Chap. IV.] NATUKAL, SELECTION. 97 



CHAPTER IV. 



KATUHAL selection; OE THE 8UBVIVAL 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 intercrosses between individuals of the same species — Cir- 

 cumstances favourable and unfavourable to the results of 

 Natural Selection, namely, intercrossing, isolation, number of 

 individuals — Slow action — Extinction caused by Natural Se- 

 lection — 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 — Ex- 

 plains the grouping of all organic beings — Advance in organ- 

 isation — 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, which 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 

 occurring 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 or- 

 ganisation becomes in some degree plastic. But the 

 variability, which we almost universally meet with in 



