608 



THE AMERICAN NATURALIST 



[Vol. L 



brown, equality being expected. The yellows also may be 

 divided into two groups, in one of which the eyes and fur 

 contain black pigment, while in the other the correspond- 

 ing parts contain brown pigment. In both of these yellow 

 groups the amount of black or brown pigment varied. 

 Again translating Mr. Whittier's descriptions into terms 

 of the numerical scale, we have the following distribution : 



Grade... 1, 2, 3, 4, 5, 6, Total 



(1) The black series— Frequency. .. 0, 4, 8, 3, 8, 2 (?), 25 



(2) The brown series— Frequency. . 13, 1, 4, 16, 1, 2, 37 



It was frequently found to be impossible to determine 

 by inspection alone whether a particular yellow animal 

 belonged to the black or the brown series, because yellow 

 fur containing a small amount of black pigment closely 

 resembles that which contains a considerable amount of 

 brown pigment. Consequently these back-cross young 

 (produced by an F ± sable mated with brown) had to be 

 tested themselves, either by inter se matings or by cross- 

 ing with browns. The classification of the back-cross 

 young in the above tables is based partially on breeding 

 tests and in the cases where these were lacking classifica- 

 tion is based on inspection at the age of three weeks. It 

 is uncertain whether any individuals were obtained from 

 the Fj sable X brown cross which showed the full inten- 

 sity of pure-bred black-and-tans (grade 6), although two 

 animals are recorded in the notes as black-and-tan with- 

 out qualifying terms. 



As a result of back-crossing with browns the Fj sables 

 (out of black-and-tan X red) and testing the young pro- 

 duced by crossing them with browns, two graded series of 

 yellow mice may be recognized as follows. 



