The Geuctic Factors in the Development of the Housemouse, etc. 115 



black cavies, rabbits and mice the difference is not nearly so easily 

 seen, and there exists a further complication in the fact that young 

 mice and rabbits are generally of a colour different from that which 

 they will finally assume. The colour of the black animal without h, 

 ABCDEFgh, of the "blue" cavy of the fancy and the fade black rabbits 

 and mice is almost exactly that of the house -rat, Mus rattus, 



I have often seen that hu animals, when adult, were nearly as 

 fully coloured as the Hh or HH ones of the same litter, but in such 

 a case the latter ones could be distinguished at a young stage, for 

 they did not pass through a "rusty" stage. 



The agouti individuals of Mus rattus do not have the same colour 

 as agouti decumanus or of agouti cavies or mice. It might therefore 

 be possible that in this species the factor H is absent. 



It has to be stated here that long-haired individuals both of the 

 rabbit and the cavy have a much lighter coloured appearance than 

 the shorthaircd ones. I don't know whether this is due to an actual 

 lessening of pigment in the hair or to a different relation of the hard 

 an the woolly hairs of the coat. A longhaired (angora) black rabbit, 

 or (Peruvian) black cavy, even if it contains H may therefore look 

 as if it did not. And a longhaired black cavy without H (ABCDEFgh) 

 looks positively slaty blue. Of course the only criterium as to whether 

 a cavy or mouse contains H or not, must be its behaviour in appro- 

 piate crosses. Yellow cavies without h (AbCDEFGh) or (AbCDEFgh) are 

 cream. The difference between AbCDEFGH and AbCDEFGh mice is not 

 nearly so great as in cavies. Still I believe that most AbCDEFGh mice 

 would be called cream by fanciers. 



I was given the numbers of h and ii young in three litters of one 

 Hh chocolate and one hh chocolate rabbit. Of nineteen young, eleven 

 were A.ABBccDDEEFFggHh, the remaining 8 being AABBccDDEEFFgghh. 

 For cavies. Miss Sollas, who calls this factor c, has given numerous 

 instances of matings of Hh animals inter se and with hh. In mice, 

 I counted the fully coloured (Hh) and the pale (hh) young produced 

 in litters having one Hh and one hh parent. I find that from 109 

 young, 51 are fully coloured (Hh) and 58 are pale (hh). The expected 

 ratio is 54.5 : 54.5. 



Proofs will be given at the end of this paper for the transmission 

 of this factor H, independantly from factors c, D, e and g. 



The numbers 9, 10, 12, 19, 21, and 22 on the plate are forms 

 without II, all the others have the factor. 



