CHAPTER IV 

 Natural Selection ; or the Survival of the Fittest 



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 the results of 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 — 

 Advance in organisation — Low forms preserved — Convergence of 

 Character — Indefinite multiplication of species — Summary. 



HOW will the struggle for existence, briefly discussed 

 in the last chapter, act in regard to variation? Can 

 the principle of selection, v^'hich we have seen is so 

 potent in the hands of man, apply under nature? I think 

 we shall see that it can act most efficiently. Let the endless 

 number of slight variations and individual differences occur- 

 ring in our domestic productions, and, in a lesser degree, in 

 those under nature, be borne in mind ; as well as the strength 

 of the hereditary tendency. Under domestication, it may be 

 truly said that the whole organisation becomes in some degree 

 plastic. But the variability, which we almost universally 

 meet with in our domestic productions, js not directly pro- 

 duced, as Hooker and Asa Gray have well remarked, by man ; 

 he can neither originate varieties, nor prevent their occur- 

 rence ; he can only preserve and accumulate such as do occur. 

 Unintentionally he exposes organic beings to new and chang- 

 ing conditions of life, and variability ensues; but similar 

 changes of conditions might and do occur under nature. Let 

 it also 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; and conse- 



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