18 THE INHERITANCE OF COLOR IN MICE. 



group of enzymes is responsible for the production of each pigment we must 

 suppose that in mice at least three such enzymes or groups of enzymes exist. 

 To determine which of these conditions occurs in mice is not a problem for the 

 biologist, but for the chemist. The biologist must confine his attention to 

 determining the number of distinct agencies at work in pigment formation 

 irrespective of their chemical nature. These agencies, because of their physio- 

 logical behavior, the biologist chooses to call "factors," and attempts to learn 

 what he can about their functions in the evolution of color varieties. 



FORMATIVE AND DISTRIBUTIVE COLOR FACTORS. 



As has already been stated, three pigments (yellow, brown, and black) are 

 obser^^able in mice. Yellow has been found, by Riddle, to be the lowest in the 

 scale of mammalian pigments. Nevertheless, in mice no case has been recorded 

 where yellow alone is present. The eye always has, in addition, brown or 

 black pigment granules, while in many cases both are present. The same fact 

 holds good for the coat, so far as observation goes. 



These three pigments — ^yellow, brown, and black — are, however, discon- 

 tinuous stages. If they are not the products of distinct enzymes they are the 

 result of distinct grades of activity of a single enzyme. Which of these conditions 

 holds makes no difference at present, so we may, for purposes of discussion, 

 refer their causation to three distinct factors, Y for yellow, Br for brown, and 

 B for black. 



Certain animals possess yellow and brown pigment to the exclusion of 

 black. In such animals, with no evidence of variation in the total amount of 

 pigmentation, there occur varieties in which the proportion of brown to yel- 

 low in the coat varies greatly. As the brown increases in amount the yellow 

 decreases. It is fair, then, to assume that something is acting to convert the 

 lower-grade pigment (yellow) into the higher grade (brown) . This, we suppose, 

 is the factor Br already mentioned. 



The activity of such a factor may, however, be affected by other factors 

 which determine the amount of brown formed and its location. Experiment 

 shows that these assumed factors governing the development of brown are 

 independent of the brown-forming factor Br, for they can be transferred without 

 visible alteration to animals with black, and vice versa. Such factors we may 

 call distributive. 



Black is the highest grade of mammalian pigment and doubtless, as Riddle 

 states, is a higher oxidation stage of brown; for brown pigment is often, if not 

 always, seen in black animals, but never the reverse. We may, therefore, 

 assume a distinct agency for the conversion of brown into black and call this 

 factor B. If by a given cross this factor B is added to a certain distributional 

 stage of brown, the corresponding distributional stage of black is obtained, 

 showing that the factor B is independent of the distributive factors. 



Such factors as Y, Br, and B, indicating as they do distinct qualitative 

 steps in pigment formation, we may call formative factors, in somewhat the 

 same way as the factors controlling the distribution and amount of pigment 

 have been considered distributive factors. 



