174 PHYSICAL BASIS OF HEREDITY 



difficult to interpret that Guyer's conclusions do not seem 

 to me as yet to be on as secure grounds as those of Seller 's 

 for moths. Both descriptions give, however, the bases for 

 a consistent explanation of sex-linked inheritance in this 

 type (WZ-ZZ). 



Since we do not know as yet whether the same or dif- 

 ferent sex factors are involved in the Drosophila and in 

 the Abraxas types, it seems best not to use the same sym- 



fVZ zz 



IV z zz 



Fig. 73. — Scheme showing the relation of the eex-chromoeomeB of the moth (and of the bird) 



in Bex-determination. WZ-ZZ type. 



bols in both for the sex-factors. If in both types a single 

 sex-factor is concerned, and if it is the same in both, the 

 conditions that make for a female in one case and for a 

 male in the other must be due to a difference in the 

 rest of the hereditary complex that reverses the reaction. 

 It would appear simpler to assume that the sex-factor itself 

 is different in the two cases. If there is more than one 

 factor for sex, the two types may have some in com- 

 mon, but the theoretical situation would remain the same. 

 For our present purpose these possible distinctions are 

 of no importance. 



