Chap. XIV. RECAPITULATION. 467 



unintentionally exposes organic beings to new conditions 

 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 Ins own benefit or pleasure. He may do 

 this methodically, or he may do it unconsciously by pre- 

 serving the individuals most useful to him at the time, 

 without any thought of altering the breed. It is cer- 

 tain that he can largely influence the character of a 

 breed by selecting, in each successive generation, indi- 

 vidual differences so slight as to be quite inappreciable 

 by an uneducated eye. This process of selection has 

 been the great agency in the production of the most 

 distinct and useful domestic breeds. That many of the 

 breeds produced by man have to a large extent the cha- 

 racter 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 preservatioD 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 exist- 

 ence 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 effects of a succession of peculiar seasons, and by 

 the results of naturalisation, as explained in the third 

 chapter. More individuals are born than can possibly 

 survive. A grain in the balance will determine which 

 individual shall live and which shall die, — which variety 

 or species shall increase in number, and which shall 

 decrease, or finally become extinct. As the indi- 



