IV] Epistatic and Hypostatic 79 



colour is absent the Individual will be devoid of colour. 

 The term is thus used correctly to denote the relation 

 between allelomorphic features belonging to the same pair. 

 But confusion will be introduced if we extend the same 

 term to the relationship between various determining factors 

 which belong to distinct allelomorphic pairs. 



Hitherto we have spoken of the determiner for such a 

 colour as grey in rabbits and mice as "dominant" over 

 the colours lower in the scale, such as black or chocolate. 

 Nevertheless we are here dealing with a relationship quite 

 different In order from that subsisting between the coloured 

 and the albino. Pending a more precise knowledge of the 

 nature of this relationship it will be enough to regard those 

 factors which prevent others from manifesting their effects 

 as higher, and the concealed factors as lower. In accord- 

 ance with this suggestion the terms epistatic and hypostatic 

 may conveniently be introduced. We shall then speak of 

 the determiner for grey as epistatic to that for black ; that 

 for black as epistatic to the determiner for chocolate, and 

 so on. 



When the facts are thus clearly represented we perceive 

 that the variation by which, for example, a black mouse came 

 originally into existence, consisted in the omission of the de- 

 terminer for grey. The chocolate mouse similarly owed Its 

 origin to the successive omission of the determiner for black. 



The important question what the effect of the grey 

 determiner, for example, actually is, remains undecided. 

 A further serious difficulty also arises in regard to the 

 relation of the coXowv yellow to the other colours. Neither 

 of these points is yet satisfactorily understood In the case 

 of mice. The recent papers of Castle (53) and of Hurst^ 

 have made the phenomena in rabbits comparatively clear, 

 though even there, however, an unexplained difficulty 

 remains. The special problems raised by the behaviour of 

 yellow pigment in these animals will be discussed in a sub- 

 sequent section! (see Chap. vii). 



* Read at the Internal. Congr. Zool. Boston, 1907 : not yet published, 

 t According to the number of factors involved and to the definiteness 

 with which their several combinations can be distinguished, an indefinite 

 variety of ratios may of course be produced in T^o families. Some of the 

 I most interesting are those in which some of the heterozygous combinations 

 can be distinguished from the homozygous dominants. (See for examples 

 Shull, 242.) 



