THE DETERMINATION OF SEX 295 



ternal conditions, as in the case of Seller's moths and 

 Riddle's doves. When, however, the male is hetero- 

 gametic, it would be possible to control sex only by 

 some agency which would differentially aid or inhibit 

 the progress of one of the two kinds of sperm peculiar 

 to this kind of an organism in its approach to, or 

 penetration of, the ovum. 



