110 
NATURAL SELECTION. 
Chap. IV. 
liave any means of knowing that any one region has 
as yet got its maximum of species. Probably no region 
is as yet fully stocked, for at the Cape of Good Hope, 
where more species of plants are crowded together than 
in any other quarter of the world, some foreign plants 
have become naturalised, without causing, as far as we 
know, the extinction of any natives. 
Furthermore, the species wliich are most nume¬ 
rous in individuals will have the best chance of pro¬ 
ducing within any given period favourable variations. 
We have evidence of this, in the facts given in the 
second chapter, showing that it is the common species 
which afford the greatest number of recorded varieties, 
or incipient species. Hence, rare species will be less 
quickly modified or improved within any given period, 
and they will consequently be beaten in the race for life 
by the modified descendants of the commoner species. 
From these several considerations I think it in¬ 
evitably follows, that as new species in the course of 
time are formed through natural selection, others will 
become rarer and rarer, and finally extinct. The forms 
which stand in closest competition with those under¬ 
going modification and improvement, will naturally 
suffer most. And we have seen in the chapter on the 
Struggle for Existence that it is the most closely-allied 
forms,—varieties of the same species, and species of 
the same genus or of related genera,—which, from 
having nearly the same structure, constitution, and 
habits, generally come into the severest competition 
with each other. Consequently, each new variety or 
species, during the progress of its formation, will gene¬ 
rally press hardest on its nearest kindred, and tend to 
exterminate them. We see the same process of exter¬ 
mination amongst our domesticated productions, through 
the selection of improved forms by man. Many curious 
