The Genetic Factors in the Development ef the Housemouse, etc. 123 



blue animals with this factor become blue and tan, the markings 

 being much lighter yellow than in the black and tan. The relation 

 of black and tan to agouti has, for as far as I know, not yet been 

 studied, and it is therefore impossible to state whether this factor is 

 or is not present in the wild rabbit. It is well known that black 

 and tan markings occur in certain \ arieties of dogs, goats, donkeys, 

 and possibly cows. 



Factor P. 



Animals having this factor (which seems to be found in rabbits 

 only), have a coat, in which after moult some of the hairs, or only 

 the tips of some hairs turn white. This factor is present in the 

 so-called "dark-silver" rabbit. The colour of this variety is therefore 

 "dominant" to black, whereas the colour of the medium silver rabbit 

 is due to the absence of F. Light silver rabbits seem to be ffi'P. 



These fifteen genetic factors are the only ones I know of which 

 have ever been studied in domestic rodents, as having an influence 

 on the colour. Other factors studied by me are one which by its 

 presence or absence makes the difference between a normal and a 

 waltzing mouse, and one other influencing the size of the animal. 

 In the light of Plate's observations on the occurrance of mice with 

 deformed tails, "Knickschwanzmäuse" in his cultures, it might be of 

 interest to state that in the beginning, when I used cages having a 

 sleeping-compartment, the door of which could be shut at will by a 

 metal slide, mice with deformed tails occurred not infrequently, but 

 that after I used cages without nest-compartment, hardly any de- 

 formed tails were found. 



In the formulae I have only included A, B, c. D, E, F, G and H, 

 which factors I know from personal study, as can be seen from the 

 tables. The factor L, although I have worked with it quite exten- 

 sively, does not alter the colour of the pigmented coat itself, and 

 therefore, not to make the formulae unnecessarily long, I did not 

 include it in them in the body of the paper. 



I will now give some instances of Mutation which I have witnessed. 



Mutations. 



I. Loss of the factor G. 



I have elsewhere described this mutation at some length, but as 

 I see that Goldschmidt is the only author who seems to have under- 



