of natural selection. Meanwhile I am engaged
only in presenting the general arguments which support the theory, and
therefore mention these objections to one of them merely _en passant_.
And I do so in order to pledge myself effectually to dispose of them
later on, so that for the purposes of my present argument both these
objections may be provisionally regarded as non-existent; which means,
in other words, that we may provisionally regard the analogy between
artificial selection and natural selection as everywhere logically
intact.

* * * * *

To sum up, then, the results of the foregoing exposition thus far, what
I hold to be the three principal, or most general, arguments in favour
of the theory of natural selection, are as follows.

First, there is the _a priori_ consideration that, if on independent
grounds we believe in the theory of evolution at all, it becomes obvious
that natural selection _must_ have had _some_ part in the process. For
no one can deny the potent facts of heredity, variability, the struggle
for existence, and survival of the fittest. But to admit these facts is
to admit natural selection as a principle which must be, at any rate,
one of the factors of organic evolution, supposing such evolution to
have taken place. Next, when we turn from these _a priori_
considerations, which thus show that natural selection _must_ have been
concerned to some extent in the process of evolution, we find in organic
nature evidence _a posteriori_ of the extent to which this principle
_has_ been thus concerned. For we find that among all the countless
millions of adaptive structures which are to be met with in organic
nature, it is an invariable rule that they exist in relation to the
needs of the particular species which present them: they never have any
primary reference to the needs of other species. And as this
extraordinarily large and general fact is exactly what the theory of
natural selection would expect, the theory is verified by the fact in an
extraordinarily cogent manner. In other words, the fact goes to prove
that in _all_ cases where adaptive structures or instincts are
concerned, natural selection must have been either the sole cause at
work, or, at the least, an influence controlling the operation of all
other causes.

Lastly, an actually experimental verification of the theory has been
furnished on a gigantic scale by the operations of breeders, fanciers,
and