HEREDITY AND SEX 185 



and is more likely to perpetuate itself in the long run 

 than a homozygous race that is from the nature of the 

 case suited to a more limited range of external conditions." 

 Whatever may be the real nature and purpose of fertili- 

 zation, it is certainly true, as Wilson 1 remarks, that " the 

 paternal germ-cell is the carrier of something which 

 incites the egg to development, and thus constitutes the 

 fertilizing element in the narrower sense." 



172. Secondary sexual characters. The sexes in 

 the higher animals are differentiated, not alone by the 

 possession of radically different essential organs of repro- 

 duction, but also by the possession of so-called secondary 

 sexual characters. The more brilliant plumage of the 

 male bird, the horns of the ram, and the greater develop- 

 ment of the head and horns of the bull are examples 

 of secondary sexual characters. Darwin regarded the 

 secondary sexual characters as of great significance in 

 sexual selection. As a result of sexual selection he believed 

 that " generally, the most vigorous males, those which 

 are best fitted for their places in nature, will leave most 

 progeny." 



173. Secondary sexual characters and vigor. Breed- 

 ers of the domestic animals have long regarded the degree 

 of development of the secondary sexual characters as 

 an index of sexual vigor. Many have held that a male 

 with the secondary sexual characters strongly developed 

 was not only prepotent in the transmission of purely 

 sexual characters but also in other characters which are 

 desirable to man. Direct evidence is not available to 

 show that because an animal is strongly developed in the 

 secondary sexual characters, he is therefore prepotent 



1 Wilson, "The Cell," p. 230 (1911). 



2 Morgan, "Heredity and Sex," p. 101. 



