80 NATURAL SELECTION. Chap. IV. 



CHAPTEE 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 Selection — On the generality of inter- 

 crosses 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 Cha- 

 racter, 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. 



How 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 con- 

 ditions 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 (remem- 



