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Show 468 RECAPITULA1'ION. CHAP. XIV. viduals of the same species come in all ref?pects into the closest competition with each other, t~e s~ruggle will generally be most severe between them ; It will be almost equally severe between the varieties of t~e same species, and next in severity between the spemes of the same O'enus. But the struggle will often be very severe be~ ween beings most remote in the scale of nature. The sli()'htest advantage in one being, at any age or during an~ season, over those with which it comes into competition, 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 conditions 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 expected 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 pia y. 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. gre~t result by adding up mere individual differences In h1s do1nestic productions; and every one admits that there are at least individual differences in species under nature. But, besides such differences, all naturalists CHAP. XIV. RECAPITULATION. 469 have admitted the existence of varieties, which they think s~fficiently distinct to be worthy of record in systematic works. No one can draw any clear distinction between individual differences and slight varieties· o~ between m?re plainl! marked varieties and sub-spe~ Cies, and species. Let It be observed how naturalists differ ~n the ran~ which they assign to the many representative forms In Europe and North America. If then we have under nature variability and a powerful agent always ready to act and select, why should we doubt that variations in any way useful to beings, under their excessively complex relations of life, would be preserved, accumulated, and inherited? Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products? What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature,-favouring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. The theory of natural selection, even if we looked no further than this, seems to me to be in itself probable. I have already recapitulated, as fairly as I could, the opposed difficulties and objections : now let us turn to the special facts and arguments in favour of the theory. On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws. On this same view we can understand how it is that in each region |