270 BREEDING 



1:2:1 in the second hybrid generation, and so on — 

 are, as it were, a secondary display set off by certain 

 varietal characters becoming inherited by the sexual 

 mode described in this paragraph. The heterozygote, 

 together with the ordinary (or homozygous) type 

 {xy + xx) first came into existence in connection 

 with the inheritance of sexual characters. And it 

 will readily be admitted that an extremely simple 

 way of bringing about the production of the two 

 sexes in equal numbers would be the institution of 

 one sex (A), half of whose germ- cells contained the 

 factors for one sex, whilst the other half contained 

 those for the other ; and of another sex (B) all of 

 whose germ-cells were indifferentiated as regards sex. 

 The original mode of inheritance, of which the 

 Mendelian is a later development, and which has pos- 

 sibly persisted without much modification as the sexual 

 mode, may, therefore, be expressed in the formula — 



DR X RR = 50 % DR + 50 % RR 



Or, more generally — 



xy X icaj = 50 % jc?/ + 50 % xx 



The familiar 



DD X RR 



I 

 DR 



1 DD 2 DR 1 RR 



DD 1 DD 2 DR 1 RR RR 



is a secondary development, and only appeared when 



