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, hares or 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 wath 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 w^ere 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 



