420 THE ORIGIN OF SPECIES 



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 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 in- 

 extricable 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 na- 

 ture. In the survival of favored 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 com- 

 mon 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 naturalized in 

 new countries. More individuab are born than can possibly sur- 

 vive. 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 be- 

 tween 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 compe- 

 tition, 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 suc- 

 cessfully 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 or- 

 ganic beings have varied under nature, in the same way as they 



