RECAPITULATION AND CONCLUSION 421 



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 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. But the 

 struggle will often be very severe between beings most 

 remote in the scale of nature. The slightest advantage 

 in one being, at any age or during any season, over 

 those with which it comes into competition, or better 

 adaptation in however slight a degree to the sur- 

 rounding physical conditions, will turn the balance. 



With animals having separated sexes there will in 

 most cases be a struggle between the males for posses- 

 sion of the females. The most vigorous individuals, or 

 those which have most successfully struggled with their 

 conditions of life, will generally leave most progeny. 

 But success will often depend on having special weaponp 

 or means of defence, or on the charms of the males ; and 

 the slightest advantage will lead to victory. 



As geology plainly proclaims that each land has 

 undergone great physical changes, we might have ex- 

 pected that organic beings would have varied under 

 nature, in the same way as they generally have varied 

 under the changed conditions of domestication. And if 

 there be 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 quite incapable of proof, that the amount of variation 

 under nature is a strictly limited quantity. Man, 

 though acting on external characters alone and often 

 capriciously, can produce within a short period a great 

 result by adding up mere individual differences in his 

 domestic productions ; and every one admits that there 

 are at least individual differences in species under 

 nature. But, besides such differences, all naturalists 

 have admitted the existence of varieties, which they 



