702 THE DESCENT OF MAN, 



the females are unadorned. AVhen tlie sexes differ in more 

 important structures it is the male which is provided with 

 special sense-organs for discovering the female, with loco- 

 motive organs for reaching her, and often with prehensile 

 organs for holding her. These various structures for charm- 

 ing or securing the female are often developed in the male 

 during only part of the year; namely, the breeding-season. 

 They have in many cases been more or less transferred to 

 the females; and in the latter case they often appear in her 

 as mere rudiments. They are lost or never gained by tlie 

 males after emasculation. Generally they are not developed 

 in the male during early youth, but appear a short time 

 before the age for reproduction. Hence, in most cases the 

 young of both sexes resemble each other; and the female 

 somewhat resembles her young offspring throughout life. 

 In almost every great class a few anomalous cases occur, 

 where there has been an almost complete transposition of 

 the characters proper to the two sexes; the females assum- 

 ing characters which properly belong to the males. This 

 surprising uniformity in the laws regulating the differences 

 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. 



Sexual selection depends on the success of certain indi- 

 viduals over others of the same sex, in relation to the prop- 

 agation of the species; while natural selection depends on 

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

 eral 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 selection is closely analogous to that which man unin- 

 tentionally, yet effectually, brings to bear on his domesti- 

 cated productions, when he preserves during a long period 

 the most pleasing or useful individuals, without any wish to 

 modify the breed. 



The laws of inheritance 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 



