DISTRIBUTIVE COLOR FACTORS. 37 



pigment granules in the hair to such a degree that we should expect the dark 

 pigment in the hairs of dark-eyed " sooty," yellows, and " sables " to be entirely 

 lost in corresponding pink-eyed forms. 



We have seen that the factor which produces "yellow" mice is dominant 

 to its absence and is (in function) an inhibitor or restrictor of brown and black. 

 It is interesting to compare with this condition the conditions observed in 

 rabbits and guinea-pigs. 



In rabbits Castle (19076) has shown the occurrence of two types of "yel- 

 low" animals, both of which are hypostatic to non-yellow forms. The process 

 of restriction, if it be such, that produces the yellow type of coat in rabbits is 

 not as extensive as that which produces the clear yellow of mice. Thus all 

 yellow rabbits which lack the factor A are, as Castle has shown, sooty; that is, 

 they have considerable black pigment in the hair, which produces not a clear 

 yellow, but a dull or dingy yellow coat. Such a condition, varying or fluc- 

 tuating as it does in sooty yellows of the same gametic composition, may 

 be called due to an imperfectly recessive character; that is to say, a condition 

 which shows varying amounts of a dominant character on extracted recessive 

 individuals, may be considered as showing imperfect recessiveness. Such a 

 condition would be more explicable on the grounds that "yellow" in rabbits 

 appeared as a restriction of black and brown than as a loss of those substances. 

 Yellow in rabbits would then be, according to a nomenclature and system sug- 

 gested by Dr. Castle, due to a semi-potent restrictive factor. By semi-potent is 

 meant a positive character which must be present in all the gametes of an indi- 

 vidual, for its manifestation in the zygote. 



With such semi-potent characters may be contrasted uni-potent characters, 

 which cause visible effect in the zygote when present in one-half its gametes. 

 An example of this category of factors is seen in the factor R, in mice, which we 

 have been discussing. 



In guinea-pigs, however, we seem to have a different condition, in that 

 sooty-yellow animals are very rarely formed. Thus yellow guinea-pigs ex- 

 tracted as recessive from a black race are, in a great majority of cases, clear 

 yellows, whether they are "reds" or light "creams." The occurrence of such 

 clear yellows is more explicable on a theory demanding loss of brown and black 

 pigment from the skin and hair than from a process of restriction, for it has 

 been seen that restriction is at times an extremely imperfect process, subject to 

 frequent fluctuations in degree of activity, while loss is or should be a clear-cut 

 process, removing the possibility of frequent fluctuations by completely remov- 

 ing the materials from which such fluctuations arise. 



In studying "yellow" color varieties similarity of color should not have suf- 

 ficient value in analysis to argue a common origin of all yellow forms. Discus- 

 sion of such a question as the origin and physiological significance of different 

 types of yellow must, at least for the present, be based on generalities, since defi- 

 nite evidence is wanting. On the evidence given by such general facts, the writer 

 has above outlined a possible explanation of the observed conditions, fully realiz- 

 ing, however, that at best such an explanation is incomplete and unsatisfactory. 



