xliv 



Obituary Notices of Felloivs deceased. 



problem of the tortoise-shell cat, which he had coiitinuously kept in mind 

 since the early days of Mendelism. As mentioned above, he Iiad shown 

 that, as a rule, the heterozygous combination of orange with black is 

 tortoise in the female but orange in the male, a distinction which has not 

 yet been factorially represented. But very rarely a male is produced 

 having the characteristic tortoise-shell distribution of colour. From exten- 

 sive inquiries among breeders, and from some direct observations of his 

 own, Doncaster came on the interesting and suggestive fact that these rare 

 tortoise-shell tom-cats are almost if not quite always sterile. 



In his last paper of all, he put forward the original but by no means 

 extravagant notion that perhaps the tortoise-shell tom is a free-martin, 

 owing its peculiarity to intra-uterine influence of other female embryos. 

 This conjecture was made in consequence of Lillie's surprising discovery 

 as to the nature of the bovine free-martin. Most of these subjects were 

 discussed in his useful text-book, ' The Determination of Sex,' 1914, but 

 of course in the six years that he lived after that publication much progress 

 was made. In regard to the chromosome hypothesis his views were at that 

 time in a transitional stage. It may be noticed, for instance, that in 

 discussing the descent of colour-blindness he does not develop the cyto- 

 logical argument. Normal colour-vision is represented as depending on the 

 presence in the male of a single factor N, the loss of which produces in him 

 colour-blindness. The normal female is homozygous in N, the transmitting 

 female, whose sight is normal, being Nn, like the normal man. This was the 

 notation which he had proposed in 1911, and it has been adopted as an 

 improvement on all previous suggestions. But in man the normal distribu- 

 tion of the sex-factor must be the same as that of N. The inference that 

 the two factors, the one for colour-vision, the other for sex, are transmitted 

 in collocation beomes inevitable ; though till man is proved to have a distinct 

 sex-chromosome, the nature of the collocation might be left to the imagina- 

 tion. Throughout the book also he never loses sight of the somewhat 

 ill-defined though unquestionable evidence as to the possible modification of 

 sex-ratios by influences of some different order, a circumstance which has 

 hitherto not been reconciled with cytological appearances. 



But of the various modes of attack on genetical problems his mind turned 

 perhaps more naturally to cytology than to any other. Laboratory methods 

 were congenial to him. He came to regard the empirical results of experi- 

 mental breeding more and more as a stimulus to microscopical search for 

 some visible basis of difference to which genetical diversities could be 

 referred. The element of apparent fundamentality which he found in 

 cytology very strongly appealed to Doncaster's analytical mind, and he 

 was therefore from the first greatly attracted by the theory of linkage 

 propounded by Morgan. In the clear and excellent ' Cytology,' which he 

 published shortly before his death, he declares himself an adherent, a 

 judgment which, from a student so slow to form decisions, has special value. 

 At a moment when the claims of cytology are acquiring such prominence. 



