Chap. XV. Recapitulation. 41 1 



methodically, or lie 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 

 educated eye. This unconscious process of selection has been the 

 <rreat 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 aborigi- 

 nally 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 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 naturalised in 

 new countries. More individuals are born than can possibly- 

 survive. A grain in the balance may determine which individuals 

 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 during any season, over those with which they come 

 into competition, or better adaptation in however slight a degree to 

 the surrounding physical conditions, will, in the long run, turn the 

 balance. 



With animals having separated sexes, there will be in most cases 

 a struggle between the males for the possession of the females. 

 The most vigorous males, or those which have most successfully 

 struggled with their conditions of life, will generally leave most 

 progeny. But success will often depend on the males haying 

 special weapons, or means of defence, or charms ; and a slight 

 advantage will lead to victory. 

 As geology plainly proclaims that each land has undergone great 



