SEX INHERITANCE 105 



mosomes in these eggs, depends in P. caryaecaulis on 

 an initial chromosomal difference between the types 

 of migrants, but in P. fallax on some environmental 

 influence. The difference between the two kinds of 

 sexual individuals, i.e., sex itself, is determined in 

 both cases by the distribution of the chromosomes, 

 however that distribution may itself be conditioned. 



It is scarcely necessary to speak of other cases, in 

 which, although an internal mechanism is known to 

 exist for producing equal numbers of males and 

 females, yet more individuals of one or the other 

 sex may actually appear. For instance, it has been 

 shown repeatedly in Drosophila that when a sex 

 linked lethal factor is present in the sex chromosome, 

 any male that contains this lethal X chromosome 

 will perish. The females, on the other hand, will 

 live, because they contain in addition another X 

 chromosome, having the dominant normal allelo- 

 morph of the lethal factor. These lethal factors 

 may be factors that cause abnormalities in organs 

 essential to the life of the individuals and destroy the 

 individual in this sense. The changed ratios do not 

 at all affect the theory that there exists in Drosophila 

 an internal sex-determining mechanism, although 

 were the cases not actually worked out, the abnormal 

 ratios might have seemed to disprove the theory of 

 the sex chromosomes. 



We can imagine other ways in which even in the 

 presence of a regulating sex mechanism the actual 

 ratio of males to females might be changed from 

 equality to some other ratio. For instance, since the 



