No. 632] SABLE VARIETIES OE MICE 255 



(wild). The average grade of these F 2 agoutis is 2.2, 

 slightly lower than the F 1 grade while two new grades 

 have been added to the distribution, grade 1 (wild) and 

 grade 5 (very dark agouti). A few back crosses were 

 made of F x agoutis with wild, and of Fj yellows with wild. 

 These showed likewise segregation of undarkened yel- 

 lows and undarkened agoutis and lower mean grades than 

 F,— 2.5 for yellows and 1.5 for agoutis. 



The results thus far had established the presence of a 

 factor or factors for darkness, the formation of inter- 

 mediates in Fj when crossed with animals lacking it, and 

 the incomplete segregation of parental types in F 2 . They 

 had not answered our questions regarding Mendelian be- 

 havior or number of factors involved. This failure may 

 have been due to incorrect observation, to the failure of 

 the grading scale to distinguish between the types pro- 

 duced or to the actual non-appearance of the types ex- 

 pected. By constant application and regrading it was 

 believed that the error from the first two reasons was low. 

 To consider the third, one must recall what is known of 

 the origin of the black-and-tan variety. It has been for 

 some years a standard breed of the English fancy, built 

 up probably through the constant selection by breeders 

 of the points it now possesses — clear yellow belly and 

 intensely black back with a sheen not duplicated in any 

 other variety or in the hybrids of black-and-tans with 

 other varieties. Since the variety breeds quite true these 

 points must be heritable, and one can hardly expect to ex- 

 tract the pure type of black-and-tan from a cross with 

 wild without practising upon the segregates a selection 

 similar to that which perfected the variety. This involves 

 the supposition that the factors causing the darkness of 

 the black-and-tan are very numerous and extremely small 

 in individual effect. 



The F 2 darkened agoutis were chosen as the starting 

 point of a selection for darkness which lasted through 

 several generations. These agoutis were easier to grade 

 because they were non-yellow; their litters were larger 

 for the same reason, and it was also desired to know 



