Lectures on Evolution This is Essay #3 from "Science and Hebrew Tradition"
by
Thomas Henry Huxley
Processed by D.R. Thompson
Lectures on Evolution
This is 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 orderof 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.
LECTURES ON EVOLUTION
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.
LECTURES ON EVOLUTION
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
(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.
(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.
(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.
(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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