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120 THE ORIGIN OF SPECIES 



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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 intercrosses be- 

 tween individuals of the same species — Circumstances favorable and 

 unfavorable to the results of Natural Selection ; namely, intercross- 

 ing, 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 naturalization — Action of 

 Natural Selection, through Divergence of Character, and Extinction, on 

 the descendants from a common parents — Explains the grouping of all 

 organic beings — Advance in organization — Low forms preserved— 

 Convergence of character — Indefinite multiplication of species — Sum- 

 mary 



HOW "WILL the struggle for existence, briefly dis- 

 cussed in the last chapter, act in regard to varia- 

 tion ? 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 effi- 

 ciently. Let the endless number of slight variations and 

 individual differences occurring in our domestic produc- 

 tions, 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 organization becomes in some degree plastic. 

 But the variability, which we almost universally meet 

 with in our domestic productions, is not directly pro- 

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

 man; he can neither originate varieties, nor prevent their. 



