398 
GENERAL SUMMARY 
Part II. 
and such 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 lon^ 
tune 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- 
