484 REGAPITULATION. 



his own benefit or pleasure. He may do this methodically, 

 or he may do it unconsciously by preserving the individ- 

 uals most useful or pleasing to him without any intention 

 of altering the breed. It is certain that he can largely 

 influence the character of a breed by selecting, in each suc- 

 cessive generation, individual differences so slight as to be 

 inappreciable except by an educated eye. This unconscious 

 process of selection has been the great agency in the for- 

 mation of the most distinct and useful domestic breeds. 

 That many breeds produced by man have to a large extent 

 the character of natural species, is shown by the inextri- 

 cable doubts whether many of them are varieties or aborig- 

 inally 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 individuals and 

 races, during the constantly recurrent Struggle for Exist- 

 ence, 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 common 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 certain individuals, at any age or 

 during any season, over those with which they come into 

 competition, or better adaptation in however slight a 

 degree to the surrounding physical conditions, will, in the 

 long run, turn the balance. 



With animals having separated sexes, there will be in 

 most cases a struggle between the males for the possession 



