418 RECAPITULATION. Chap. XIV. 



Now let us lum to the other side of the argument. Under 

 domestication Ave see much variability, caused, or at least ex- 

 cited, liy changed conditions of life. This variability is gov- 

 erned by many complex laws — by correlation, by use and dis- 

 use, and by the definite action of the surrounding conditions. 

 There is much difficulty in ascertaining hoAV largely our do- 

 mestic productions have been modified ; but we may safely in- 

 fer that the amount has been large, and that modifications can 

 be inherited for long periods. As long as the conditions of 

 life remain the same, we have reason to believe that a modifi- 

 cation, Avhich has already been inherited for many generations, 

 may continue to be inherited for an almost infinite number of 

 generations. On the other hand, Ave haAO evidence that vari- 

 ability, AA-hen it has once come into play, does not cease under 

 domestication for a very long period; for noAV A'arieties are 

 still occasionally produced by our oldest domesticated produc- 

 tions. 



Man does not actually produce variability ; he only unin- 

 tentionally exposes organic beings to ncAV conditions of life, 

 and then Nature acts on the organization, and causes .A'ari abil- 

 ity. But man can and docs select the A-ariations given to him 

 by Nature, and thus accumulate them in any desired manner. 

 lie thus adapts animals and plants for his OAvn benefit or 

 pleasure. He may do this methodically, or he may do it un- 

 consciously, by preserving the indiA'iduals most useful or pleas- 

 ing to hini Avithout any intention of altering the breed. It is 

 certain that he can largely influence the character of a breed 

 Ijy selecting, in each successive generation, individual differ- 

 ences so slight as to be inap{)reciable except by an educated 

 eye. This process of selection has been the great agency in 

 f lie formation 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 inextricable 

 doubts AA'hether many of them are Aarietics or aboriginally dis- 

 tinct species. 



There is no obvious reason why the principles Avhich have 

 acted so cfficientl}^ under domestication should not act under 

 Nature. In the survival of favored individuals and races, dur- 

 ing the constantly-recurrent Struggle for Existence, Ave see a 

 jKnverful and ever-acting form of Selection. The struggle for 

 existence inevitably follows from the high geometrical ratio of 

 increase Avhich is common to all organic beings. This high 

 rate of increase is proved by calculation — l)y the rapid increase 



