72 MENDELISM CHAP. 



and until further experiments have been devised and car- 

 ried out it is not possible to decide which is the correct 

 view. 



Besides these comparatively rare cases where the 

 heterozygote cannot be said to bear a closer resemblance 

 to one parent more than to the other, there are cases in 

 which it is often possible to draw a visible distinction be- 

 tween the heterozygote and the pure dominant. There 

 are certain white breeds of poultry, notably the White 

 Leghorn, in which the white behaves as a dominant to 

 colour. But the heterozygous whites made by crossing 

 the dominant white birds with a pure coloured form (such 

 as the Brown Leghorn) almost invariably show a few col- 

 oured feathers or ''ticks" in their plumage. The domi- 

 nance of white is not quite complete, and renders it pos- 

 sible to distinguish the pure from the impure dominant 

 without recourse to breeding experiments. 



This case of the dominant white fowl opens up another 

 interesting problem in connection with dominance. By 

 accepting the " Presence and Absence " hypothesis we are 

 committed to the view that the dominant form possesses 

 an extra factor as compared with the recessive. The 

 natural way of looking at this case of the fowl is to regard 

 white as the absence of colour. But were this so, colour 

 should be dominant to white, which is not the case. We 

 are therefore forced to suppose that the absence of colour 

 in this instance is due to the presence of a factor whose 



