REPRODUCTION 87 



proportions just mentioned and present con- 

 ditions of great interest. The twenty-five per 

 cent of white individuals, when bred among 

 themselves, have been proved to be pure whites 

 like their white grandparent. The seventy- 

 five per cent of black individuals, when tested 

 in a like fashion, have been shown to be, in 

 the case of twenty-five per cent, pure black like 

 their black grandparent, and, in the case of 

 the remaining fifty per cent, black but with 

 the ability to produce white offspring exactly 

 as their parents did. Thus the grandchildren 

 of the original pure black and white mating 

 fall into three classes: twenty-five per cent 

 pure white, twenty-five per cent pure black ; 

 and fifty per cent black, but capable of pro- 

 ducing a definite proportion of white off- 

 spring. 



You are familiar with the explanation that 

 Mendel offered for this remarkable state of 

 affairs, an explanation that is still generally 

 accepted. Briefly it is to the effect that each 

 reproductive element, egg cell or sperm cell, 

 can carry only one of any pair of characteris- 

 tics, and that in this respect each reproduc- 

 tive cell, be it either egg or sperm, is pure. 

 If we assume this segregation of characteris- 



