K. ('. PUNNKTT 47 



bringing about, cunijilcti' )>igiii('iit:iti(>ii, |)assing into the condition 

 X', so that animals which are liomo/.ygoiis for the factor after the 

 change has occurred (X'X') display the Himalayan pattern in place 

 of self-colour. Again, as the result of some other unknown change 

 it becomes X" and loses the property of bringing about any pig- 

 mentation, so that animals homozygous for the factor in this condition 

 (X"X") are albinos. Nevertheless both X' and X" remain equivalent 

 to X and to one another in the mechanism of factorial distribution, 

 i.e. any one of the three is allelomorphic to any other one'. 



Of the cases cited by Sturtevant in support of his contention 

 three, viz. the Himalayan i-abbit case, the case of the green, varie- 

 gated, and cliloriiia foliage in Aquilegiu, and that of the red, eosin, and 

 white eye in Drusophila, are certainly favourable to it. In all of them 

 there is a series of three forms of diminishing pigmentation, and the 

 genetic relations are similar in all'-^. 



In this connection the rabbit case dealt with in the earlier part 

 of this paper is of interest, as it differs in some respects from the group 

 of cases just mentioned. On the presence and absence hypothesis the 

 two factors D and E are, so far as is known, completely coupled, and 

 the three gametic possibilities are therefore DE, dE, and de. To bring 

 this case into line with those discussed above we should have to refer 

 these three possibilities respectively to the three allelomorphs X, X', and 

 X". The subjoined table represents the zygotic formulae of the various 

 possible combinations on these two hypotheses, both when the agouti 

 factor is present and when it is absent. 



' Sturtevant does not suggest what the nature of these changes in the factor may be— 

 whether some internal change involving merely a rearrangement of its constituents and 

 analogous to isomerism in chemistry, or the definite loss of some constituent in each case. 

 The latter view would practically mean the application of the presence and absence 

 hypothesis to certain of the constituents of the factor itself. 



- The case of Emerson's beans also cited by Sturtevant may perhaps be of a similar 

 nature, but until the relation of the green-leaved-yellow-podded to the yellow-leaved- 

 yellow-podded form has been investigated this cannot be regarded as certain. 



