260 THE AMERICAN NATURALIST [Vol. LIV 



the black pigment on which they operate in the pure 

 black-and-tan and transferred to brown pigmented yel- 

 lows where their action is similar. Their independence 

 of yellow was illustrated by their action on agouti mice- 



T t 8kvd<-»Ue Xbrov 



"^W C arm 



The intermediate nature of F x in the black-and-tan X 

 brown cross and subsequent segregation of dark and un- 

 darkened forms is not as clear as in the agouti crosses, 

 for it is probable that the brown parent contributed fac- 

 tors comparable to the darkeners which have acted to 

 make both F x and F 2 darker than if the darkening factors 

 had come only from the black-and-tan parent. The 

 browns used were derived from a yellow variety known 

 as red. Both the reds and their brown recessives are 

 more intensely colored than any other yellows or browns, 

 and in fact have given evidence in later experiments of 

 differing from ordinary varieties by intensity factors 

 similar to the darkening factors of black-and-tan. 



