398 GENERAL SUMMAEY Part II. 



and sucli widely separated classes, is intelligible if we 

 admit the action throughout all the higher divisions 

 of the animal kingdom of one common cause, namely 

 sexual selection. 



Sexual selection depends on the success of certain 

 individuals over others of the same sex in relation to 

 the propagation of the species ; whilst natural selection 

 depends on the success of both sexes, at all ages, in re- 

 lation 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 male sex, in 

 order to drive away or kill their rivals, the females 

 remaining passive; whilst 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 selection is closely analogous to that which man 

 unintentionally, yet effectually, brings to bear on his 

 domesticated productions, when he continues for a long 

 time choosing the most pleasing or useful individuals, 

 without any wish to modify the breed. 



The laws of inheritance determine whether charac- 

 ters gained through sexual selection by either sex shall 

 be transmitted to the same sex, or to both sexes ; as 

 well as the age at which they shall be developed. It 

 appears that variations which arise late in life are com- 

 monly transmitted to one and the same sex. Vari- 

 ability 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 

 species, and through natural selection in relation to the 

 general purposes of life. Hence secondary sexual cha- 



