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 
