DISTRIBUTIVE COLOR FACTORS. 25 



phic characters, "density" is subject to considerable fluctuation. Thus we 

 may have a gradual series of yellow mice, all possessing the factor D, but 

 varying in depth of pigmentation from an extremely light cream-color to a 

 deep rich orange. Microscopic examination of the hair of these various forms 

 shows in "cream" animals a distinctly smaller amount of pigment than in the 

 hairs of the darker grades of "dilute" yellow animals. Dilute animals also 

 vary in depth of color. For example, certain dilute browns may be of a dis- 

 tinctly lighter color than others. We know from the behavior of "density" and 

 "dilution" in crosses that they are allelomorphic to each other, but we must 

 admit that they are fluctuating characters and not stable in the sense of the 

 formative qualitative factors Y, Br, and B. The differences in depth of color 

 in the case of any color variety, either intense or dilute, are more easily dis- 

 cerned in those animals which have a preponderance of light pigment, such as 

 yellows, or in those possessing reduction in amount of pigment due to some 

 modification, such as dilute dark-eyed and intense pink-eyed mice. 



In this connection it is interesting to note the occurrence of certain "coat 

 patterns" on mice, reported by Morgan (1911a). He describes, among others, 

 patterns consisting of distinct regions of lighter and darker color. He further 

 states that when the light hairs are examined under the microscope they show 

 a lessening of pigment granules in different regions, "a lessening that is a 

 characteristic feature of the so-called dilute condition." But as reduction in 

 the amount of pigment occurs in the case of the "light" densely pigmented 

 and the "pink-eyed" forms, as well as in the dilute forms, evidences of the sort 

 oft'ered by Morgan can not be considered to establish his conclusion. 



Is it not possible that these cases are those of intensely pigmented animals 

 showing on their coat somatic segregation of certain of the fluctuations that 

 are seen in dense pigmentation? Such "hghtness" would not be in any way 

 correlated with dilution, and would appear in homozygous animals as well as in 

 heterozygous ones. 



The writer suggested (19116) that such patterns as were described might 

 not be a criterion of the heterozygosity of their possessor, but due entirely to 

 the physiological condition of the animal at moulting. Morgan in reply (1911c) 

 stated that the cases he described were not "moulting" patterns, which were, 

 as he stated, " too famihar to * * * call for comment." Yet in his first 

 paper (1911a) he mentions certain "gray" animals, and figures two, which, as 

 he states, "show areas of lighter and darker color, and these may come and go 

 at each moult.' ^ 



The writer has obtained in a homozygous dilute brown animal (cf317) 

 regions of clearly lighter hair than the predominant body color of the individual. 

 These lighter hairs are apparently full-grown, and remained clearly distin- 

 guishable in this animal for nearly a year, after which the animal died. If this 

 individual was heterozygous he might, as Morgan states, be considered as giving 

 "evidence of his dual nature," but his gametic composition is "single;" so per- 

 haps the "evidence" in the case of the heterozygous animals reported by 

 Morgan is a matter of coincidence. 



