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THE AMERICAN NATURALIST [Vol. LIV 



quisition by the dark agoutis of the peculiar and puzzling 

 darkness from the black-and-tan race. 1 



Up to his point the data has involved crosses of dark- 

 ness with its absence — and crosses of dark animals of 

 various grades inter se. Multiple factors appear to be 

 involved and yet no evidence of Mendelian behavior has 

 been adduced except the segregation of lack of darkness 

 without accompanying segregation of darkness. 



Many additional crosses have been made in which the 

 darkeners from the sable series have been transferred 

 into other varieties with results similar to those outlined 

 above. The most extensive of these secondary exper- 

 iments involved crosses of black-and-tan with brown 

 (chocolate) mice to test the black-and-tan (yellow) gamete 

 and to separate if possible the darkening modifiers (see 

 Table III and Fig. 3). This cross brought about the 

 union in the first generation of gametes with full dark- 

 ness (black-and-tan) with a uniform set of gametes from 

 the brown race all of which supposedly lacked the dark- 

 ener. The first generation from this cross showed, as 

 was expected, the dominance of black, all young being 

 black pigmented. Approximately one half were self black 

 which on being tested proved to be heterozygous for 

 brown. The other half were a lightened black-and-tan 

 (Table III, cross 23) with some variation but none were 

 as black as pure black-and-tan. The number of dark pig- 

 ment granules in the dorsal hairs was reduced and the 

 yellow pigment substratum was thereby allowed to show 

 at bases and tips of hairs especially in the older animals. 

 From these light black-and-tans was bred an F 2 genera- 

 tion (cross 26) consisting of mice with black pigment and 



which he gave the name "now -r:iy," whirl) were noted frst in the off- 

 spring of a pair of cinnamon mi. v. Th.y ••looked like chocolates, but 



grays bred to black (heterozygous) gave some chocolates, black, new grays, 



quite accurately to cinnamon and agouti mice which I have raised from 

 crosses with one of the sable series and I have little doubt that the dark- 

 ness of Morgan's new grays was derived originally from some mouse carry- 

 ing the "darkener." 



