RECAPITULATION 287 



selecting, in each successive generation, individual differ- 

 ences so slight as to be inappreciable except by an 

 educated eye. This unconscious process of selection has 

 been the great agency in the formation of the most dis- 

 tinct and useful domestic breeds. That many breeds 

 produced by man have to a large extent the character 

 of natural species is shown by the inextricable doubts 

 whether many of them are varieties or aboriginally 

 distinct species. 



There is no reason why the principles which have 

 acted so efficiently under domestication should not have 

 acted under nature. In the survival of favored individ- 

 uals and races, during the constantly-recurrent Struggle 

 for Existence, we see a powerful and ever-acting form of 

 Selection. The struggle for existence inevitably follows 

 from the high geometrical ratio of increase which is com- 

 mon to all organic beings. This high rate of increase 

 is proved by calculation — by the rapid increase of many 

 animals and plants during a succession of peculiar 

 seasons, and when naturalized in new countries. More 

 individuals are born than can possibly survive. A grain 

 in the balance may determine which individuals shall 

 live and which shall die — which variety or species 

 shall increase in number, and which shall decrease, or 

 ■finally become extinct. As the individuals of the same 

 species come in all respects into the closest competition 

 with each other, the struggle will generally be most severe 

 between them; it will be almost equally severe between 

 the varieties of the same species, and next in severity 

 between the species of the same genus. On the other 

 hand, the struggle will often be severe between beings 

 remote in the scale of nature. The slightest advantage in 



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