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AMERICAN ADDRESSES, WITH A LECTURE ON THE STUDY OF BIOLOGY
by
THOMAS H. HUXLEY.
London: MacMillan and Co.
London: R. Clay, Sons, and Taylor, Printers, Bread Street Hill,
Queen Victoria Street.
1877
"Naturæ leges et regulæ, secundum quas omnia fiunt et ex unis
formis in alias mutantur, sunt ubique et semper eadem."
B. DE SPINOZA, _Ethices_, Pars tertia, Præfatio.
CONTENTS.
I. THREE LECTURES ON EVOLUTION (New York, September 18, 20, 22, 1876).
LECTURE I. THE THREE HYPOTHESES RESPECTING THE HISTORY OF NATURE
LECTURE II. THE HYPOTHESIS OF EVOLUTION. THE NEUTRAL AND THE
FAVOURABLE EVIDENCE
LECTURE III. THE DEMONSTRATIVE EVIDENCE OF EVOLUTION
II. AN ADDRESS ON THE OCCASION OF THE OPENING OF THE JOHNS HOPKINS
UNIVERSITY (Baltimore, September 12, 1876)
III. A LECTURE ON THE STUDY OF BIOLOGY, IN CONNECTION WITH THE LOAN
COLLECTION OF SCIENTIFIC APPARATUS (South Kensington Museum,
December 16, 1876)
NEW YORK.
LECTURES ON EVOLUTION.
LECTURE I.
THE THREE HYPOTHESES RESPECTING THE HISTORY OF NATURE.
We live in and form part of a system of things of immense diversity and
perplexity, which we call Nature; and it is a matter of the deepest
interest to all of us that we should form just conceptions of the
constitution of that system and of its past history. With relation to
this universe, man is, in extent, little more than a mathematical point;
in duration but a fleeting shadow; he is a mere reed shaken in the winds
of force. But, as Pascal long ago remarked, although a mere reed, he is
a thinking reed; and in virtue of that wonderful capacity of thought, he
has the power of framing for himself a symbolic conception of the
universe, which, although doubtless highly imperfect and inadequate as a
picture of the great whole, is yet sufficient to serve him as a chart
for the guidance of his practical affairs. It has taken long ages of
toilsome and often fruitless labour to enable man to look steadily at
the shifting scenes of the phantasmagoria of Nature, to notice what is
fixed among her fluctuations, and what is regular among her apparent
irregularities; and it is only comparatively lately, within the last few
centuries, that the conception of a universal order and of a definite
course of things, which we term the course of Nature, has emerged.
But, once originated, the conception of the constancy of the order of
Nature has become the dominant idea of modern thought. To any person who
is familiar with the facts upon which that conception is based, and is
competent to estimate their significance, it has ceased to be
conceivable that chance should have any place in the universe, or that
events should depend upon any but the natural sequence of cause and
effect. We have come to look upon the present as the child of the past
and as the parent of the future; and, as we have excluded chance from a
place in the universe, so we ignore, even as a possibility, the notion
of any interference with the order of Nature. Whatever may be men's
speculative doctrines, it is quite certain, that every intelligent
person guides his life and risks his fortune upon the belief that the
order of Nature is constant, and that the chain of natural causation is
never broken.
In fact, no belief which we entertain has so complete a logical basis as
that to which I have just referred. It tacitly underlies every process
of reasoning; it is the foundation of every act of the will. It is based
upon the broadest induction, and it is verified by the most constant,
regular, and universal of deductive processes. But we must recollect
that any human belief, however broad its basis, however defensible it
may seem, is, after all, only a probable belief, and that our widest and
safest generalizations 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 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 harps 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
framework of the earth; until, at length, in place of that framework, 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 a natural process," and "This is not a natural process;" but that
the whole might be compared to that wonderful process 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 organization 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
_à 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.
[Illustration: FIG. 1.--IDEAL SECTION OF THE CRUST OF THE EARTH.]
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 exuviæ of plants and animals.
Many of these strata are full of such exuviæ--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-beech, 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 palæozoic
formations the contrast is still more marked. Thus the circumstantial
evidence absolutely negatives the conception of the eternity of the
present condition of things. We can say with certainty that the present
condition of things has existed for a comparatively short period; and
that, so far as animal and vegetable nature are concerned, it has been
preceded by a different condition. We can pursue this evidence until we
reach the lowest of the stratified rocks, in which we lose the
indications of life altogether. The hypothesis of the eternity of the
present state of nature may therefore be put out of court.
We now come to what I will term Milton's hypothesis--the hypothesis that
the present condition of things has endured for a comparatively short
time; and, at the commencement of that time, came into existence within
the course of six days. I doubt not that it may have excited some
surprise in your minds that I should have spoken of this as Milton's
hypothesis, rather than that I should have chosen the terms which are
more customary, such as "the doctrine of creation," or "the Biblical
doctrine," or "the doctrine of Moses," all of which denominations, as
applied to the hypothesis to which I have just referred, are certainly
much more familiar to you than the title of the Miltonic hypothesis. But
I have had what I cannot but think are very weighty reasons for taking
the course which I have pursued. In the first place, I have discarded
the title of the "doctrine of creation," because my present business is
not with the question why the objects which constitute Nature came into
existence, but when they came into existence, and in what order. This is
as strictly a historical question as the question when the Angles and
the Jutes invaded England, and whether they preceded or followed the
Romans. But the question about creation is a philosophical problem, and
one which cannot be solved, or even approached, by the historical
method. What we want to learn is, whether the facts, so far as they are
known, afford evidence that things arose in the way described by Milton,
or whether they do not; and, when that question is settled, it will be
time enough to inquire into the causes of their origination.
In the second place, I have not spoken of this doctrine as the Biblical
doctrine. It is quite true that persons as diverse in their general
views as Milton the Protestant and the celebrated Jesuit Father Suarez,
each put upon the first chapter of Genesis the interpretation embodied
in Milton's poem. It is quite true that this interpretation is that
which has been instilled into every one of us in our childhood; but I do
not for one moment venture to say that it can properly be called the
Biblical doctrine. It is not my business, and does not lie within my
competency, to say what the Hebrew text does, and what it does not
signify; moreover, were I to affirm that this is the Biblical doctrine,
I should be met by the authority of many eminent scholars, to say
nothing of men of science, who, at various times, have absolutely denied
that any such doctrine is to be found in Genesis. If we are to listen to
many expositors of no mean authority, we must believe that what seems so
clearly defined in Genesis--as if very great pains had been taken that
there should be no possibility of mistake--is not the meaning of the
text at all. The account is divided into periods that we may make just
as long or as short as convenience requires. We are also to understand
that it is consistent with the original text to believe that the most
complex plants and animals may have been evolved by natural processes,
lasting for millions of years, out of structureless rudiments. A person
who is not a Hebrew scholar can only stand aside and admire the
marvellous flexibility of a language which admits of such diverse
interpretations. But assuredly, in the face of such contradictions of
authority upon matters respecting which he is incompetent to form any
judgment, he will abstain, as I do, from giving any opinion.
In the third place, I have carefully abstained from speaking of this as
the Mosaic doctrine, because we are now assured upon the authority of
the highest critics, and even of dignitaries of the Church, that there
is no evidence that Moses wrote the Book of Genesis, or knew anything
about it. You will understand that I give no judgment--it would be an
impertinence upon my part to volunteer even a suggestion--upon such a
subject. But, that being the state of opinion among the scholars and the
clergy, it is well for the unlearned in Hebrew lore, and for the laity,
to avoid entangling themselves in such a vexed question. Happily, Milton
leaves us no excuse for doubting what he means, and I shall therefore be
safe in speaking of the opinion in question as the Miltonic hypothesis.
Now we have to test that hypothesis. For my part, I have no prejudice
one way or the other. If there is evidence in favour of this view, I am
burdened by no theoretical difficulties in the way of accepting it; but
there must be evidence. Scientific men get an awkward habit--no, I won't
call it that, for it is a valuable habit--of believing nothing unless
there is evidence for it; and they have a way of looking upon belief
which is not based upon evidence, not only as illogical, but as immoral.
We will, if you please, test this view by the circumstantial evidence
alone; for, from what I have said, you will understand that I do not
propose to discuss the question of what testimonial evidence is to be
adduced in favour of it. If those whose business it is to judge are not
at one as to the authenticity of the only evidence of that kind which is
offered, nor as to the facts to which it bears witness, the discussion
of such evidence is superfluous.
But I may be permitted to regret this necessity of rejecting the
testimonial evidence the less, because the examination of the
circumstantial evidence leads to the conclusion, not only that it is
incompetent to justify the hypothesis, but that, so far as it goes, it
is contrary to the hypothesis.
The considerations upon which I base this conclusion are of the simplest
possible character. The Miltonic hypothesis contains assertions of a
very definite character relating to the succession of living forms. It
is stated that plants, for example, made their appearance upon the third
day, and not before. And you will understand that what the poet means by
plants are such plants as now live, the ancestors, in the ordinary way
of propagation of like by like, of the trees and shrubs which flourish
in the present world. It must needs be so; for, if they were different,
either the existing plants have been the result of a separate
origination since that described by Milton, of which we have no record,
nor any ground for supposition that such an occurrence has taken place;
or else they have arisen by a process of evolution from the original
stocks.
In the second place, it is clear that there was no animal life before
the fifth day, and that, on the fifth day, aquatic animals and birds
appeared. And it is further clear that terrestrial living things, other
than birds, made their appearance upon the sixth day, and not before.
Hence, it follows that, if, in the large mass of circumstantial evidence
as to what really has happened in the past history of the globe we find
indications of the existence of terrestrial animals, other than birds,
at a certain period, it is perfectly certain that all that has taken
place since that time must be referred to the sixth day.
In the great Carboniferous formation, whence America derives so vast a
proportion of her actual and potential wealth, in the beds of coal which
have been formed from the vegetation of that period, we find abundant
evidence of the existence of terrestrial animals. They have been
described, not only by European but by your own naturalists. There are
to be found numerous insects allied to our cockroaches. There are to be
found spiders and scorpions of large size, the latter so similar to
existing scorpions that it requires the practised eye of the naturalist
to distinguish them. Inasmuch as these animals can be proved to have
been alive in the Carboniferous epoch, it is perfectly clear that, if
the Miltonic account is to be accepted, the huge mass of rocks extending
from the middle of the Palæozoic 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 upon 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 exuviæ
of marine animals; and if the view which is entertained by Principal
Dawson and Dr. Carpenter respecting the nature of the _Eozoön_ 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 _Eozoön_
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 this: 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 give 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.
LECTURE 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 to me 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
_Globigerinæ_ can be traced down to the _Globigerinæ_ 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.