oic rocks, they present no such modifications
as can safely be considered evidence of progressive modification.

Facts of this kind are undoubtedly fatal to any form of the doctrine of
evolution which postulates the supposition that there is an intrinsic
necessity, on the part of animal forms which have once come into
existence, to undergo continual modification; and they are as distinctly
opposed to any view which involves the belief, that such modification
may occur, must take place, at the same rate, in all the different types
of animal or vegetable life. The facts, as I have placed them before
you, obviously directly contradict any form of the hypothesis of
evolution which stands in need of these two postulates.

But, one great service that has been rendered by Mr. Darwin to the
doctrine of evolution in general is this: he has shown that there
are two chief factors in the process of evolution: one of them is the
tendency to vary, the existence of which in all living forms may
be proved by observation; the other is the influence of surrounding
conditions upon what I may call the parent form and the variations which
are thus evolved from it. The cause of the production of variations is
a matter not at all properly understood at present. Whether variation
depends upon some intricate machinery--if I may use the phrase--of the
living organism itself, or whether it arises through the influence of
conditions upon that form, is not certain, and the question may, for
the present, be left open. But the important point is that, granting the
existence of the tendency to the production of variations; then, whether
the variations which are produced shall survive and supplant the parent,
or whether the parent form shall survive and supplant the variations, is
a matter which depends entirely on those conditions which give rise to
the struggle for existence. If the surrounding conditions are such that
the parent form is more competent to deal with them, and flourish in
them than the derived forms, then, in the struggle for existence,
the parent form will maintain itself and the derived forms will be
exterminated. But if, on the contrary, the conditions are such as to be
more favourable to a derived than to the parent form, the parent form
will be extirpated and the derived form will take its place. In the
first case, there will be no progression, no change of structure,
through any imaginable series of ages; in the second place ther