40 MENDELISM CHAP. 



The rose and the pea factors are separate entities, 

 and each when present alone produces a perfectly 

 distinct and characteristic effect upon the single 

 comb, turning it into a rose or a pea as the case 

 may be. But when both are present in the same 

 zygote their combined effect is to produce the 

 walnut comb, a comb which is quite distinct from 

 either and in no sense intermediate between them. 

 The question of the influence of factors upon one 

 another did not present itself to Mendel because 

 he worked with characters which affected different 

 parts of the plant. It was unlikely that the factor 

 which led to the production of colour in the flower 

 would affect the shape of the pod, or that the height 

 of the plant would be influenced by the presence or 

 absence of the factor that determined the shape of 

 the ripe seed. But when several factors can modify 

 the same structure it is reasonable to suppose that 

 they will influence one another in the effects which 

 their simultaneous presence has upon the zygote. 

 By themselves the pea and the rose factors each 

 produce a definite modification of the single comb, 

 but when both are present in the zygote, whether as 

 a single or double dose, the modification that results 

 is quite different to that produced by either when 

 present alone. Thus we are led to the conception 

 of characters which depend for their manifestation 

 on more than one factor in the zygote, and in the 

 present chapter we may consider a few of the 

 phenomena which result from such interaction be- 

 tween separate and distinct factors. 



One of the most interesting and 'instructive cases 

 in which the interaction between separate factors has 



