Title: Lectures on Evolution - Essay #3 from "Science and Hebrew Tradition"
Author: Huxley, Thomas Henry, 1825-1895
Language: English
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LECTURES ON EVOLUTION

ESSAY #3 FROM "SCIENCE AND HEBREW TRADITION"


By Thomas Henry Huxley



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:--

     "The sixth, and of creation last, arose
     With evening harp and matin, when God said,
     'Let the earth bring forth soul living in her kind,
     Cattle and creeping things, and beast of the earth.
     Each in their kind!' The earth obeyed, and, straight
     Opening her fertile womb, teemed at a birth
     Innumerous living creatures, perfect forms,
     Limbed and full-grown. Out of the ground uprose,
     As from his lair, the wild beast, where he wons
     In forest wild, in thicket, brake, or den;
     Among the trees in pairs they rose, they walked;
     The cattle in the fields and meadows green;
     Those rare and solitary; these in flocks
     Pasturing at once, and in broad herds upsprung.
     The grassy clods now calved; now half appears
     The tawny lion, pawing to get free
     His hinder parts--then springs, as broke from bonds,
     And rampant shakes his brinded mane; the ounce,
     The libbard, and the tiger, as the mole
     Rising, the crumbled earth above them threw
     In hillocks; the swift stag from underground
     Bore up his branching head; scarce from his mould
     Behemoth, biggest born of earth, upheaved
     His vastness; fleeced the flocks and bleating rose
     As plants; ambiguous between sea and land,
     The river-horse and scaly crocodile.
     At once came forth whatever creeps the ground,
     Insect or worm."

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.

     Fig. 1.--Ideal Section of the Crust of the Earth.

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.

     Fig. 2.--Tracks of Brontozoum.

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.

     Fig. 3--Hesperornis regalis (Marsh)

     Fig. 4--Hesperornis regalis (Marsh)
     (Side and upper views of half the lower jaw; side and end views
     of a vertebra and a separate tooth.)

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.

     Figure. 5--Ichthyornis Dispar (Marsh). Side and upper views
     of half the lower jaw; and side and end views of a
     vertebra.

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.

     Fig. 6.--Bird. Ornithoscelidan. Crocodile. The letters have
     the same signification in all the figures. _Il.,_ Ilium;
     _a._ anterior end; _b._ posterior end; _Ia._ ischium; _Pb.,_
     pubis; _T,_ tibia; _F,_ fibula; _As.,_ astragalus; _Ca.,_
     calcaneum; I, distal portion of the tarsus; i., ii., iii.,
     iv., metatarsal bones.

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._

     Fig. 7.--Restoration of Compsognathus Longipes

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.

     Fig. 8.--Pterodactylus Spectabilis (Von Meyer).

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).

     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:

[Footnote 1: 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.]

[Footnote 2: 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.]

[Footnote 3: 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.]

[Footnote 4: 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.]





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