THEORY OF EVOLUTION 



55 



sic case of the combs of fowls. If a bird with a 

 rose comb is bred to one with a pea comb (fig. 

 23), the offspring have a comb different from 

 either. It is called a walnut comb. If two 

 such individuals are bred tliey give 9 walnut, 



i 



X 



"t 



r 



\ 



Fio. ~'.i. Cross lietweeii pea and rose combed fowls. (Cliarts 

 of Baur and Goldschmidt.) 



3 rose, 3 pea, 1 single. This proportion shows 

 tliat tlie grandparental types differed in re- 

 spect to two pairs of characters. 



A fourth case is shown in the fruit fly, where 

 an ebony fly with long wings is mated to a grey 

 fly with vestigial wings (fig. 24). The off- 



