RECAPITULATION. 455 



viduals shall live, and which shall die — which variety of 

 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 individ- 

 uals, 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 under- 

 gone 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 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 species present individual differ- 

 ences. But, beside such differences, all naturalists admit 

 that natural varieties exist, which are considered suffi- 

 ciently distinct to be worthy of record in systematic works. 

 No one has drawn any clear distinction between individual 

 differences and slight varieties ; or between more plainly 

 marked varieties and sub-species and species. On separate 

 continents, and on different parts of the same continent, 

 when divided by barriers of any kind, and on out-lying 

 islands, what a multitude of forms exist, which some experi- 



