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Show 76 STRUGGLE FOR EXISTENCE. [CHAP. III. period of its _life, during ~o~e1seh~: t~ :!~[;I':'f~:Jfr£ each generation or at In m.va s, we reflect on this and to suffer great destruction. Whe~ h f 11 b r f. 1 a console ourselves With t e u e Ie ' ~~~g{h!'w':~ :r fature is not incessant, that no. fear is fe~~ that death is generally prompt, and that t~e VIgorous, t healthy, and the happy survive and multiply. 0HAP. IV.] NATURAL SELECTION. '11 CHAPTER IV. NATURAL SELECTION. Natural Selection~its power compared with man's selection-its power on characters of trifling importance-its power at all ages and on both sexes-Sexual SelectionOn the generality of intercrosses between individuals of the same species-Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals-Slow action-Extinction caused by Natural Selection-Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation-Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent-Explains the Grouping of all organic beings. I-Iow will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation ? Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature ? I think we shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary ; and how strong the hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to . their physical conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations ? If such do occur, can we doubt (remembering that many more individuals are born th~n can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? |