4 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



variates within the same continuous series, and (6) by cross-breeding between 

 different types. There is no evidence that one of these methods has effects 

 less permanent than the other. So far, then, as these experiments go, they 

 support the Darwinian view rather than that of De Vries. 



VARIATIONS OF RATS IN COAT-COLOR AND COAT-PATTERN. 



Variations in the pigmented coat of rodents are of two principal sorts: 

 (i) Variations in the character of the pigments found in the coat, and (2) 

 variations in the distribution of those pigments. The character of the 

 pigmentation in the wild rodent is nearly always complex. Two or three 

 pigments are associated together in the same hair, but they differ in their 

 regional distribution, so that a grayish or brown "ticked" coat results, incon- 

 spicuous against many backgrounds. The coat of the house-mouse (Bate- 

 son, 103) and that of the wild guinea-pig (Castle, 105) contain three optically 

 different pigments yellow, brown, and black. These all coexist in the 

 same hair. In certain fancy varieties of these rodents, a single pigment is 

 present without the others, or the distribution of the pigments is such that 

 only one sort is conspicuous. Animals pigmented thus are known as black, 

 chocolate, and yellow (or red) varieties. If all three pigments are absent 

 from the coat and likewise from the retina of the eye, a condition known as 

 total albinism obtains. 



In rats, rabbits, and certain other rodents, black and yellow self-varieties 

 are well known, but no pure chocolate animal has been observed. 



Total albinism and the several "self" conditions of pigmentation are all 

 mutually alternative in inheritance. 



Variations in pigment distribution on the body result commonly either 

 from entire absence of pigment from certain regions of the body, in which 

 case the coat has white markings, or from the occurrence of different pig- 

 ments singly in different body-regions, in which case the body bears spots of 

 different colors. Both sorts of variation may occur simultaneously, in which 

 case the body is spotted with pigments of different sorts and with white. 



The color-varieties of rats are fewer and simpler than those of mice, rab- 

 bits, and guinea-pigs. Aside from albinos, there are only two "self" (i. e., 

 uniformly colored) varieties, namely, gray (or brown, the color of the wild 

 Mus decumanus) and black. Gray is a Mendelian dominant in relation to 

 black. 



As regards coat-pattern, there occur two conditions of partial albinism, 

 which differ from each other only in degree, but which may be obtained 

 each probably in a pure (homozygous) condition. These two patterns may be 

 called "Irish" and "hooded." Each occurs either with gray or with black 

 pigmentation. The "Irish" of fanciers, as described by Doncaster (:o6), 



