262 BREEDING 



cases, may end in its becoming a female ; but it is 

 incapable of making tbe female undergo even the 

 smallest change in the direction of becoming a male. 



The general conclusion derived from these facts 

 is that the male contains within his organisation the 

 potentiality of both sexes, only one of which — ^the 

 male — is manifested under ordinary circumstances; 

 though under the exceptional ones just described 

 it may be suppressed and the female given the oppor- 

 tunity of appearing ; whilst the female contains only 

 the potentiality of that sex to which she ostensibly 

 belongs. This conclusion, stated in Mendelian terms, 

 is that the male is a heterozygote, and the female 

 a homozygote, which agrees with the Mendelian 

 theory of the constitution of the sexes in so far as 

 it asserts that one of them is heterozygous and the 

 other homozygous, and differs from it in that it 

 makes the male heterozygous and the female homo- 

 zygous. 



In the case of the next piece of evidence bearing 

 on the constitution of the sexes we come to very 

 close quarters with the processes which determine 

 sex, and the course of heredity in general — ^to the 

 germ-cells themselves. The weakness in Mendel's 

 theory to account for his simplest results is that the 

 existence of factors in the gametes is no more than 

 an assumption. The assertion that they exist rests 

 on an a priori basis. They have never been seen. 

 In the case to be considered now, the statement that 

 one sex is heterozygous for sex, and that half of its 

 germ-cells carry a thing which determines a particular 



