No. 632] 



SABLE VARIETIES OF MICE 



251 



must rather be related to the formative genes for black 

 and brown pigment, so that the number of black and 

 brown pigment granules is increased in proportion to the 

 number of modifying genes present. I have examined 

 microscopically hairs from the mid-dorsal region of a 

 black-sable mouse which was intermediate in color be- 

 tween black-and-tan and yellow, from pure black-and-tan 

 and from clear yellow mice of the same age. The cause 

 of the difference among the hair colors of these three 

 forms was clearly the varying number of black pigment 

 granules. In the yellow hair the dark granules were ex- 

 tremely rare and poorly denned, appearing in many cases 

 as partially mixed with the diffuse yellow ground color. 

 In the sable hair the black granules were more numerous, 

 occurring singly in the distal third of the shaft, while in 

 the proximal two thirds the concentration was greater. 

 Here the granules were large, one granule usually ex- 

 tending across the medullary space. In some cases two 

 granules appeared side by side, and in rare instances I 

 noted rows of three across the hair. In the hairs from 

 the black-and-tan the concentration of granules was three 

 to ten times as great as in the sable hair ; the whole shaft 

 was filled with closely packed small black granules. One 

 row was rare; two was common; the rule was three or 

 four rows, while I sometimes found rows of six small 

 granules packed closely into the width of the hair. 



If, as I have stated, there exist in mice genes determin- 

 ing the quantitative increase of dark pigments, it should 

 be possible by experiment to test their existence and to 

 determine whether they are Mendelian in behavior or not 

 and whether they are simple or multiple. The data which 

 follows is submitted as a test of the above questions. 



According to the provisional hypothesis, black-and-tan 

 heing the darkest member of the series should genetically 

 contain the greatest number of modifying genes. The 

 presence of such genes should become apparent if black- 

 and-tan were crossed with a race containing the dark pig- 

 ments but lacking entirely any of the modifying genes. 

 These conditions were satisfied only by pure wild bouse 



