THEORY OF REVERSION 225 



second hybrid generations in the two cases (9 purple, 

 3 pink, and 4 white in flower colour, and 9 purple - 

 spotted grey, 3 grey, and 4 white in seed- coat) had 

 not given the clue. In the case of the seed-coat 

 this interpretation is relatively obvious, inasmuch 

 as the two dominant characters, the purple-spot and 

 the grey background, can be seen as distinct and 

 separate things when present in the same zygote ; 

 whereas in the case of flower colour the one is super- 

 imposed on the other, the blue on the pink, or vice 

 versa, in such a way as to afiord no indication that 

 the resultant blend is compounded of two distinct 

 things. 



The two pairs of factors involved in this case 

 are blue (B) and absence of blue (b), and pink (P) 

 and absence of pink (p). Blue bears the same relation 

 to pink as purple-spot does to grey — namely, that 

 the blue factor is unable to manifest itself in the 

 absence of pink. It can only come into being in a 

 zygote in which pink also exists, so that blue never 

 appears as such, because it can only exist in the 

 presence of pink, and the two together make purple. 

 But pink can exist in the absence of blue, just as 

 grey can exist in the absence of purple-spot. 



If blue were not dependent on the presence of 

 pink for its development, the second hybrid genera- 

 tion, the types composing which are shown in 

 Plate IV, would have been 9 purple, 3 pink, 3 

 blue, and 1 white. Also the cross would have been 

 between a pink and blue, or between a purple and 

 white. 



