— 



6 2 Natural Selection. Chap. iv\ 



CHAPTEE 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 

 seX es — Sexual Selection — On the generality of intercrosses between 

 individuals of the same species — Circumstances favourable and unfa- 

 vourable 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 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 organisation becomes in some degree plastic. 

 But the variability, which we almost universally meet with in our 

 domestic productions, is not directly produced, as Hooker and 

 Asa Gray have well remarked, by man ; he can neither originate 

 varieties, nor prevent their occurrence ; he can only preserve and 

 accumulate such as do occur. Unintentionally he exposes organic 

 beings to new and changing 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 consequently 

 what infinitely varied diversities of structure might be of use to 

 each being under changing conditions of life. Can it, then, be 

 thought improbable, seeing that variations useful to man have 



