MORGAN, COAT COLORS IN MICE 101 



gray-looking black-chocolate mice often show a stratification of these 

 colors in many of the hairs. The outer end is chocolate and the base 

 black. The yellow band of the gray mouse alone is missing. In other 

 words, the ticking factor has reappeared and affects the distribution of 

 the only two colors present, namely, black and chocolate. The mice may 

 be said to be "grays without yellow." On the other hand, these mice 

 often turn into black mice, often in spots, so that some of the curious pat- 

 terns shown in Plate VII, figs. 2, 3, 4, appear temporarily at least. These 

 patterns usually last several weeks or months and then generally change 

 to a uniform coat or to a different arrangement of the color area. Con- 

 versely these same or other mice often change from a uniform color to 

 a black-chocolate spotted animal. The condition of the animal — its 

 "physiological state" — induced by change in the environment at the time 

 of moulting, is probably the cause of these differences. It seems probable 

 that the "ticking factor" is able to assert itself only when the conditions 

 are of a particular kind. 



When the hair of these mice is studied under the microscope, a very 

 complicated series of relations becomes manifest. Many of the hairs are 

 ticked as described, but to various degrees; others may be uniformly 

 black, etc. Among the mice that have originated through crosses between 

 strong and dilute races, I have often met with color changes in the coat, 

 often local, sometimes general, that are related in some way to the pres- 

 ence of the two factors, that stand for dilute and strong color ( Plate VII, 

 fig. 1). Gray mice may at times also show areas of lighter and darker 

 color, and these may come and go with each molt (Plate VIII, figs. 1, 2). 



When examined under the microscope, the light hairs show a lessening 

 of the pigment granules in different regions, a lessening that is a charac- 

 teristic feature of the so-called dilute condition. Here again we can only 

 refer these effects to the conditions, often transitory, that affect the 

 heterozygous animals. This variability raises the question as to whether 

 the ticking factor is in reality present in the "waltzer-chocolate" hybrids. 

 May not the result be expressed in terms of strong and dilute ? I am not 

 disinclined to such an interpretation, provided that black and chocolate 

 bear this relation to each other. This question will also be deferred for 

 treatment to the final conclusions, but one further question may be briefly 

 mentioned here. The supposed origin in China of the waltzing mice, the 

 peculiar shape of the head, their size and the proportions of the body 

 strongly suggest that they belong to a different race (or species?) from 

 that from which our common domesticated mice have arisen. If this can 

 be proven, it may be that they have not lost entirely the ticking factor, 

 but that black is epistatic. By the introduction of chocolate, the possi- 



