380 GENERAL SUMMARY [Part II. 



organs for holding lier. These various structures for 

 securing or charming the female are often developed in 

 the male during only part of the year, namely, the breed- 

 ing-season. They have in many cases been transferred in 

 a greater or less degree to the females ; and in the latter 

 case they appear in her as mere rudiments. They are lost 

 by the 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 resembles her young oflfspring throughout 

 life. In almost every great class a few anomalous cases 

 occur in which there has been an almost complete trans- 

 position of the characters proper to the two sexes ; the fe- 

 males assuming 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 throughout all the higher divisions of the animal 

 kingdom of one common cause, namely, sexual selection. 

 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 de- 

 pends 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 male sex, 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 part- 

 ners. 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 



