SEXUAL DIMORPHISM 



423 



the males and females of a species of one of the lower phyla 

 the difference is most frequently a difference of size. The 

 female is usually larger. This is regarded as due to the great 

 demands made upon the female organism in the development 

 of the relatively large mass of egg substance. Among Mammals 

 the male is usually the larger, if there is any 

 marked difference in size. The males of the 

 fur seal and sea lion are about four times as 

 large as the female. In such cases the males 

 fight for the possession of the females, and 

 consequently size and strength are the deter- 

 mining factors in the struggle for existence 

 among the males. Why the size of the male 

 parent should be inherited by the male off- 

 spring and not by the female is an interesting 

 problem which still awaits solution. 



857. Sexual Selection. — In some species of 

 Biids also the males are the larger, and this 

 occurs again in those cases in which the males 

 contend in battle for the possession of the 

 females. Generally, however, the male bird is 

 distinguished from the female by his greater 

 beauty or his superior ability as a vocalist. 

 This introduces us to a particular form of 

 natural selection, called sexual selection. At 

 the time of mating the males vie with each 

 other by displaying their beautiful plumage or 

 singing their best songs. This is done in the 

 presence of the female, and is evidently intended 

 to win her favor. Following this courtship the birds mate in 

 pairs (monogamy), apparently according as the appearance or 

 performance of the male pleases the female. That is to say, 

 in this case the selecting which determines the male parentage 

 of the next generation is done by the female, and the survival 



Fig. 260 . — 

 Queen of Termes 

 obesus. Natural 

 size. (From 

 Folsom after 

 Hagen). It is 

 said that in some 

 cases the queen 

 attains the size 

 of thirty thou- 

 sand workers. 



