THE FACTORIAL HYPOTHESIS 



217 



three rose: three pea: one single. Since single comb 

 was not present in either of the grandparental strains, 





'1^\>\"v\". 



\o 



Fig. 64. 



' d 

 -Combs of fowls, a, Single; 6, pea; c, rose; (i, walnut; e, Breda. 



how then can its appearance in this cross be explained? 

 The difficulty was met as follows: The ratio shows 



