1^4 THE DOCTRINE OF DESCENT. 



waged as that between the black rat (Mus rattus) and 

 the brown rat (Mus decumanus); and we far more fre- 

 quently imagine that harmonious intercourse exists be- 

 tween the members of the same species sharing the 

 same habitation, as, for instance, hares or deer, than 

 that they are anxiously striving to maintain existence. 

 Yet this is not the case. The two great motive powers, 

 the preservation of the individual and the preservation 

 of the species, are unremitting instigations to warfare, 

 and under their influence every living being, plants in- 

 clusive, joins in conflict with its congeners of the imme- 

 diate vicinity. 



In this competition for nutriment, combined with de- 

 fence 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, advantages, mental as 

 well as bodily, which aided these individuals 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 gen- 

 erations of becoming established and increased by re- 

 peated 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 externa] 

 habit, size, and strength of the individual, but, owing 

 to the actual variability and plasticity of the organic 



