R C. PUNNETT 307 



animal with pigmented eyes (grade 18). This last form does not appear 

 to have occurred in Castle's experiments \ nor has it turned up in my 

 own, though both of us have met with animals figured as grade 17. 

 As the result of his experiments Castle distinguishes four types, viz. : 



(1) Self-colour, without any trace of white. 



(2) Dark Dutch, in which he includes animals of grades 1-7. 



(3) " Tan " Dutch, a form of Dutch originating from a cross between 

 yellow Dutch and Black-and-tan. The members of the strain varied 

 between grades 2-5, the great majority belonging to grades 3 and 4. 

 The grades used in describing these animals are the same as those used 

 for the Dark Dutch. There is however a qualitative difference in that 

 a Tan Dutch has more white on the nose and blaze with less on the 

 collar than the corresponding grade of Dark Dutch. The character is 

 probably that described some years ago as the " New Style " Dutch-, to 

 distinguish it from the " Old Style " which is the typical animal of the 

 fancy. 



(4) White Dutch, which may be of grades 15-17. 



These four types are regarded by Castle as due to a system of 

 multiple allelomorphs. Animals with a pattern grade of 8-14 would 

 appear to be regarded either as heterozygous forms, or as due to "mutual 

 modification " by which " the several members of this allelomorphic series 

 tend, as a result of crosses, to become more like one another." Such 

 " mutual modification " Castle states " need not be regarded as change 

 in the nature of a single gene, but merely as equalization of the residual 

 heredity additional to the single genes which produce monohybrid 

 ratios^" At the same time Castle does not appear to be quite clear 

 whether " mutual modification " will account for the appearance of all of 

 the various grades that are not covered by his four type forms, for he 

 states that there may be "possibly many other types or conditions 

 of white-spotting which with sufficiently accurate observation might 

 be distinguished from each other*": though whether he intends this to 

 apply to forms which appeared in the course of his experiments, or 

 whether he is referring to the possibility of other forms appearing 

 which have so far not been met with, is not clear. 



What we want at present to determine in connection with these 



cases of continuous series is whether they can be expressed satisfactorily 



^ A white rabbit with pigmented (blue) eyes is however known : cf. EinfUhrung in die 

 experimentelle Vererbungslehre, von E. Baar, 2 Aufl. Beriin, 1914, Fig. 28a, p. 75. 



2 The Book of the Rabbit, by L. Upcott Gill, 1881, p. 51. 



3 Op. cit. p. 19. * Op. cit. p. 18. 



