INHERITANCE OF SEX 257 



and this circumstance would be slender evidence for 

 the truth of this theory. The peculiar interest and 

 value as evidence of cases like that of the currant 

 moth lies in the fact that in them the character of 

 sex has, as it were, become entangled with and 

 entered into permanent relations with a varietal 

 colour- character, the consequences of which insigni- 

 ficant little event has been to withdraw the veil 

 which has all this time hidden the real difference 

 between the sexes from our view. 



Bearing in mind the presence and absence hypo- 

 thesis of the nature of the two characters of a 

 Mendelian pair, it will be observed that, according 

 to the theory we are considering, the female who 

 is heterozygous for sex is characterised by the ^presence 

 of the factor in virtue of which she is female, whilst 

 the male lacks this factor altogether. Even the 

 female, being a hybrid, has only one dose of this 

 factor, whatever it is. The male is male in virtue 

 of the fact that he has none at all. Half of the 

 germ cells of the female contain this factor ; the 

 other half lack it. All the germ- cells of the male 

 lack it. 



Now this factor, which is present in the female 

 alone, has been identified with the thing which checks 

 the development of certain characters which attain 

 to their full development in the male, in cases of sex 

 limited inheritance. In the case of the horns in 

 sheep there is something in the female hybrid resulting 

 from a cross between a horned and hornless individual 

 which inhibits the development of horns, which attain 



R 



