RECAPITULATION AND CONCLUSION 507 



Variability is not actually caused by man ; he only uninten- 

 tionally exposes organic beings to new conditions 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, individual 

 differences so slight as to be inappreciable except by an edu- 

 cated 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 favoured individuals and races, 

 during the constantly-recurrent Struggle for Existence, we 

 see a powerful and ever-acting form of Selection. The 

 struggle for existence inevitably follows from the high geo- 

 metrical 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 naturalised in new 

 countries. More individuals are born than can possibly sur- 

 vive. A grain in the balance may determine which indi- 

 viduals shall live and which shall die,— which variety or 

 species shall increase in number, and which shall decrease, 

 or finally become extinct. As the individuals of the same 

 species come in all respects into the closest competition with 

 each other, the struggle will generally be most severe between 

 them; it will be almost equally severe between the varieties 

 of the same species, and next in severity between the species 

 of the same genus. On the other hand the struggle will often 

 be severe between beings remote in the scale of nature. The 

 slightest advantage in certain individuals, at any age or dur- 



