in Hybridisation 73 



constitution, represent in equal numbers all constant forms 

 which result from the combination of the characters when 

 united in fertilisation. 



The difference of the forms among the progeny of the 

 hybrids, as well as the respective ratios of the numbers in 

 which they are observed, find a sufficient explanation in the 

 principle above deduced. The simplest case is afforded by 

 the developmental series of each pair of differentiating 

 characters. This series is represented by the expression 

 A+2Aa + a, in which A and a signify the forms with 

 constant differentiating characters, and A a the hybrid 

 form of both. It includes in three different classes four 

 individuals. In the formation of these, pollen and egg 

 cells of the form A and a take part on the average equally 

 in the fertilisation ; hence each form [occurs] twice, since 

 four individuals are formed. There participate consequently 

 in the fertilisation- 



The pollen cells A + A + a + a 

 The egg cells A+A+a + a. 



It remains, therefore, purely a matter of chance which 

 of the two sorts of pollen will become united with each 

 separate egg cell. According, however, to the law of 

 probability, it will always happen, on the average of many 

 cases, that each pollen form A and a will unite equally 

 often with each egg cell form A and a, consequently one 

 of the two pollen cells A in the fertilisation will meet with 

 the egg cell A and the other with an egg cell a, and so 

 likewise one pollen cell a will unite with an egg cell A, 

 and the other with egg cell a. 



Pollen cells A A a a 



Egg cells A A a a 



