420 ON THE ORIGIN OF SPECIES 



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 wholly cease ; for new varieties are still 

 occasionally produced by our most anciently domesti- 

 cated productions. 



Man does not actually produce variability ; he only 

 unintentionally exposes organic beings to new condi- 

 tions of life, and then nature acts on the organisation, 

 and causes variability. But man can and does select 

 the variations given to him by nature, and thus 

 accumulate 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 

 to him at the time, without any thought of altering the 

 breed. It is certain that he can largely influence the 

 character of a breed by selecting, in each successive 

 generation, individual differences so slight as to be 

 quite inappreciable by an uneducated eye. This process 

 of selection has been the great agency in the produc- 

 tion of the most distinct and useful domestic breeds. 

 That many of the breeds produced by man have to a 

 large extent the character of natural species, is shown 

 by the inextricable doubts whether very many of them 

 are varieties or aboriginal species. 



There is no obvious reason why the principles which 

 have acted so efficiently under domestication should 

 not have acted under nature. In the preservation of 

 favoured individuals and races, during the constantly- 

 recurrent Struggle for Existence, we see the most 

 powerful and ever-acting means 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, or 

 when naturalised in a new country. More individuals 

 are born than can possibly survive. A grain in the 



