386 KECAPITULATION. [CHAP. XV 



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 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 abori- 

 ginally 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 con- 

 stantly-recurrent Struggle for Existence, we see a powerful and 

 ever-acting form of Selection. The struggle for existence inevi- 

 tably 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 natu- 

 ralised in new countries. More individuals are born than -)an 

 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 

 having 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 physical changes, we might have expected to find that 

 organic beings have varied under nature, in the same way as 

 they have varied under domestication. And if there has been any 

 variability under nature, it would be an unaccountable fact if 

 natural selection had not come into play. It has often been 

 asserted, but the assertion is incapable of proof, that the amount 



