46 INHERITANCE IN RABBITS 



hairs, where yellow then shows. A "ticked" gray coat 

 results. When this factor is present the under surfaces 

 of the rabbit (tail, belly) are unpigmented (white). The 

 symbol, A, stands for agouti, this factor having first been 

 demonstrated in the "agouti" guinea-pig. (See Castle, 

 :o7.) 



U. A factor for uniformity of pigmentation (in distinction from 

 spotting with white, S). 



E. A factor governing the extension of black and of brown pig- 

 mentation, but not of yellow. When most restricted in 

 distribution the black or brown pigments are found in the 

 eye and in the skin of the extremities only, but not in the 

 hair, when more extended they occur also in the hair 

 generally. 



DEVELOPMENT OF THE FACTOR HYPOTHESIS. 



Scientific hypotheses, to be of service, should be as simple as possible. 

 Therefore no unnecessary assumptions should be made. To assume 

 the existence in gray rabbits of eight independent color factors requires 

 justification. 



The General Color Factor, C. 



The existence of a color factor (C) was first suggested by Cuenot (: 03) 

 to explain how it is that albinos transmit in crosses the particular colors 

 which were borne by their pigmented ancestors. This common color 

 factor being acted upon by specific substances (perhaps color enzymes) 

 produces specific pigments, such as black, brown, or yellow. No hypoth- 

 esis simpler than this has been suggested, nor any other which adequately 

 accounts for the observed facts. 



The Specific Pigment Factors, B, Br, and Y. 



The existence of separate factors for black and for brown pigmentation 

 is shown beyond question by the results of crossing black with brown 

 varieties, in guinea-pigs and in mice. In rabbits a brown variety is not 

 known to us personally, though we have been informed that such a vari- 

 ety exists in continental Europe. 



The existence of a separate factor (Y) for yellow pigmentation can 

 scarcely be questioned, in view of the fact that the yellow pigmentation 

 is as regards distribution quite independent of both black and brown, 

 remaining extended throughout the fur when they are restricted to the 

 eyes and the skin of the extremities. 



The Intensity Factor, I or D. 



The existence of an intensity factor was first announced by Bateson 

 (: 06) as having been demonstrated by Miss Durham in the case of mice. 



For guinea-pigs and rabbits we are able to confirm completely Miss 

 Durham's discovery. Since dilution or concentration of pigment is a 



