36 THE METHODS AND 



of the interpretation of sexual difference sug- 

 gested by our experimental work as of some 

 practical moment, I do not imply that as in 

 the other instances I have given, the know- 

 ledge is likely to be of immediate use to our 

 species ; but only that if true it makes a con- 

 tribution to the stock of human ideas which 

 no one can regard as insignificant. 



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 



