Chap. XV.] RECAPITULATION. 277 



productions have been modified; but we may safely 

 infer that the amount has been large, and that modifica- 

 tions can be inherited for long periods. As long as the 

 conditions of life remain the same, we have reason to 

 believe that a modification, which has already been 

 inherited for many generations, may continue to be 

 inherited for an almost infinite number of generations. 

 On the other hand, we have evidence that variability 

 when it has once come into play, does not cease under 

 domestication for a very long period ; nor do we know 

 that it ever ceases, for new varieties are still occasionally 

 produced by our oldest domesticated productions. 



Variability is not actually caused by man ; he only 

 unintentionally exposes organic beings to new condi- 

 tions of life, and then nature acts on the organisation 

 and causes it to vary. But man can and does select the 

 variations given to him by nature, and thus accumulates 

 them in any desired manner. He thus adapts animals 

 and plants for his own benefit or pleasure. He may 

 do this methodically, or he may do it unconsciously by 

 preserving the individuals 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 successive generation, indi- 

 vidual differences so slight as to be inappreciable except 

 by an educated eye. This unconscious process of selec- 

 tion has been the great agency in the 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 inextric- 

 able doubts whether many of them are varieties or 

 aboriginally distinct species. 



There is no reason why the principles which have 



