234 Heredity. 



more fundamental and general than the great exposure 

 of the male, through the intensity of the sexual passion, 

 to the influence of selection; for the parental instinct is 

 fully as important for the welfare of the race as the 

 sexual instinct, and the former is, as a rule, most devel- 

 oped in the female, just as the latter is greatest in the 

 male, and it might be expected to lead to the selection 

 and modification of females, as the latter passion does to 

 the modification of males. 



The Theory of Heredity Furnishes the Only Adequate 

 Explanation. 



We must acknowledge that the great body of facts de- 

 tailed in the beginning of this chapter have no adequate 

 explanation, except on the hypothesis that a part which 

 is present, or functional, or most important in the male 

 alone, is very much more likely than a part which is 

 limited to females in the same way, to give rise to heredi- 

 tary variations. The facts receive a ready explanation on 

 the hypothesis that there is an especial adaptation for 

 the transmission to the egg of gemmules thrown off by 

 the cells of the male body, while their transmission in the 

 female is not thus provided for, but is due to accident. 

 According to this view we must, in animals where the 

 sexes have long been separated, look to the cells of the 

 male body for the origin of a large proportion of the 

 variations which have gradually been accumulated in. 

 the past to give species their present character; and we 

 must regard secondary sexual characters as differing 

 from ordinary specific characteristics, simply in being 

 especially useful to one sex, usually the male, or in being 

 disadvantageous to the other sex, so that natural selec- 

 tion has developed them to a greater degree in one sex 

 than in the other. 



