144 THE DOCTRINE OF DESCENT. 
waged as that between the black rat (Mus rattus) 
and the brown rat (Mus decumanus); and we far more 
frequently imagine that harmonious intercourse exists 
between the members of the same species sharing the 
-same habitation, as, for instance, the hare and the deer, 
than that they are anxiously striving to maintain exist- 
ence. Yet this is not the case. The two great motive 
powers, the preservation of the individual and the pre- 
servation of the species, are unremitting instigations to 
warfare, and under their influence every living being, 
plants inclusive, joins in conflict with its congeners of 
the immediate vicinity. 
In this competition for nutriment, combined with 
defence against all possible enemies and other rivals 
for the remaining privileges of existence, the strongest 
gains the advantage, or the most crafty, the most 
skilful—in short, the one that can measure itself against 
its rivals armed with any sort of superiority. Not 
only in the struggle for mates, but on every occasion 
of competition, the weaker individuals are beaten off, 
and a selection of the strongest and the best takes place. 
But the primarily slight,—often scarcely perceptible, ad- 
vantages, mental as well as bodily, which aided these in- 
dividuals to conquer and survive the other members of 
the species who were weaker and destitute of accidental 
advantages, have a prospect of being transmitted, and 
in the following generations of becoming established 
and increased by repeated selection. This selection is 
therefore a natural and necessary course of things ; and 
it applies, not in a merely general and vague manner, 
as in the external habit, size, and strength of the indi- 
vidual, but, owing to the actual variability and plasticity 
