SEXUAL SELECTION IN RELATION TO MAN 791 



ferences between the sexes in so many and such widely 

 separated classes is intelligible if we admit the action of 

 one common cause, namely, sexual selection. 

 y<. Sexual selection depends on the success of certain in- 

 dividuals over others of the same sex, in relation to the 

 propagation of the species; while natural selection depends 

 on the success of both sexes, at all ages, in relation to the 

 general conditions of life. The sexual struggle is of two 

 kinds: in the one it is between the individuals of the same 

 sex, generally the males, in order to drive away or kill their 

 rivals, the females remaining passive; while in the other^ 

 the struggle is likewise between the individuals of the same 

 sex, in order to excite or charm those of the opposite sex, 

 generally the females, which no longer remain passive, but 

 select the more agreeable partners. This latter kind of se- 

 lection is closely analogous to that which man unintention- 

 ally, yet effectually, brings to bear on his domesticated pro- 

 ductions, when he preserves during a long period the most 

 pleasing or useful individuals, without any wish to modify 

 the breed. 



The laws of inneritance determine whether characters 

 gained through sexual selection by either sex shall be trans- 

 mitted to the same sex, or to both, as well as the age at 

 which they shall be developed. It appears that variations 

 arising late in life are commonly transmitted to one and the 

 same sex. Variability is the necessary basis for the action 

 of selection, and is wholly independent of it. It follows 

 from this, that variations of the same general nature have 

 often been taken advantage of and accumulated through 

 sexual selection in relation to the propagation of the spe- 

 cies, as well as through natural selection in relation to the 

 general purposes of life. Hence secondary sexual charac- 

 ters, when equally transmitted to both sexes, can be distin- 

 guished from ordinary specific characters only by the light 

 of analogy. The modifications acquired through sexual se- 

 lection are often so strongly pronounced that the two sexes 

 have frequently been ranked as distinct species, or even as 



