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Title: Lectures on Evolution
Essay #3 from "Science and Hebrew Tradition"
Author: Thomas Henry Huxley
Release Date: December 3, 2008 [EBook #2629]
Last Updated: January 20, 2013
Language: English
Character set encoding: ISO-8859-1
*** START OF THIS PROJECT GUTENBERG EBOOK LECTURES ON EVOLUTION ***
Produced by D. R. Thompson, and David Widger
|
I. THE THREE HYPOTHESES RESPECTING THE HISTORY OF NATURE II. THE HYPOTHESIS OF EVOLUTION. THE NEUTRAL AND THE FAVOURABLE |
"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.
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. 2But 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.
AsCompsognathus 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.
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.
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