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THE AMERICAN NATURALIST [Vol. LIV 



the mice used in my experiments. Miss Durham de- 

 scribed "black, blue, chocolate and silver fawn mice which 

 differ only from the ordinary forms by having yellow 

 bellies," but they subsequently always moulted into ordi- 

 nary sables which have a "dark black or brown streak 

 down the middle of the dorsal region while the rest of the 

 mouse is yellow. ' ' Black-and-tan however does not moult 

 to ordinary sable. Even at the age of twenty months 

 black-and-tan is entirely black except for the yellow belly 

 and yellow ticking on flanks and muzzle. Since these 

 mice were different from ordinary sables, they were 

 given a name to indicate the difference. Moreover when 

 they were crossed with various non-black-and-tan varie- 

 ties there were produced in the second generation mice 

 resembling the black-and-tan parent, others intermediate 

 between black-and-tan and yellow, and conforming to 

 Miss Durham's description of sable. The latter were 

 called black sable or brown sable to indicate the color of 

 their non-yellow pigment. I am in sympathy with On- 

 slow's desire to prevent a duplication of terms but I be- 

 lieve that the names employed are required by the pres- 

 ence of tvpes which differ both genetically and somat- 

 ically. 



Sooty yellows may likewise be included in the sable 

 series since these mice appear when black-and-tans or 

 sables are crossed with non-sable varieties. Sooties can 

 not be distinguished simply as yellows which are hetero- 

 zygous for black, for yellow mice which carry black may 

 show no trace of sootiness. Factors additional to the 

 black gamete are involved in the production of sooties 

 and these factors are present not only in the sooties but 

 in the blacks which they produce. Little (1916) used 

 blacks derived from sooties in crosses with wild agouti 

 and obtained agoutis which were much darker dorsally 

 than any wild agoutis. I have obtained similar results 

 from such crosses. Certain blacks with which yellows 

 have been crossed to produce sooties evidently carry 

 some of the factors for darkness which appear in greater 

 concentration in sables and black-and-tan as well as in 



