38 THE METHODS AND 



In the light of Mendelian knowledge, when 

 a family consists of more than one type the 

 fact means that the germ-cells of one or other 

 parent must certainly be of more than one 

 kind. In the case of sex the members of the 

 family are thus of two kinds, and the pre- 

 sumption is overwhelming that this distinction 

 is due to a difference among the germ-cells. 

 Next, since for all practical purposes the 

 numbers of the two sexes produced are 

 approximately equal, sex exhibits the special 

 case in which a family consists of two types 

 represented in equal numbers, half being 

 male, half female. But I called your attention 

 to the fact that equality of types results when 

 one parent was cross-bred in the character 

 concerned, having received one dose only of 

 the factor on which it depends. So we may 

 feel fairly sure that the distinction between the 

 sexes depends on the presence in one or other 



