CHAPTER VI 



Sexual Selection 



The origin of secondary sexual characters. — Darwin's hypothesis ; 

 objections formulated against it. — The numerical propor- 

 tion of males and females. — The aesthetic sense of the fe- 

 male. — Ornamental characters of animals with external fer- 

 tilisation. — New theories: means of identification, desire to 

 frighten enemies, surplus energy in the male, internal se- 

 cretions of genital organs. 



CERTAIN animal characters proved quite a puz- 

 zle to Darwin; they could not be explained by 

 natural selection as they are not apparently useful 

 for the preservation of the species. Such are the 

 many characters which constitute the outward differ- 

 ences between sexes and are not directly useful, either 

 for the act of reproduction or for the bringing up of 

 the young. 



These characters depend sometimes upon the dif- 

 ference between the male's and the female's mode of 

 life, the one leading a roving, the other a sedentary 

 life (f. e. certain parasitic crustaceans) ; in such cases, 

 sexual dimorphism can be explained by natural se- 

 lection. On the other hand, the bright colouring of 

 many birds (parrots, peacocks, humming-birds), but- 



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