672 



THE AMERICAN NATURALIST 



[Vol.L 



darkener which acts on the black pigment of a yellow 

 mouse. 



Yellow, Cuenot showed, is an allelomorph of agouti and 

 non-agouti. Black-and-tan, in turn, is not an alternative 

 form of agouti like the light-bellied gray mouse, but a 

 yellow, and hence should be allelomorphic to agouti. 

 And such it is as far as its yellow component is con- 

 cerned, but not as regards its darkener. The Fj young 

 from a cross of black-and-tan by wild agouti vary in 

 darkness about a mode midway between black-and-tan 

 and agouti. Black-and-tan we may regard as full dark- 

 ness and assign to it an arbitrary grade of 6. Wild 

 agouti we may regard as entire absence of this darkness 

 and assign to it the grade 1. ¥ 1 from a cross of these two 

 has consisted of two sorts of young. (1) The first sort 

 may be considered as the result of a union of the YD 

 gamete from black-and-tan with the agouti gamete. These 

 mice have been called sable agoutis ; since they have the 

 general pattern of sables. The bellies are yellow; the 

 darkness of the dorsal hairs is variable through the same 

 range as that of the sables, while all hairs on flanks, head 

 and parts of the back are agouti ticked. These yellow Fj 

 young graded on the sable scale show the following distri- 

 bution : 



Grade 2, 3, 4, 5, Total 



Frequency 1, 15, 7, 4, 27 



(2) The second sort of young may s be called non- 

 yellow and referred to a union of the BD gamete from 

 black-and-tan with the agouti gamete. These are simply 

 much-darkened agoutis. The belly is gray like the wild 

 agouti and the flanks are agouti ticked. The head and 

 middle of the back are covered by hairs which are black 

 for most of their length, a very narrow yellow band being 

 present near the tip, or in some cases lacking entirely in 

 an area of hairs down the center of the back. This type 

 is known as dark agouti and has also been graded accord- 

 ing to darkness on a scale parallel but not exactly equiva- 



