454 RECAPITULATION. 



•ounding conditions. There is much difficulty in ascer- 

 taining how largely our domestic productions have been 

 modified ; but we may safely infer 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 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 domesti- 

 cation 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. 



Variabilit}'- is not actually caused by man ; he only unin- 

 tentionally exposes organic beings to new conditions of 

 life, and then nature acts on the organization 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 genera- 

 tion, 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 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 

 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 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 indi- 



