Chap. IV.] NATURAL SELECTION 97 



CHAPTER IV. 



Natural Selection ; ok the Sueviyal 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 — Circum- 

 stances favourable and unfavourable to the results of Natural 

 Selection, namely, intercrossing, isolation, number of individuals 

 — Slow action — Extinction caused by Natural Selection — Diver- 

 gence 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 de- 

 scendants 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, 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 



