134 HEREDITY [CH. 



factor, confined to the female, which prevents horns 

 from developing even though the factor for horned- 

 ness is present. Some such inhibiting factor, confined 

 to one sex, may possibly explain sexual dimorphism 

 in general. It may be of course that in some cases the 

 inheritance of sexual characters is limited like that of 

 the variety lacticolor in the Currant Moth ; if the var. 

 lacticolor were dominant instead of recessive, we 

 should have an instance of ordinary sexual dimorphism 

 in this case, for all females are normally heterozygous 

 and would therefore show one character, all males 

 homozygous, and would show the other. Some con- 

 dition of this kind will probably be found to apply to 

 the remarkable cases of certain butterflies which have 

 two or three discontinuous and very distinct forms of 

 the female, but only one kind of male, and in which all 

 the different forms have been bred from eggs laid by 

 one female insect. The few breeding experiments that 

 have yet been made with these species (Papilios or 

 * Swallow-tails ' from Africa and India) suggest that 

 a combination of alternative dominance and sex- 

 limited inheritance will probably be found to explain 

 them. It seems reasonable to suppose, therefore, that 

 not only the determination of sex itself, but the 

 difficult problems of sexual dimorphism and the 

 inheritance of secondary sexual characters, will all be 

 found on analysis to fall into line with the simple 

 principles of Mendelian Heredity. 



