No. 599] THE GENETIC BEHAVIOR OF MICE 673 



lent with the sable scale. Grade 1 was taken as ordinary 

 wild agouti ; grade 6 was taken as a gray-bellied black in 

 which the agouti pattern had been lost and in which the 

 darkness was equivalent roughly to that of the black-and- 

 tan. In grade 2 the extent of black in each hair is in- 

 creased, and the wide yellow band diminished; in grade 3 

 the yellow is left only in a narrow band; in grade 4 the 

 yellow ticking is lost from hairs in a streak down the 

 center of the back and in grade 5 the area of all black hairs 

 is extended to cover the whole back, ticking being limited 

 to the sides of the body. On the basis of such a scale the 

 Fj dark agoutis were distributed as follows : 



These ¥ x dark agoutis bred inter se have produced 155 

 young, of which 109 have been dark agouti and 46 black, 

 indicating that the ¥ x dark agoutis were heterozygous for 

 black. The distribution of 58 of these F 2 dark agoutis is 

 as follows : 



By using as parents the darkest of these agoutis re- 

 gardless of generation, dark agoutis were obtained, which 

 when three weeks old approximated the grade 6. lhey 

 resemble all-black mice with gray bellies except for occa- 

 sional ticked hairs on their flanks. 



It will be remembered that all darkness in these dark 

 agoutis was acquired originally from the BT) gamete ot a 

 black-and-tan mouse, since the range of darkness m u > 

 wild agouti used has never been above grade 1. Carer u 

 grading of the voung from matings among dark agouu 

 should then furnish information as to the variation in h 

 "darkener." If the "darkener" is a multiple ^ ^° 

 matings should afford it opportunity to senate oi < 

 delize. A tabulation of matings among all classes of dark 



