52 THEORY OF EVOLUTION 



even when, as in this case, there is a perfectly 

 graded series from one end to the other, by 

 testing out individually enough of the flies to 

 show that one-fourth of them never produce 

 any descendants but ebonies, one-fourth never 

 any but sooties, and one-half of them give rise 

 to both ebony and sooty. 



MENDEL'S SECOND DISCOVERY INDEPENDENT 

 ASSORTMENT 



Besides his discovery that there are pairs of 

 characters that disjoin, as it were, in the germ 

 cells of the hybrid (law of segregation) Men- 

 del made a second discovery which also has 

 far-reaching consequences. The following case 

 illustrates Mendel's second law. 



If a pea that is yellow and round is crossed 

 to one that is green and wrinkled (fig. 21), all 

 of the offspring are yellow and round. Inbred, 

 these give 9 yellow round, 3 green round, 3 

 yellow wrinkled, 1 green wrinkled. All the 

 yellows taken together are to the green as 3: 1. 

 All the round taken together are to the wrin- 

 kled as three to one ; but some of the yellows are 

 now wrinkled and some of the green are now 



