190 EVOLUTION AND ANIMAL LIFE 



"The process of breaking up into the parent forms is thus con- 

 tinued in each successive generation, the same numerical law being 

 followed so far as has yet been observed. 



"Mendel made further experiments with Pisum sativum, crossing 

 pairs of varieties which differed from each other in two characters, 

 and the results, though necessarily much more complex, showed that 

 the law exhibited in the simpler case of pairs differing in respect of one 

 character operated here also. 



"In the case of the union of varieties AB and ab differing in two 

 distinct pairs of characters, A and a, B and b, of which A and B are 

 dominant, a and b recessive, Mendel found that in the first cross-bred 

 generation there was only one class of offspring, really AaBb. 



"But by reason of the dominance of one character of each pair 

 these first crosses were hardly if at all distinguishable from AB. 



"By letting the AaBb's fertilize themselves, only four classes of 

 offspring seemed to be produced, namely: 

 " AB showing both dominant characters. 

 "Ab showing dominant A and recessive b. 

 " aB showing recessive a and dominant B. 

 " ab showing both recessive characters a and b. 

 "The numerical ratio in which these classes appeared was also 

 regular and approached the ratio 



QAB : 3Ab : 3aB : lab. 



"But on cultivating these plants and allowing them to fertilize 

 themselves, it was found that the members of the 

 Ratios 



1 ab class produce only ab's. 



( 1 aB class may produce either all aB's, 



(2 or both aB's and ab's. 

 o j 1 Ab class may produce either all Ab's 

 (2 or both Ab's and ab's. 



1 AB class may produce either all AB's 



2 or both AB's and Ab's, 

 2 or both AB's and aB's, 



4 or all four possible classes again, 



namely, AB's, Ab's, aB's, and ab's, 



and the average number of members of each class will approach the 

 ratio 1 : 3 : 3 : 9 as indicated above. 



"The details of these experiments and of others like them made 

 with three pairs of differentiating characters are all set out in Mendel's 

 memoir." 



