48 THEORY OF EVOLUTION 



(fig. 18) is the same in principle as in the pre- 

 ceding cases. The only difference between 

 them is that the hybrid which contains both the 

 yellow and the green factors is in appearance 



P\/tK.\7.-> ( J 



FIG. 18. Diagram illustrating the history of the factors in 

 the cross shown in Fig. 17. 



not intermediate, but like the yellow parent 

 stock. Yellow is said therefore to be dominant 

 and green to be recessive. 



Another example where one of the con- 

 trasted characters is dominant is shown by the 

 cross of Drosophila with vestigial wings to 

 the wild type with long wings (fig. 19). The 

 F! flies have long wings not differing from 

 those of the wild fly, so far as can be observed. 

 When two such flies are inbred there result 

 three long to one vestigial. 



