R. C. PUNNETT 311 



grade, such as Spotted and White Dutch, often exhibit heterochromia 

 iridis. In the lowest grades (i.e. those with most white) this may 

 not infrequently extend so far as to result in completely pale blue, 

 or " wall " eyes'. Even in typical Dutch, cases occur in which one or both 

 eyes are blue-, though more frequently the iris is patched. I have 

 hitherto met with no case of heterochromia in a rabbit which contains 

 P. A White Dutch, to which P has been added, is of a grade of pig- 

 mentation very like that of a Typical Dutch, and its eyes are normal 

 brown. When such animals, heterozygous for P, are mated back to 

 White Dutch, they produce approximately equal numbers of White 

 Dutch and of animals near Typical Dutch in appearance. Of those 

 that I have so far bred in this way, the white have always been hetero- 

 chromic, whereas those containing P have never been heterochromic. 

 This relation of P to heterochromia promises to be of much assistance 

 in the analysis of the higher grades of pigmentation. 



Whether P produces a similar eflfect in a single and a double dose is 

 a point which I have not yet been able to decide, but experiments with 

 this object are now in progress^ The bearing of the demonstration of 

 this factor upon the nature of the higher grades of pigmentation (i.e. 

 those with more colour) is however sufficiently clear. Corresponding to 

 each lower grade we must suppose that there is a higher grade, differing 

 from the former in that it also contains P; and if certain lower grades 

 breed true there is no reason why we should not eventually find a coiTe- 

 sponding series of higher grades also breeding true. The self-coloured 

 rabbit, the highest grade of all, I am at present inclined to regard as a 

 Typical Dutch which is also homozygous for P. 



Regarded from this point of view Castle's procedure of lumping all 

 the higher grades together as Dark Dutch is justified in so far as these 

 higher grades contain a factor which is not present in the lower grades. 

 It does not however take into account the fact that they may vary 

 greatly with respect to those other genetic factors to which are due 

 the diflferences between Typical, Spotted, and White Dutch, and their 

 various heterozygous forms. 



With this brief outline of the interpretation I would suggest, an 



* The White Dutch animal figured on PI. 2, Fig. 19 of Castle's memoir evidently has a 

 ' ' wall " eye. So also has the white rabbit figured by Baur to which reference was made earlier. 



■•* Cf. Bond, C. J., Journ. Genet. Vol. ii, 1912, p. 111. 



' Evidence is accumulating that PP animals are more pigmented than the corre- 

 sponding Pp forms. This has been allowed for in Fig. 2 (p. 312) except in the case 

 of PPttss rabbits. The grade of such animals is probably about 3 — 5 instead of 6 — 8 as 

 given in Fig. 2. 



