DISTRIBUTIVE COLOR FACTORS. 23 



ability to form either a greater or less total amount of pigment ; or (2) by the 

 restriction or loss of one or more pigments from certain portions of the pelage, 

 thus producing " coat patterns." The existence of two distinct factors of each 

 of these types has been proved by experiment. These will be presently 

 described. 



For a basis from which to calculate increase or diminution in the total 

 amount of pigmentation (the first category of modifications mentioned), it is 

 desirable to have, as a standard, some known stock of as nearly constant 

 pigmentation as possible. For this purpose none seems better than the wild 

 house-mouse, Mus musculus. There is, to be sure, variation among the wild 

 individuals of this species, but on the whole it forms as satisfactory and constant 

 a basis as can be obtained. To the factor producing the degree of pigmentation 

 observed in the common house-mouse we may give the designation D (density 

 of pigmentation). 



This factor D affects the distribution of pigment granules of the three sorts, 

 yellow, brown, and black, but there is experimental evidence that another factor 

 exists which determines the amount of brown and black granules present with- 

 out affecting the number of yellow granules in the coat. To this factor we may 

 give the designation P (plenitude or fulness of brown and black pigmentation) . 

 There is a complete series of color varieties as described by Castle and the 

 writer (1909) , in which simultaneous modifications of both these factors exist, as 

 well as two other series in each of which one only of the two factors is modified. 



Of the second category of distributive factors, those which cause local 

 restriction or loss of certain pigments, there are two clear examples in mice. 

 The first of these is seen in the case of the wild type, where the so-called "agouti" 

 pattern exists. This pattern results from the activity of a factor determining 

 the order in which pigments are laid down in the individual hair, and also 

 producing a differentiation between hairs of the dorsal and ventral surfaces. 

 This factor we may designate A (agouti) . A factor of this sort produces the 

 "ticked" coat characteristic of almost all wild rodents. In the absence of ^4., 

 animals are self-colored.* 



The second factor of this category is a restrictive factor which almost 

 excludes black and brown pigment from the hair and skin, thus leaving a pre- 

 ponderance of yellow granules in those localities and producing the so-called 

 "yellow" varieties of mice. This factor we may call R (restriction). In its 

 absence colored animals are either agouti if A is present, or self-colored if A 

 is absent.* 



From this very brief description it may be seen that the presence of certain 

 distributive factors, or modifications of them, determines what color varieties 

 of mice shall be formed. These distributive factors determine either the total 

 amount of pigmentation or the relative amounts of j^ellow, brown, and black 

 that are visible. It is, then, obvious that they are factors dealing with "quan- 

 tity" of pigment, not with "quality" of pigment, as do the formative factors 

 Y, Br, and B. Since the "distributive" factors D, P, A, and R deal with 



*This ignores spotted forms, which will be considered separately. 



