on the one hand, and a just estimation of the
importance of the imperfection of the geological record on the other, all
difficulty is removed from the kind of evidence to which I have adverted;
and that we are justified in believing that all such cases are examples of
what I have designated negative or indifferent evidence—that is to
say, they in no way directly advance the hypothesis of evolution, but they
are not to be regarded as obstacles in the way of our belief in that
doctrine.

I now pass on to the consideration of those cases which, for reasons which
I will point out to you by and by, are not to be regarded as demonstrative
of the truth of evolution, but which are such as must exist if evolution
be true, and which therefore are, upon the whole, evidence in favour of
the doctrine. If the doctrine of evolution be true, it follows, that,
however diverse the different groups of animals and of plants may be, they
must all, at one time or other, have been connected by gradational forms;
so that, from the highest animals, whatever they may be, down to the
lowest speck of protoplasmic matter in which life can be manifested, a
series of gradations, leading from one end of the series to the other,
either exists or has existed. Undoubtedly that is a necessary postulate of
the doctrine of evolution. But when we look upon living Nature as it is,
we find a totally different state of things. We find that animals and
plants fall into groups, the different members of which are pretty closely
allied together, but which are separated by definite, larger or smaller,
breaks, from other groups. In other words, no intermediate forms which
bridge over these gaps or intervals are, at present, to be met with.

To illustrate what I mean: Let me call your attention to those vertebrate
animals which are most familiar to you, such as mammals, birds, and
reptiles. At the present day, these groups of animals are perfectly
well-defined from one another. We know of no animal now living which, in
any sense, is intermediate between the mammal and the bird, or between the
bird and the reptile; but, on the contrary, there are many very distinct
anatomical peculiarities, well-defined marks, by which the mammal is
separated from the bird, and the bird from the reptile. The distinctions
are obvious and striking if you compare the definitions of these great
groups as they now exist.

The same may be said of many of the subordinate groups, or orders, into
which these great classes are divided. At the present time, for example,
there are numerous forms of non-ruminant pachyderms, or what we may call
broadly, the pig tribe, and many varieties of ruminants. These latter have
their definite characteristics, and the former have their distinguishing
peculiarities. But there is nothing that fills up the gap between the
ruminants and the pig tribe. The two are distinct. Such also is the case
in respect of the minor groups of the class of reptiles. The existing
fauna shows us crocodiles, lizards, snakes, and tortoises; but no
connecting link between the crocodile and lizard, nor between the lizard
and snake, nor between the snake and the crocodile, nor between any two of
these groups. They are separated by absolute breaks. If, then, it could be
shown that this state of things had always existed, the fact would be
fatal to the doctrine of evolution. If the intermediate gradations, which
the doctrine of evolution requires to have existed between these groups,
are not to be found anywhere in the records of the past history of the
globe, their absence is a strong and weighty negative argument against
evolution; while, on the other hand, if such intermediate forms are to be
found, that is so much to the good of evolution; although, for reasons
which I will lay before you by and by, we must be cautious in our estimate
of the evidential cogency of facts of this kind.

It is a very remarkable circumstance that, from the commencement of the
serious study of fossil remains, in fact, from the time when Cuvier began
his brilliant researches upon those found in the quarries of Montmartre,
palaeontology has shown what she was going to do in this matter, and what
kind of evidence it lay in her power to produce.

I said just now that, in the existing Fauna, the group of pig-like animals
and the group of ruminants are entirely distinct; but one of the first of
Cuvier’s discoveries was an animal which he called the Anoplotherium,
and which proved to be, in a great many important respects, intermediate
in character between the pigs, on the one hand, and the ruminants on the
other. Thus, research into the history of the past did, to a certain
extent, tend to fill up the breach between the group of ruminants and the
group of pigs. Another remarkable animal restored by the great French
palaeontologist, the Palaeotherium, similarly tended to connect
together animals to all appearance so different as the rhinoceros, the
horse, and the tapir. Subsequent research has brought to light multitudes
of facts of the same order; and at the present day, the investigations of
such anatomists as Rutimeyer and Gaudry have tended to fill up, more and
more, the gaps in our existing series of mammals, and to connect groups
formerly thought to be distinct.

But I think it may have an especial interest if, instead of dealing with
these examples, which would require a great deal of tedious osteological
detail, I take the case of birds and reptiles; groups which, at the
present day, are so clearly distinguished from one another that there are
perhaps no classes of animals which, in popular apprehension, are more
completely separated. Existing birds, as you are aware, are covered with
feathers; their anterior extremities, specially and peculiarly modified,
are converted into wings by the aid of which most of them are able to fly;
they walk upright upon two legs; and these limbs, when they are considered
anatomically, present a great number of exceedingly remarkable
peculiarities, to which I may have occasion to advert incidentally as I go
on, and which are not met with, even approximately, in any existing forms
of reptiles. On the other hand, existing reptiles have no feathers. They
may have naked skins, or be covered with horny scales, or bony plates, or
with both. They possess no wings; they neither fly by means of their
fore-limbs, nor habitually walk upright upon their hind-limbs; and the
bones of their legs present no such modifications as we find in birds. It
is impossible to imagine any two groups more definitely and distinctly
separated, notwithstanding certain characters which they possess in
common.

As we trace the history of birds back in time, we find their remains,
sometimes in great abundance, throughout the whole extent of the tertiary
rocks; but, so far as our present knowledge goes, the birds of the
tertiary rocks retain the same essential characters as the birds of the
present day. In other words, the tertiary birds come within the definition
of the class constituted by existing birds, and are as much separated from
reptiles as existing birds are. Not very long ago no remains of birds had
been found below the tertiary rocks, and I am not sure but that some
persons were prepared to demonstrate that they could not have existed at
an earlier period. But, in the course of the last few years, such remains
have been discovered in England; though, unfortunately, in so imperfect
and fragmentary a condition, that it is impossible to say whether they
differed from existing birds in any essential character or not. In your
country the development of the cretaceous series of rocks is enormous; the
conditions under which the later cretaceous strata have been deposited are
highly favourable to the preservation of organic remains; and the
researches, full of labour and risk, which have been carried on by
Professor Marsh in these cretaceous rocks of Western America, have
rewarded him with the discovery of forms of birds of which we had hitherto
no conception. By his kindness, I am enabled to place before you a
restoration of one of these extraordinary birds, every part of which can
be thoroughly justified by the more or less complete skeletons, in a very
perfect state of preservation, which he has discovered. This Hesperornis
(Fig. 3), which measured between five and six feet in length, is
astonishingly like our existing divers or grebes in a great many respects;
so like them indeed that, had the skeleton of Hesperornis been
found in a museum without its skull, it probably would have been placed in
the same group of birds as the divers and grebes of the present day. 1
But Hesperornis differs from all existing birds, and so far
resembles reptiles, in one important particular—it is provided with
teeth. The long jaws are armed with teeth which have curved crowns and
thick roots (Fig. 4), and are not set in distinct sockets, but are lodged
in a groove. In possessing true teeth, the Hesperornis differs from
every existing bird, and from every bird yet discovered in the tertiary
formations, the tooth-like serrations of the jaws in the Odontopteryx
of the London clay being mere processes of the bony substance of the jaws,
and not teeth in the proper sense of the word. In view of the
characteristics of this bird we are therefore obliged to modify the
definitions of the classes of birds and reptiles. Before the discovery of
Hesperornis, the definition of the class Aves based upon our
knowledge of existing birds might have been extended to all birds; it
might have been said that the absence of teeth was characteristic of the
class of birds; but the discovery of an animal which, in every part of its
skeleton, closely agrees with existing birds, and yet possesses teeth,
shows that there were ancient birds which, in respect of possessing teeth,
approached reptiles more nearly than any existing bird does, and, to that
extent, diminishes the hiatus between the two classes.

The same formation has yielded another bird, Ichthyornis (Fig. 5),
which also possesses teeth; but the teeth are situated in distinct
sockets, while those of Hesperornis are not so lodged. The latter
also has such very small, almost rudimentary wings, that it must have been
chiefly a swimmer and a diver like a Penguin; while Ichthyornis has
strong wings and no doubt possessed corresponding powers of flight. Ichthyornis
also differed in the fact that its vertebrae have not the peculiar
characters of the vertebrae of existing and of all known tertiary birds,
but were concave at each end. This discovery leads us to make a further
modification in the definition of the group of birds, and to part with
another of the characters by which almost all existing birds are
distinguished from reptiles.

Apart from the few fragmentary remains from the English greensand, to
which I have referred, the Mesozoic rocks, older than those in which Hesperornis
and Ichthyornis have been discovered, have afforded no certain
evidence of birds, with the remarkable exception of the Solenhofen slates.
These so-called slates are composed of a fine grained calcareous mud which
has hardened into lithographic stone, and in which organic remains are
almost as well preserved as they would be if they had been imbedded in so
much plaster of Paris. They have yielded the Archaeopteryx, the
existence of which was first made known by the finding of a fossil
feather, or rather of the impression of one. It is wonderful enough that
such a perishable thing as a feather, and nothing more, should be
discovered; yet, for a long time, nothing was known of this bird except
its feather. But by and by a solitary skeleton was discovered which is now
in the British Museum. The skull of this solitary specimen is
unfortunately wanting, and it is therefore uncertain whether the Archaeopteryx
possessed teeth or not. 2 But the remainder of the skeleton
is so well preserved as to leave no doubt respecting the main features of
the animal, which are very singular. The feet are not only altogether
bird-like, but have the special characters of the feet of perching birds,
while the body had a clothing of true feathers. Nevertheless, in some
other respects, Archaeopteryx is unlike a bird and like a reptile.
There is a long tail composed of many vertebrae. The structure of the wing
differs in some very remarkable respects from that which it presents in a
true bird. In the latter, the end of the wing answers to the thumb and two
fingers of my hand; but the metacarpal bones, or those which answer to the
bones of the fingers which lie in the palm of the hand, are fused together
into one mass; and the whole apparatus, except the last joints of the
thumb, is bound up in a sheath of integument, while the edge of the hand
carries the principal quill-feathers. In the Archaeopteryx, the
upper-arm bone is like that of a bird; and the two bones of the forearm
are more or less like those of a bird, but the fingers are not bound
together—they are free. What their number may have been is
uncertain; but several, if not all, of them were terminated by strong
curved claws, not like such as are sometimes found in birds, but such as
reptiles possess; so that, in the Archaeopteryx, we have an animal
which, to a certain extent, occupies a midway place between a bird and a
reptile. It is a bird so far as its foot and sundry other parts of its
skeleton are concerned; it is essentially and thoroughly a bird by its
feathers; but it is much more properly a reptile in the fact that the
region which represents the hand has separate bones, with claws resembling
those which terminate the forelimb of a reptile. Moreover, it has a long
reptile-like tail with a fringe of feathers on each side; while, in all
true birds hitherto known, the tail is relatively short, and the vertebrae
which constitute its skeleton are generally peculiarly modified.

Like the Anoplotherium and the Palaeotherium, therefore, Archaeopteryx
tends to fill up the interval between groups which, in the existing world,
are widely separated, and to destroy the value of the definitions of
zoological groups based upon our knowledge of existing forms. And such
cases as these constitute evidence in favour of evolution, in so far as
they prove that, in former periods of the world’s history, there were
animals which overstepped the bounds of existing groups, and tended to
merge them into larger assemblages. They show that animal organisation is
more flexible than our knowledge of recent forms might have led us to
believe; and that many structural permutations and combinations, of which
the present world gives us no indication, may nevertheless have existed.

But it by no means follows, because the Palaeotherium has much in
common with the horse, on the one hand, and with the rhinoceros on the
other, that it is the intermediate form through which rhinoceroses have
passed to become horses, or vice versa; on the contrary, any such
supposition would certainly be erroneous. Nor do I think it likely that
the transition from the reptile to the bird has been effected by such a
form as Archaeopteryx. And it is convenient to distinguish these
intermediate forms between two groups, which do not represent the actual
passage from the one group to the other, as intercalary types, from
those linear types which, more or less approximately, indicate the
nature of the steps by which the transition from one group to the other
was effected.

I conceive that such linear forms, constituting a series of natural
gradations between the reptile and the bird, and enabling us to understand
the manner in which the reptilian has been metamorphosed into the bird
type, are really to be found among a group of ancient and extinct
terrestrial reptiles known as the Ornithoscelida. The remains of
these animals occur throughout the series of mesozoic formations, from the
Trias to the Chalk, and there are indications of their existence even in
the later Palaeozoic strata.

Most of these reptiles, at present known, are of great size, some having
attained a length of forty feet or perhaps more. The majority resembled
lizards and crocodiles in their general form, and many of them were, like
crocodiles, protected by an armour of heavy bony plates. But, in others,
the hind limbs elongate and the fore limbs shorten, until their relative
proportions approach those which are observed in the short-winged,
flightless, ostrich tribe among birds.

The skull is relatively light, and in some cases the jaws, though bearing
teeth, are beak-like at their extremities and appear to have been
enveloped in a horny sheath. In the part of the vertebral column which
lies between the haunch bones and is called the sacrum, a number of
vertebrae may unite together into one whole, and in this respect, as in
some details of its structure, the sacrum of these reptiles approaches
that of birds.

But it is in the structure of the pelvis and of the hind limb that some of
these ancient reptiles present the most remarkable approximation to birds,
and clearly indicate the way by which the most specialised and
characteristic features of the bird may have been evolved from the
corresponding parts of the reptile.

In Fig. 6, the pelvis and hind limbs of a crocodile, a three-toed bird,
and an ornithoscelidan are represented side by side; and, for facility of
comparison, in corresponding positions; but it must be recollected that,
while the position of the bird’s limb is natural, that of the crocodile is
not so. In the bird, the thigh bone lies close to the body, and the
metatarsal bones of the foot (ii., iii., iv., Fig. 6) are, ordinarily,
raised into a more or less vertical position; in the crocodile, the thigh
bone stands out at an angle from the body, and the metatarsal bones (i.,
ii., iii., iv., Fig. 6) lie flat on the ground. Hence, in the crocodile,
the body usually lies squat between the legs, while, in the bird, it is
raised upon the hind legs, as upon pillars.

In the crocodile, the pelvis is obviously composed of three bones on each
side: the ilium (Il.), the pubis (Pb.), and the ischium (Is.).
In the adult bird there appears to be but one bone on each side. The
examination of the pelvis of a chick, however, shows that each half is
made up of three bones, which answer to those which remain distinct
throughout life in the crocodile. There is, therefore, a fundamental
identity of plan in the construction of the pelvis of both bird and
reptile; though the difference in form, relative size, and direction of
the corresponding bones in the two cases are very great.

But the most striking contrast between the two lies in the bones of the
leg and of that part of the foot termed the tarsus, which follows upon the
leg. In the crocodile, the fibula (F) is relatively large and its
lower end is complete. The tibia (T) has no marked crest at its
upper end, and its lower end is narrow and not pulley-shaped. There are
two rows of separate tarsal bones (As., Ca., &c.) and four
distinct metatarsal bones, with a rudiment of a fifth.

In the bird, the fibula is small and its lower end diminishes to a point.
The tibia has a strong crest at its upper end and its lower extremity
passes into a broad pulley. There seem at first to be no tarsal bones; and
only one bone, divided at the end into three heads for the three toes
which are attached to it, appears in the place of the metatarsus.

In the young bird, however, the pulley-shaped apparent end of the tibia is
a distinct bone, which represents the bones marked As., Ca., in the
crocodile; while the apparently single metatarsal bone consists of three
bones, which early unite with one another and with an additional bone,
which represents the lower row of bones in the tarsus of the crocodile.

In other words, it can be shown by the study of development that the
bird’s pelvis and hind limb are simply extreme modifications of the same
fundamental plan as that upon which these parts are modelled in reptiles.

On comparing the pelvis and hind limb of the ornithoscelidan with that of
the crocodile, on the one side, and that of the bird, on the other (Fig.
6), it is obvious that it represents a middle term between the two. The
pelvic bones approach the form of those of the birds, and the direction of
the pubis and ischium is nearly that which is characteristic of birds; the
thigh bone, from the direction of its head, must have lain close to the
body; the tibia has a great crest; and, immovably fitted on to its lower
end, there is a pulley-shaped bone, like that of the bird, but remaining
distinct. The lower end of the fibula is much more slender,
proportionally, than in the crocodile. The metatarsal bones have such a
form that they fit together immovably, though they do not enter into bony
union; the third toe is, as in the bird, longest and strongest. In fact,
the ornithoscelidan limb is comparable to that of an unhatched chick.

Taking all these facts together, it is obvious that the view, which was
entertained by Mantell and the probability of which was demonstrated by
your own distinguished anatomist, Leidy, while much additional evidence in
the same direction has been furnished by Professor Cope, that some of
these animals may have walked upon their hind legs as birds do, acquires
great weight. In fact, there can be no reasonable doubt that one of the
smaller forms of the Ornithoscelida, Compsognathus, the almost
entire skeleton of which has been discovered in the Solenhofen slates, was
a bipedal animal. The parts of this skeleton are somewhat twisted out of
their natural relations, but the accompanying figure gives a just view of
the general form of Compsognathus and of the proportions of its
limbs; which, in some respects, are more completely bird-like than those
of other Ornithoscelida.

We have had to stretch the definition of the class of birds so as to
include birds with teeth and birds with paw-like fore limbs and long
tails. There is no evidence that Compsognathus possessed feathers;
but, if it did, it would be hard indeed to say whether it should be called
a reptilian bird or an avian reptile.

As Compsognathus walked upon its hind legs, it must have made
tracks like those of birds. And as the structure of the limbs of several
of the gigantic Ornithoscelida, such as Iguanodon, leads to
the conclusion that they also may have constantly, or occasionally,
assumed the same attitude, a peculiar interest attaches to the fact that,
in the Wealden strata of England, there are to be found gigantic
footsteps, arranged in order like those of the Brontozoum, and
which there can be no reasonable doubt were made by some of the Ornithoscelida,
the remains of which are found in the same rocks. And, knowing that
reptiles that walked upon their hind legs and shared many of the
anatomical characters of birds did once exist, it becomes a very important
question whether the tracks in the Trias of Massachusetts, to which I
referred some time ago, and which formerly used to be unhesitatingly
ascribed to birds, may not all have been made by ornithoscelidan reptiles;
and whether, if we could obtain the skeletons of the animals which made
these tracks, we should not find in them the actual steps of the
evolutional process by which reptiles gave rise to birds.

The evidential value of the facts I have brought forward in this Lecture
must be neither over nor under estimated. It is not historical proof of
the occurrence of the evolution of birds from reptiles, for we have no
safe ground for assuming that true birds had not made their appearance at
the commencement of the Mesozoic epoch. It is, in fact, quite possible
that all these more or less avi-form reptiles of the Mesozoic epochs are
not terms in the series of progression from birds to reptiles at all, but
simply the more or less modified descendants of Palaeozoic forms through
which that transition was actually effected.

We are not in a position to say that the known Ornithoscelida are
intermediate in the order of their appearance on the earth between
reptiles and birds. All that can be said is that, if independent evidence
of the actual occurrence of evolution is producible, then these
intercalary forms remove every difficulty in the way of understanding what
the actual steps of the process, in the case of birds, may have been.

That intercalary forms should have existed in ancient times is a necessary
consequence of the truth of the hypothesis of evolution; and, hence, the
evidence I have laid before you in proof of the existence of such forms,
is, so far as it goes, in favour of that hypothesis.

There is another series of extinct reptiles which may be said to be
intercalary between reptiles and birds, in so far as they combine some of
the characters of both these groups; and which, as they possessed the
power of flight, may seem, at first sight, to be nearer representatives of
the forms by which the transition from the reptile to the bird was
effected, than the Ornithoscelida.

These are the Pterosauria, or Pterodactyles, the remains of which
are met with throughout the series of Mesozoic rocks, from the lias to the
chalk, and some of which attained a great size, their wings having a span
of eighteen or twenty feet. These animals, in the form and proportions of
the head and neck relatively to the body, and in the fact that the ends of
the jaws were often, if not always, more or less extensively ensheathed in
horny beaks, remind us of birds. Moreover, their bones contained air
cavities, rendering them specifically lighter, as is the case in most
birds. The breast bone was large and keeled, as in most birds and in bats,
and the shoulder girdle is strikingly similar to that of ordinary birds.
But, it seems to me, that the special resemblance of pterodactyles to
birds ends here, unless I may add the entire absence of teeth which
characterises the great pterodactyles (Pteranodon) discovered by
Professor Marsh. All other known pterodactyles have teeth lodged in
sockets. In the vertebral column and the hind limbs there are no special
resemblances to birds, and when we turn to the wings they are found to be
constructed on a totally different principle from those of birds.

There are four fingers. These four fingers are large, and three of them,
those which answer to the thumb and two following fingers in my hand—are
terminated by claws, while the fourth is enormously prolonged and
converted into a great jointed style. You see at once, from what I have
stated about a bird’s wing, that there could be nothing less like a bird’s
wing than this is. It was concluded by general reasoning that this finger
had the office of supporting a web which extended between it and the body.
An existing specimen proves that such was really the case, and that the
pterodactyles were devoid of feathers, but that the fingers supported a
vast web like that of a bat’s wing; in fact, there can be no doubt that
this ancient reptile flew after the fashion of a bat.

Thus, though the pterodactyle is a reptile which has become modified in
such a manner as to enable it to fly, and therefore, as might be expected,
presents some points of resemblance to other animals which fly; it has, so
to speak, gone off the line which leads directly from reptiles to birds,
and has become disqualified for the changes which lead to the
characteristic organisation of the latter class. Therefore, viewed in
relation to the classes of reptiles and birds, the pterodactyles appear to
me to be, in a limited sense, intercalary forms; but they are not even
approximately linear, in the sense of exemplifying those modifications of
structure through which the passage from the reptile to the bird took
place.

III. THE DEMONSTRATIVE EVIDENCE OF EVOLUTION

The occurrence of historical facts is said to be demonstrated, when the
evidence that they happened is of such a character as to render the
assumption that they did not happen in the highest degree improbable; and
the question I now have to deal with is, whether evidence in favour of the
evolution of animals of this degree of cogency is, or is not, obtainable
from the record of the succession of living forms which is presented to us
by fossil remains.

Those who have attended to the progress of palaeontology are aware that
evidence of the character which I have defined has been produced in
considerable and continually-increasing quantity during the last few
years. Indeed, the amount and the satisfactory nature of that evidence are
somewhat surprising, when we consider the conditions under which alone we
can hope to obtain it.

It is obviously useless to seek for such evidence except in localities in
which the physical conditions have been such as to permit of the deposit
of an unbroken, or but rarely interrupted, series of strata through a long
period of time; in which the group of animals to be investigated has
existed in such abundance as to furnish the requisite supply of remains;
and in which, finally, the materials composing the strata are such as to
ensure the preservation of these remains in a tolerably perfect and
undisturbed state.

It so happens that the case which, at present, most nearly fulfils all
these conditions is that of the series of extinct animals which culminates
in the horses; by which term I mean to denote not merely the domestic
animals with which we are all so well acquainted, but their allies, the
ass, zebra, quagga, and the like. In short, I use “horses” as the
equivalent of the technical name Equidae, which is applied to the
whole group of existing equine animals.

The horse is in many ways a remarkable animal; not least so in the fact
that it presents us with an example of one of the most perfect pieces of
machinery in the living world. In truth, among the works of human
ingenuity it cannot be said that there is any locomotive so perfectly
adapted to its purposes, doing so much work with so small a quantity of
fuel, as this machine of nature’s manufacture—the horse. And, as a
necessary consequence of any sort of perfection, of mechanical perfection
as of others, you find that the horse is a beautiful creature, one of the
most beautiful of all land-animals. Look at the perfect balance of its
form, and the rhythm and force of its action. The locomotive machinery is,
as you are aware, resident in its slender fore and hind limbs; they are
flexible and elastic levers, capable of being moved by very powerful
muscles; and, in order to supply the engines which work these levers with
the force which they expend, the horse is provided with a very perfect
apparatus for grinding its food and extracting therefrom the requisite
fuel.

Without attempting to take you very far into the region of osteological
detail, I must nevertheless trouble you with some statements respecting
the anatomical structure of the horse; and, more especially, will it be
needful to obtain a general conception of the structure of its fore and
hind limbs, and of its teeth. But I shall only touch upon those points
which are absolutely essential to our inquiry.

Let us turn in the first place to the fore-limb. In most quadrupeds, as in
ourselves, the fore-arm contains distinct bones called the radius and the
ulna. The corresponding region in the horse seems at first to possess but
one bone. Careful observation, however, enables us to distinguish in this
bone a part which clearly answers to the upper end of the ulna. This is
closely united with the chief mass of the bone which represents the
radius, and runs out into a slender shaft which may be traced for some
distance downwards upon the back of the radius, and then in most cases
thins out and vanishes. It takes still more trouble to make sure of what
is nevertheless the fact, that a small part of the lower end of the bone
of the horse’s fore arm, which is only distinct in a very young foal, is
really the lower extremity of the ulna.

What is commonly called the knee of a horse is its wrist. The “cannon
bone” answers to the middle bone of the five metacarpal bones, which
support the palm of the hand in ourselves. The “pastern,” “coronary,” and
“coffin” bones of veterinarians answer to the joints of our middle
fingers, while the hoof is simply a greatly enlarged and thickened nail.
But if what lies below the horse’s “knee” thus corresponds to the middle
finger in ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two slender
splint-like bones, about two-thirds as long as the cannon bone, which
gradually taper to their lower ends and bear no finger joints, or, as they
are termed, phalanges. Sometimes, small bony or gristly nodules are to be
found at the bases of these two metacarpal splints, and it is probable
that these represent rudiments of the first and fifth toes. Thus, the part
of the horse’s skeleton, which corresponds with that of the human hand,
contains one overgrown middle digit, and at least two imperfect lateral
digits; and these answer, respectively, to the third, the second, and the
fourth fingers in man.

Corresponding modifications are found in the hind limb. In ourselves, and
in most quadrupeds, the leg contains two distinct bones, a large bone, the
tibia, and a smaller and more slender bone, the fibula. But, in the horse,
the fibula seems, at first, to be reduced to its upper end; a short
slender bone united with the tibia, and ending in a point below, occupying
its place. Examination of the lower end of a young foal’s shin bone,
however, shows a distinct portion of osseous matter, which is the lower
end of the fibula; so that the apparently single, lower end of the shin
bone is really made up of the coalesced ends of the tibia and fibula, just
as the apparently single, lower end of the fore-arm bone is composed of
the coalesced radius and ulna.

The heel of the horse is the part commonly known as the hock. The hinder
cannon bone answers to the middle metatarsal bone of the human foot, the
pastern, coronary, and coffin bones, to the middle toe bones; the hind
hoof to the nail; as in the fore-foot. And, as in the fore-foot, there are
merely two splints to represent the second and the fourth toes. Sometimes
a rudiment of a fifth toe appears to be traceable.

The teeth of a horse are not less peculiar than its limbs. The living
engine, like all others, must be well stoked if it is to do its work; and
the horse, if it is to make good its wear and tear, and to exert the
enormous amount of force required for its propulsion, must be well and
rapidly fed. To this end, good cutting instruments and powerful and
lasting crushers are needful. Accordingly, the twelve cutting teeth of a
horse are close-set and concentrated in the fore-part of its mouth, like
so many adzes or chisels. The grinders or molars are large, and have an
extremely complicated structure, being composed of a number of different
substances of unequal hardness. The consequence of this is that they wear
away at different rates; and, hence, the surface of each grinder is always
as uneven as that of a good millstone.

I have said that the structure of the grinding teeth is very complicated,
the harder and the softer parts being, as it were, interlaced with one
another. The result of this is that, as the tooth wears, the crown
presents a peculiar pattern, the nature of which is not very easily
deciphered at first; but which it is important we should understand
clearly. Each grinding tooth of the upper jaw has an outer wall so
shaped that, on the worn crown, it exhibits the form of two crescents, one
in front and one behind, with their concave sides turned outwards. From
the inner side of the front crescent, a crescentic front ridge
passes inwards and backwards, and its inner face enlarges into a strong
longitudinal fold or pillar. From the front part of the hinder
crescent, a back ridge takes a like direction, and also has its pillar.

The deep interspaces or valleys between these ridges and the outer
wall are filled by bony substance, which is called cement, and
coats the whole tooth.

The pattern of the worn face of each grinding tooth of the lower jaw is
quite different. It appears to be formed of two crescent-shaped ridges,
the convexities of which are turned outwards. The free extremity of each
crescent has a pillar, and there is a large double pillar
where the two crescents meet. The whole structure is, as it were, imbedded
in cement, which fills up the valleys, as in the upper grinders.

If the grinding faces of an upper and of a lower molar of the same side
are applied together, it will be seen that the opposed ridges are nowhere
parallel, but that they frequently cross; and that thus, in the act of
mastication, a hard surface in the one is constantly applied to a soft
surface in the other, and vice versa. They thus constitute a
grinding apparatus of great efficiency, and one which is repaired as fast
as it wears, owing to the long-continued growth of the teeth.

Some other peculiarities of the dentition of the horse must be noticed, as
they bear upon what I shall have to say by and by. Thus the crowns of the
cutting teeth have a peculiar deep pit, which gives rise to the well-known
“mark” of the horse. There is a large space between the outer incisors and
the front grinder. In this space the adult male horse presents, near the
incisors on each side, above and below, a canine or “tush,” which is
commonly absent in mares. In a young horse, moreover, there is not
unfrequently to be seen in front of the first grinder, a very small tooth,
which soon falls out. If this small tooth be counted as one, it will be
found that there are seven teeth behind the canine on each side; namely,
the small tooth in question, and the six great grinders, among which, by
an unusual peculiarity, the foremost tooth is rather larger than those
which follow it.

I have now enumerated those characteristic structures of the horse which
are of most importance for the purpose we have in view.

To any one who is acquainted with the morphology of vertebrated animals,
they show that the horse deviates widely from the general structure of
mammals; and that the horse type is, in many respects, an extreme
modification of the general mammalian plan. The least modified mammals, in
fact, have the radius and ulna, the tibia and fibula, distinct and
separate. They have five distinct and complete digits on each foot, and no
one of these digits is very much larger than the rest. Moreover, in the
least modified mammals, the total number of the teeth is very generally
forty-four, while in horses, the usual number is forty, and in the absence
of the canines, it may be reduced to thirty-six; the incisor teeth are
devoid of the fold seen in those of the horse: the grinders regularly
diminish in size from the middle of the series to its front end; while
their crowns are short, early attain their full length, and exhibit simple
ridges or tubercles, in place of the complex foldings of the horse’s
grinders.

Hence the general principles of the hypothesis of evolution lead to the
conclusion that the horse must have been derived from some quadruped which
possessed five complete digits on each foot; which had the bones of the
fore-arm and of the leg complete and separate; and which possessed
forty-four teeth, among which the crowns of the incisors and grinders had
a simple structure; while the latter gradually increased in size from
before backwards, at any rate in the anterior part of the series, and had
short crowns.

And if the horse has been thus evolved, and the remains of the different
stages of its evolution have been preserved, they ought to present us with
a series of forms in which the number of the digits becomes reduced; the
bones of the fore-arm and leg gradually take on the equine condition; and
the form and arrangement of the teeth successively approximate to those
which obtain in existing horses.

Let us turn to the facts, and see how far they fulfil these requirements
of the doctrine of evolution.

In Europe abundant remains of horses are found in the Quaternary and later
Tertiary strata as far as the Pliocene formation. But these horses, which
are so common in the cave-deposits and in the gravels of Europe, are in
all essential respects like existing horses. And that is true of all the
horses of the latter part of the Pliocene epoch. But, in deposits which
belong to the earlier Pliocene and later Miocene epochs, and which occur
in Britain, in France, in Germany, in Greece, in India, we find animals
which are extremely like horses—which, in fact, are so similar to
horses, that you may follow descriptions given in works upon the anatomy
of the horse upon the skeletons of these animals—but which differ in
some important particulars. For example, the structure of their fore and
hind limbs is somewhat different. The bones which, in the horse, are
represented by two splints, imperfect below, are as long as the middle
metacarpal and metatarsal bones; and, attached to the extremity of each,
is a digit with three joints of the same general character as those of the
middle digit, only very much smaller. These small digits are so disposed
that they could have had but very little functional importance, and they
must have been rather of the nature of the dew-claws, such as are to be
found in many ruminant animals. The Hipparion, as the extinct
European three-toed horse is called, in fact, presents a foot similar to
that of the American Protohippus (Fig. 9), except that, in the Hipparion,
the smaller digits are situated farther back, and are of smaller
proportional size, than in the Protohippus.

The ulna is slightly more distinct than in the horse; and the whole length
of it, as a very slender shaft, intimately united with the radius, is
completely traceable. The fibula appears to be in the same condition as in
the horse. The teeth of the Hipparion are essentially similar to
those of the horse, but the pattern of the grinders is in some respects a
little more complex, and there is a depression on the face of the skull in
front of the orbit, which is not seen in existing horses.

In the earlier Miocene, and perhaps the later Eocene deposits of some
parts of Europe, another extinct animal has been discovered, which Cuvier,
who first described some fragments of it, considered to be a Palaeotherim.
But as further discoveries threw new light upon its structure, it was
recognised as a distinct genus, under the name of Anchitherium.

In its general characters, the skeleton of Anchitherium is very
similar to that of the horse. In fact, Lartet and De Blainville called it
Palæotherium equinum or hippoides; and De Christol, in 1847,
said that it differed from Hipparion in little more than the
characters of its teeth, and gave it the name of Hipparitherium.
Each foot possesses three complete toes; while the lateral toes are much
larger in proportion to the middle toe than in Hipparion, and
doubtless rested on the ground in ordinary locomotion.

The ulna is complete and quite distinct from the radius, though firmly
united with the latter. The fibula seems also to have been complete. Its
lower end, though intimately united with that of the tibia, is clearly
marked off from the latter bone.

There are forty-four teeth. The incisors have no strong pit. The canines
seem to have been well developed in both sexes. The first of the seven
grinders, which, as I have said, is frequently absent, and, when it does
exist, is small in the horse, is a good-sized and permanent tooth, while
the grinder which follows it is but little larger than the hinder ones.
The crowns of the grinders are short, and though the fundamental pattern
of the horse-tooth is discernible, the front and back ridges are less
curved, the accessory pillars are wanting, and the valleys, much
shallower, are not filled up with cement.

Seven years ago, when I happened to be looking critically into the bearing
of palaentological facts upon the doctrine of evolution, it appeared to me
that the Anchitherium, the Hipparion, and the modern horses,
constitute a series in which the modifications of structure coincide with
the order of chronological occurrence, in the manner in which they must
coincide, if the modern horses really are the result of the gradual
metamorphosis, in the course of the Tertiary epoch, of a less specialised
ancestral form. And I found by correspondence with the late eminent French
anatomist and palaeontologist, M. Lartet, that he had arrived at the same
conclusion from the same data.

That the Anchitherium type had become metamorphosed into the Hipparion
type, and the latter into the Equine type, in the course of that
period of time which is represented by the latter half of the Tertiary
deposits, seemed to me to be the only explanation of the facts for which
there was even a shadow of probability. 3

And, hence, I have ever since held that these facts afford evidence of the
occurrence of evolution, which, in the sense already defined, may be
termed demonstrative.

All who have occupied themselves with the structure of Anchitherium,
from Cuvier onwards, have acknowledged its many points of likeness to a
well-known genus of extinct Eocene mammals, Palaeotherium. Indeed,
as we have seen, Cuvier regarded his remains of Anchitherium as
those of a species of Palaeotherium. Hence, in attempting to trace
the pedigree of the horse beyond the Miocene epoch and the Anchitheroid
form, I naturally sought among the various species of Palaeotheroid
animals for its nearest ally, and I was led to conclude that the Palaeotherium
minus (Plagiolophus) represented the next step more nearly than any
form then known.

I think that this opinion was fully justifiable; but the progress of
investigation has thrown an unexpected light on the question, and has
brought us much nearer than could have been anticipated to a knowledge of
the true series of the progenitors of the horse.

You are all aware that, when your country was first discovered by
Europeans, there were no traces of the existence of the horse in any part
of the American Continent. The accounts of the conquest of Mexico dwell
upon the astonishment of the natives of that country when they first
became acquainted with that astounding phenomenon—a man seated upon
a horse. Nevertheless, the investigations of American geologists have
proved that the remains of horses occur in the most superficial deposits
of both North and South America, just as they do in Europe. Therefore, for
some reason or other—no feasible suggestion on that subject, so far
as I know, has been made—the horse must have died out on this
continent at some period preceding the discovery of America. Of late years
there has been discovered in your Western Territories that marvellous
accumulation of deposits, admirably adapted for the preservation of
organic remains, to which I referred the other evening, and which
furnishes us with a consecutive series of records of the fauna of the
older half of the Tertiary epoch, for which we have no parallel in Europe.
They have yielded fossils in an excellent state of conservation and in
unexampled number and variety. The researches of Leidy and others have
shown that forms allied to the Hipparion and the Anchitherium
are to be found among these remains. But it is only recently that the
admirably conceived and most thoroughly and patiently worked-out
investigations of Professor Marsh have given us a just idea of the vast
fossil wealth, and of the scientific importance, of these deposits. I have
had the advantage of glancing over the collections in Yale Museum; and I
can truly say that, so far as my knowledge extends, there is no collection
from any one region and series of strata comparable, for extent, or for
the care with which the remains have been got together, or for their
scientific importance, to the series of fossils which he has deposited
there. This vast collection has yielded evidence bearing upon the question
of the pedigree of the horse of the most striking character. It tends to
show that we must look to America, rather than to Europe, for the original
seat of the equine series; and that the archaic forms and successive
modifications of the horse’s ancestry are far better preserved here than
in Europe.

Professor Marsh’s kindness has enabled me to put before you a diagram,
every figure in which is an actual representation of some specimen which
is to be seen at Yale at this present time (Fig. 9).

The succession of forms which he has brought together carries us from the
top to the bottom of the Tertiaries. Firstly, there is the true horse.
Next we have the American Pliocene form of the horse (Pliohippus);
in the conformation of its limbs it presents some very slight deviations
from the ordinary horse, and the crowns of the grinding teeth are shorter.
Then comes the Protohippus, which represents the European Hipparion,
having one large digit and two small ones on each foot, and the general
characters of the fore-arm and leg to which I have referred. But it is
more valuable than the European Hipparion for the reason that it is
devoid of some of the peculiarities of that form—peculiarities which
tend to show that the European Hipparion is rather a member of a
collateral branch, than a form in the direct line of succession. Next, in
the backward order in time, is the Miohippus, which corresponds
pretty nearly with the Anchitherium of Europe. It presents three
complete toes—one large median and two smaller lateral ones; and
there is a rudiment of that digit, which answers to the little finger of
the human hand.

The European record of the pedigree of the horse stops here; in the
American Tertiaries, on the contrary, the series of ancestral equine forms
is continued into the Eocene formations. An older Miocene form, termed Mesohippus,
has three toes in front, with a large splint-like rudiment representing
the little finger; and three toes behind. The radius and ulna, the tibia
and the fibula, are distinct, and the short crowned molar teeth are
anchitherioid in pattern.

But the most important discovery of all is the Orohippus, which
comes from the Eocene formation, and is the oldest member of the equine
series, as yet known. Here we find four complete toes on the front limb,
three toes on the hind limb, a well-developed ulna, a well-developed
fibula, and short-crowned grinders of simple pattern.

Thus, thanks to these important researches, it has become evident that, so
far as our present knowledge extends, the history of the horse-type is
exactly and precisely that which could have been predicted from a
knowledge of the principles of evolution. And the knowledge we now possess
justifies us completely in the anticipation, that when the still lower
Eocene deposits, and those which belong to the Cretaceous epoch, have
yielded up their remains of ancestral equine animals, we shall find,
first, a form with four complete toes and a rudiment of the innermost or
first digit in front, with, probably, a rudiment of the fifth digit in the
hind foot; 4
while, in still older forms, the series of the digits will be more and
more complete, until we come to the five-toed animals, in which, if the
doctrine of evolution is well founded, the whole series must have taken
its origin.

That is what I mean by demonstrative evidence of evolution. An inductive
hypothesis is said to be demonstrated when the facts are shown to be in
entire accordance with it. If that is not scientific proof, there are no
merely inductive conclusions which can be said to be proved. And the
doctrine of evolution, at the present time, rests upon exactly as secure a
foundation as the Copernican theory of the motions of the heavenly bodies
did at the time of its promulgation. Its logical basis is precisely of the
same character—the coincidence of the observed facts with
theoretical requirements.

The only way of escape, if it be a way of escape, from the conclusions
which I have just indicated, is the supposition that all these different
equine forms have been created separately at separate epochs of time; and,
I repeat, that of such an hypothesis as this there neither is, nor can be,
any scientific evidence; and, assuredly, so far as I know, there is none
which is supported, or pretends to be supported, by evidence or authority
of any other kind. I can but think that the time will come when such
suggestions as these, such obvious attempts to escape the force of
demonstration, will be put upon the same footing as the supposition made
by some writers, who are I believe not completely extinct at present, that
fossils are mere simulacra, are no indications of the former existence of
the animals to which they seem to belong; but that they are either sports
of nature, or special creations, intended—as I heard suggested the
other day—to test our faith.

In fact, the whole evidence is in favour of evolution, and there is none
against it. And I say this, although perfectly well aware of the seeming
difficulties which have been built up upon what appears to the uninformed
to be a solid foundation. I meet constantly with the argument that the
doctrine of evolution cannot be well founded, because it requires the
lapse of a very vast period of time; while the duration of life upon the
earth thus implied is inconsistent with the conclusions arrived at by the
astronomer and the physicist. I may venture to say that I am familiar with
those conclusions, inasmuch as some years ago, when President of the
Geological Society of London, I took the liberty of criticising them, and
of showing in what respects, as it appeared to me, they lacked complete
and thorough demonstration. But, putting that point aside, suppose that,
as the astronomers, or some of them, and some physical philosophers, tell
us, it is impossible that life could have endured upon the earth for as
long a period as is required by the doctrine of evolution—supposing
that to be proved—I desire to be informed, what is the foundation
for the statement that evolution does require so great a time? The
biologist knows nothing whatever of the amount of time which may be
required for the process of evolution. It is a matter of fact that the
equine forms which I have described to you occur, in the order stated, in
the Tertiary formations. But I have not the slightest means of guessing
whether it took a million of years, or ten millions, or a hundred
millions, or a thousand millions of years, to give rise to that series of
changes. A biologist has no means of arriving at any conclusion as to the
amount of time which may be needed for a certain quantity of organic
change. He takes his time from the geologist. The geologist, considering
the rate at which deposits are formed and the rate at which denudation
goes on upon the surface of the earth, arrives at more or less justifiable
conclusions as to the time which is required for the deposit of a certain
thickness of rocks; and if he tells me that the Tertiary formations
required 500,000,000 years for their deposit, I suppose he has good ground
for what he says, and I take that as a measure of the duration of the
evolution of the horse from the Orohippus up to its present
condition. And, if he is right, undoubtedly evolution is a very slow
process, and requires a great deal of time. But suppose, now, that an
astronomer or a physicist—for instance, my friend Sir William
Thomson—tells me that my geological authority is quite wrong; and
that he has weighty evidence to show that life could not possibly have
existed upon the surface of the earth 500,000,000 years ago, because the
earth would have then been too hot to allow of life, my reply is: “That is
not my affair; settle that with the geologist, and when you have come to
an agreement among yourselves I will adopt your conclusion.” We take our
time from the geologists and physicists; and it is monstrous that, having
taken our time from the physical philosopher’s clock, the physical
philosopher should turn round upon us, and say we are too fast or too
slow. What we desire to know is, is it a fact that evolution took place?
As to the amount of time which evolution may have occupied, we are in the
hands of the physicist and the astronomer, whose business it is to deal
with those questions.

I have now, ladies and gentlemen, arrived at the conclusion of the task
which I set before myself when I undertook to deliver these lectures. My
purpose has been, not to enable those among you who have paid no attention
to these subjects before, to leave this room in a condition to decide upon
the validity or the invalidity of the hypothesis of evolution; but I have
desired to put before you the principles upon which all hypotheses
respecting the history of Nature must be judged; and furthermore, to make
apparent the nature of the evidence and the amount of cogency which is to
be expected and may be obtained from it. To this end, I have not hesitated
to regard you as genuine students and persons desirous of knowing the
truth. I have not shrunk from taking you through long discussions, that I
fear may have sometimes tried your patience; and I have inflicted upon you
details which were indispensable, but which may well have been wearisome.
But I shall rejoice—I shall consider that I have done you the
greatest service which it was in my power to do—if I have thus
convinced you that the great question which we have been discussing is not
one to be dealt with by rhetorical flourishes, or by loose and superficial
talk; but that it requires the keen attention of the trained intellect and
the patience of the accurate observer.

When I commenced this series of lectures, I did not think it necessary to
preface them with a prologue, such as might be expected from a stranger
and a foreigner; for during my brief stay in your country, I have found it
very hard to believe that a stranger could be possessed of so many
friends, and almost harder that a foreigner could express himself in your
language in such a way as to be, to all appearance, so readily
intelligible. So far as I can judge, that most intelligent, and perhaps, I
may add, most singularly active and enterprising body, your press
reporters, do not seem to have been deterred by my accent from giving the
fullest account of everything that I happen to have said.

But the vessel in which I take my departure to-morrow morning is even now
ready to slip her moorings; I awake from my delusion that I am other than
a stranger and a foreigner. I am ready to go back to my place and country;
but, before doing so, let me, by way of epilogue, tender to you my most
hearty thanks for the kind and cordial reception which you have accorded
to me; and let me thank you still more for that which is the greatest
compliment which can be afforded to any person in my position—the
continuous and undisturbed attention which you have bestowed upon the long
argument which I have had the honour to lay before you.

FOOTNOTES:

1 (return)
[ The absence of any keel on
the breast-bone and some other osteological peculiarities, observed by
Professor Marsh, however, suggest that Hesperornis may be a
modification of a less specialised group of birds than that to which these
existing aquatic birds belong.]

2 (return)
[ A second specimen,
discovered in 1877, and at present in the Berlin museum, shows an
excellently preserved skull with teeth; and three digits, all terminated
by claws, in the fore limb. 1893.]

3 (return)
[ I use the word “type”
because it is highly probable that many forms of Anchitherium-like
and Hipparion-like animals existed in the Miocene and Pliocene
epochs, just as many species of the horse tribe exist now, and it is
highly improbable that the particular species of Anchitherium or Hipparion,
which happen to have been discovered, should be precisely those which have
formed part of the direct line of the horse’s pedigree.]

4 (return)
[ Since this lecture was
delivered, Professor Marsh has discovered a new genus of equine mammals (Eohippus)
from the lowest Eocene deposits of the West, which corresponds very nearly
to this description.—American Journal of Science, November,
1876.]