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THE AMERICAN NATURALIST [Vol. XLIII 



Both hypotheses account for the numerical outcome. Some 

 other criterion must decide between them. The criterion is 

 found in the recent work of Miss Florence M. Durham who has 

 pointed out that chocolate can not be dilute black, since a dilute 

 form of black is known, which is quite different from chocolate. 2 

 Let us examine her results. 



Black and chocolate are found either in a dense condition 

 when the mouse is called black or chocolate, or in a dilute state, 

 i. e., with the pigment granules scattered. Dilute black is blue 

 and dilute chocolate is silver fawn in the " fancy." Black 

 dominates blue and chocolate dominates silver fawn on the older 

 terminology. But it is known that black mice often contain 

 chocolate whose presence is obscured by the darker color, black. 

 This relation Bateson calls epistatic. In the same sense black is 

 epistatic to blue; and chocolate is epistatic to silver fawn. 



Black crossed with blue gives black only. Such heterozygous 

 blacks inbred give 3 blacks to 1 blue. Similarly chocolate 

 crossed with silver fawn gives chocolate. These inbred give 3 

 chocolate to 1 silver fawn. 



The most interesting result reported by Durham, is seen when 

 black, i, e., blue, is crossed with chocolate. The result is black, 

 because the chocolate supplies the strengthening factor and makes 

 the dilute black dense black which is epistatic to chocolate. 3 In 



'There may be two quite distinct meanings however attached to 

 ment. Durham means that the black pigment granules are sparse in the 



