468 EECAPITULATION. Chap. XIV. 



viduals 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 be- 

 tween 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 compe- 

 tition, or better adaptation in however slight a degree to 

 the surrounding physical conditions, will turn the balance. 



With animals having separated sexes there will in 

 most cases be a struggle between the males for possession 

 of the females. The most vigorous individuals, or those 

 which have most successfully struggled with their condi- 

 tions of life, will generally leave most progeny. But 

 success will often depend on having special weapons 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 



