ies which the present chapter is concerned with, and which
are conveniently summed up in the term natural selection, are as
competent to produce these so-called mechanical contrivances, and the
other cases of adaptation which are to be met with in organic nature,
as intelligent design could be. Hence, our choice as between these two
hypotheses must be governed by a study of all collateral circumstances;
that is to say, by a study of the evidences in favour of the physical
explanation. To this study, therefore, we shall now address ourselves,
in the course of the following chapters.

CHAPTER VIII.

EVIDENCES OF THE THEORY OF NATURAL SELECTION.

I will now proceed to state the main arguments in favour of the theory
of natural selection, and then, in the following chapter, the main
objections which have been urged against it.

In my opinion, the main arguments in favour of the theory are three in
number.

First, it is a matter of observation that the struggle for existence in
nature does lead to the extermination of forms less fitted for the
struggle, and thus makes room for forms more fitted. This general fact
may be best observed in cases where an exotic species proves itself
better fitted to inhabit a new country than is some endemic species
which it exterminates. In Great Britain, for example, the so-called
common rat is a comparatively recent importation from Norway, and it has
so completely supplanted the original British rat, that it is now
extremely difficult to procure a single specimen of the latter: the
native black rat has been all but exterminated by the foreign brown rat.
The same thing is constantly found in the case of imported species of
plants. I have seen the river at Cambridge so choked with the inordinate
propagation of a species of water-weed which had been introduced from
America, that considerable expense had to be incurred in order to clear
the river for traffic. In New Zealand the same thing has happened with
the European water-cress, and in Australia with the common rabbit. So it
is doubtless true, as one of the natives is said to have philosophically
remarked, "the white man's rat has driven away our rat, the European fly
drives away our fly, his clover kills our grass, and so will the Maoris
disappear before the white man himself." Innumerable other cases to the
same effect might be quoted; and they all go to establish the fact that
forms less fitted to survive succumb in their com