COLOR. 69 



The physical or chemical differences between the pigments of the 

 different fur colors are not wholly clear. According to Onslow (1915) 

 the pigments of black, brown, and yellow rabbits can not be distin- 

 guished, physically or chemically, when isolated. At first sight this 

 seems hardly possible with such apparently different colors. A result 

 thoroughly in line with this view, however, followed the matching 

 of fur colors with Ridgway's charts, much to the writer's surprise at 

 the time. Ridgway distinguishes 72 hues passing from red through 

 orange, yellow, green, blue, and purple, back to red. The yellows, 

 sepias, and browns of guinea-pigs and human brown and red hair all 

 matched colors near hue 17, "orange yellow," in the classification. 

 The differences depended merely on differing amounts of black and 

 white. Bateson (1903), indeed, found that yellow pigment is dissolved 

 from hair by potassium hydroxide very much more rapidly than brown 

 pigment, which dissolved more rapidly than black. This, however, 

 might be due merely to size or density of granules. 



This apparent qualitative difference in pigments has been attributed 

 to several causes: (1) to variations in the chromogen acted on by a 

 given enzyme, (2) to interruptions at different stages in the process of 

 oxidation of a given chromogen, (3) to specific enzymes which in each 

 case can only produce a certain result once the action on the chromo- 

 gen is begun. 



Observations of Onslow indicate that for qualitative differences, as 

 well as for the absence of pigment, enzyme and not chromogen differ- 

 ences are responsible. He could find no peroxidase in self yellows, a 

 recessive variation. There must of course have been some peroxidase 

 at some time to produce pigment at all. Perhaps the apparent absence 

 indicates a very low degree of stability in the yellow-producing enzyme. 

 Again, grays differ from blacks by a dominant factor which causes 

 yellow to appear in ticking over the back but white to appear on the 

 belly. Onslow found a tyrosinase inhibitor in the belly and compared 

 the case with that of the dominant white of the English rabbit. As 

 grays differ from self blacks by only one Mendelian factor, it would 

 seem likely that all of the changes in appearance dorsal yellow tick- 

 ing, ventral white are to be ascribed to one physiological cause. If 

 black is absent from the belly because of an enzyme inhibitor, it would 

 seem likely that black is replaced by yellow in the dorsal ticking by the 

 presence, for a certain period in the development of a hair, of the same 

 enzyme inhibitor, which, however, is in this case merely an inhibitor of 

 the black-producing reaction, not of the yellow. Reasons for which 

 yellow can appear on the back of rabbits, but not on the belly, when 

 black is inhibited will be discussed later. Thus, a recessive yellow and 

 a dominant yellow-pattern factor are both due to enzyme, not chromo- 

 gen, differences. 



