CHAPTER V 



NATURAL SELECTION AND THE STRUGGLE 

 FOR EXISTENCE; SEXUAL SELECTION 



The tendency to regard natural selection as more or less unnecessary 

 or superfluous which is so characteristic of our day, seems to grow out 

 of reverence for the all-sufficiency of the philoso])hy of evolution, and 

 pious belief that the history of li\dng things flows out of this j^hilosophy 

 as a necessary truth or axiom. — Brooks. 



La selection naturelle est un principe admirable et parfaitement 

 juste. Tout le monde est d'accord aujourdliui sur ce point. Mais 

 oil Ton n'est pas d'accord, c'est sur la liniite de sa puissance et sur la 

 question de savoir si elle peut engendrer des formes specifiques 

 nouvelles. II semble bien demontre aujourd'hui qu'elle ne le i:)eut. 



— Delage. 



Of all the various factors of organic evolution the one 

 which has been most relied on as the great determining agent 

 is that called Natural Selection, the survival of the individuals 

 best fitted for the conditions of life, with the inheritance of 

 those species-forming adaptations in which fitness lies. The 

 primal initiative is not in natural selection, but in variation, 

 germinal and individual. This may be slight variation (fluc- 

 tuation) or large deviation (saltation), but in an}- case all 

 difference in species or race must first be individi'al. The 

 impulse to change, once arisen, is continued through lierodity. 

 From natural selection arises the choice among different lines 

 of descent, the ada])tive tending to exclude the non-adaptive, 

 while traits which are neither helpful nor hurtful, but simply 

 indifferent, may be borne along by the current of adaptive 

 characters. Finally separation or isolation tends to preserve a 

 special line of heredity from being merged in the mass which 

 constitutes the parent stock or species. 



Without individual variation, no change could take j^laco; 

 all organisms would be identical in structure. Without heredity, 



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