316 The Genetics of the Dutch Rabbit 



such an interpretation, even when we make a most generous allowance 

 for " mutual modification." The absence of selfs in F^ is an argument 

 used by Castle in favour of his view; and though I agree that both Tan 

 and White Dutch must lack something present in the self-coloured 

 animal, I fail to see that this entails the creation of a series of multiple 

 allelomorphs to represent these three states of pigmentation. 



I am inclined to regard the Tan Dutch as a definite stage of higher pig- 

 mentation in the series White-Spotted-Typical Dutch, and consequently 

 lacking P. This view receives some support from a piece of information 

 which Professor Castle very kindly sent in answer to an enquiry of mine, 

 i.e. that Tarr Dutch rabbits may exhibit heterochromia iridis. If we 

 denote as N the factor that turns a Typical Dutch into a Tan, or New- 

 Style, Dutch (cf p. 307), the cross Tan Dutch x White Dutch would 

 result in F^ heterozygotes of the constitution NnTtSs, and these might 

 be expected to give results similar to those which Castle obtained, both 

 when mated together and when crossed back with White Dutch. 



(4) The cross between Dark Dutch and Tan Dutch (pp. 13—16). Two 

 outstanding features are here exhibited. In the first place the F^ 

 animals are of distinctly higher pigmentation than either of the parental 

 forms, some of them being even self-coloured. Secondly of the F.^ 

 generation, consisting of 275 individuals, about three-quarters are of 

 grades 0-3 (as many as 89 being self-coloured), while the remaining 

 quarter extends over grades 4-11. Here again the facts appear to me 

 to be entirely opposed to the interpretation in terms of multiple allelo- 

 morphs advocated by Castle; for it offers no explanation of the self- 

 coloured animals in F^ , or in Fo where they constitute about 30 % of the 

 total. Nor does it explain the considerable range in F^, extending so 

 far as grade 11. On the interpretation I have suggested these facts are 

 intelligible. The Dark Dutch animals used varied from grades 2-6. 

 Consequently, though homozygous for P, they may well have lacked 

 either T or S, or in some cases both. The ^i animals would be 

 heterozygous for P and N, and this would entail their being self- 

 coloured, or nearly so. But at the same time if lacking in T or S they 

 would be constitutionally PpNnTtSS, or PpNnTTSs, while some 

 might be PpNnTtSs^ From such animals we should expect an F.2 

 generation similar to that which Castle obtained. 



This cursory examination of Castle's data is sufficient to make 

 it clear that the hypothesis of multiple allelomorphs advocated by him 



1 Probably some were of this constitution, for when crossed with White Dutch a few 

 animals of grades 16 and 17 appeared {cf. Text-fi<j. 5, p. 15). 



