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and land； of estuary and open ocean； and； in correspondence with
these alternations； we observe the changes in the fauna and
flora to which I have referred。
But the inspection of these changes gives us no right to believe
that there has been any discontinuity in natural processes。
There is no trace of general cataclysms； of universal deluges；
or sudden destructions of a whole fauna or flora。
The appearances which were formerly interpreted in that way have
all been shown to be delusive； as our knowledge has increased
and as the blanks which formerly appeared to exist between the
different formations have been filled up。 That there is no
absolute break between formation and formation； that there has
been no sudden disappearance of all the forms of life and
replacement of them by others； but that changes have gone on
slowly and gradually； that one type has died out and another has
taken its place； and that thus； by insensible degrees； one fauna
has been replaced by another； are conclusions strengthened by
constantly increasing evidence。 So that within the whole of the
immense period indicated by the fossiliferous stratified rocks；
there is assuredly not the slightest proof of any break in the
uniformity of Nature's operations； no indication that events
have followed other than a clear and orderly sequence。
That； I say； is the natural and obvious teaching of the
circumstantial evidence contained in the stratified rocks。 I
leave you to consider how far； by any ingenuity of
interpretation； by any stretching of the meaning of language； it
can be brought into harmony with the Miltonic hypothesis。
There remains the third hypothesis； that of which I have spoken
as the hypothesis of evolution； and I purpose that； in lectures
to come； we should discuss it as carefully as we have considered
the other two hypotheses。 I need not say that it is quite
hopeless to look for testimonial evidence of evolution。 The very
nature of the case precludes the possibility of such evidence；
for the human race can no more be expected to testify to its own
origin； than a child can be tendered as a witness of its own
birth。 Our sole inquiry is； what foundation circumstantial
evidence lends to the hypothesis； or whether it lends none； or
whether it controverts the hypothesis。 I shall deal with the
matter entirely as a question of history。 I shall not indulge in
the discussion of any speculative probabilities。 I shall not
attempt to show that Nature is unintelligible unless we adopt
some such hypothesis。 For anything I know about the matter； it
may be the way of Nature to be unintelligible； she is often
puzzling； and I have no reason to suppose that she is bound to
fit herself to our notions。
I shall place before you three kinds of evidence entirely based
upon what is known of the forms of animal life which are
contained in the series of stratified rocks。 I shall endeavour
to show you that there is one kind of evidence which is neutral；
which neither helps evolution nor is inconsistent with it。
I shall then bring forward a second kind of evidence which
indicates a strong probability in favour of evolution； but does
not prove it； and； lastly； I shall adduce a third kind of
evidence which； being as complete as any evidence which we can
hope to obtain upon such a subject； and being wholly and
strikingly in favour of evolution； may fairly be called
demonstrative evidence of its occurrence。
LECTURES ON EVOLUTION
II
THE HYPOTHESIS OF EVOLUTION。 THE NEUTRAL AND
THE FAVOURABLE EVIDENCE。
In the preceding lecture I pointed out that there are three
hypotheses which may be entertained； and which have been
entertained； respecting the past history of life upon the globe。
According to the first of these hypotheses； living beings； such
as now exist； have existed from all eternity upon this earth。
We tested that hypothesis by the circumstantial evidence； as I
called it； which is furnished by the fossil remains contained in
the earth's crust； and we found that it was obviously untenable。
I then proceeded to consider the second hypothesis； which I
termed the Miltonic hypothesis； not because it is of any
particular consequence whether John Milton seriously entertained
it or not； but because it is stated in a clear and unmistakable
manner in his great poem。 I pointed out to you that the evidence
at our command as completely and fully negatives that hypothesis
as it did the preceding one。 And I confess that I had too much
respect for your intelligence to think it necessary to add that
the negation was equally clear and equally valid； whatever the
source from which that hypothesis might be derived； or whatever
the authority by which it might be supported。 I further stated
that； according to the third hypothesis； or that of evolution；
the existing state of things is the last term of a long series
of states； which； when traced back； would be found to show no
interruption and no breach in the continuity of natural
causation。 I propose； in the present and the following lecture；
to test this hypothesis rigorously by the evidence at command；
and to inquire how far that evidence can be said to be
indifferent to it； how far it can be said to be favourable to
it； and； finally； how far it can be said to be demonstrative。
From almost the origin of the discussions about the existing
condition of the animal and vegetable worlds and the causes
which have determined that condition； an argument has been put
forward as an objection to evolution； which we shall have to
consider very seriously。 It is an argument which was first
clearly stated by Cuvier in his criticism of the doctrines
propounded by his great contemporary； Lamarck。 The French
expedition to Egypt had called the attention of learned men to
the wonderful store of antiquities in that country； and there
had been brought back to France numerous mummified corpses of
the animals which the ancient Egyptians revered and preserved；
and which； at a reasonable computation； must have lived not less
than three or four thousand years before the time at which they
were thus brought to light。 Cuvier endeavoured to test the
hypothesis that animals have undergone gradual and progressive
modifications of structure； by comparing the skeletons and such
other parts of the mummies as were in a fitting state of
preservation； with the corresponding parts of the
representatives of the same species now living in Egypt。
He arrived at the conviction that no appreciable change had
taken place in these animals in the course of this considerable
lapse of time； and the justice of his conclusion is
not disputed。
It is obvious that； if it can be proved that animals have
endured； without undergoing any demonstrable change of
structure； for so long a period as four thousand years； no form
of the hypothesis of evolution which assumes that animals
undergo a constant and necessary progressive change can be
tenable； unless； indeed； it be further assumed that four
thousand years is too short a time for the production of a
change sufficiently great to be detected。
But it is no less plain that if the process of evolution of
animals is not independent of surrounding conditions； if it may
be indefinitely hastened or retarded by variations in these
conditions； or if evolution is simply a process of accommodation
to varying conditions； the argument against the hypothesis of
evolution based on the unchanged character of the Egyptian fauna
is worthless。 For the monuments which are coeval with the
mummies testify as strongly to the absence of change in the
physical geography and the general conditions of the land of
Egypt； for the time in question； as the mummies do to the
unvarying characters of its living population。
The progress of research since Cuvier's time has supplied far
more striking examples of the long duration of specific forms of
life than those which are furnished by the mummified Ibises and
Crocodiles of Egypt。 A remarkable case is to be found in your
own country； in the neighbourhood of the falls of Niagara。
In the immediate vicinity of the whirlpool； and again upon Goat
Island； in the superficial deposits which cover the surface of
the rocky subsoil in those regions； there are found remains of
animals in perfect preservation； and among them； shells
belonging to exactly the same species as those which at present
inhabit the still waters of Lake Erie。 It is evident； from the
structure of the country； that these animal remains were
deposited in the beds in which they occur at a time when the
lake extended over the region in which they are found。 This
involves the conclusion that they lived and died before the
falls had cut their way back through the gorge of Niagara；
and； indeed； it has been determined that； when these animals
lived； the falls of Niagara must have been at least six miles
furthe