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THE AMERICAN NATURALIST [Vol. XLYIII 



develop pigment at all in certain places, so that a white 

 or unpigmented area is produced. 



It is not rare among mammals to find that one or more 

 of the characteristic sorts of pigments are not produced 

 in certain individuals and probably the factor or factors 

 for these are lost altogether from the somatic and sex 

 cells alike. Such variations may be perpetuated through 

 inbreeding and so no doubt have arisen sundry domestic 

 color varieties of animals and plants. For example, in 

 the course of experiments with color varieties of the 

 house mouse (carried on some years since with Professor 

 W. E. Castle) we found that the chocolate-colored mice 

 which we bred as extracted recessives from black mice, 

 contained only chocolate pigment in their hair, whereas 

 in the black parents both black and chocolate pigments 

 were present, but the black masked a chocolate pigment. 

 Moreover, the chocolate mice always bred true to that 

 color, but if bred back to the black parents, gave black 

 youn.u- or both Mack and chocolate in Mendelian propor- 

 tions, according to the nature of the matings. The inter- 

 esting point here is that the chocolate mouse once pro- 

 duced, through the loss of its black-and-gray-pigment- 

 potentiality, can transmit no other pigment character but 

 the chocolate. What causes the occasional production of 

 an individual in which one or more of the characteristic 

 sorts of pigment is absolutely lacking is still unexplained. 

 Nevertheless it is of frequent occurrence not only among 

 domesticated species, in which the natural conditions of 

 life are so greatly modified, but also in species in a state 

 of nature. 



A skunk normally marked, but chocolate instead of 

 black, a raccoon likewise of normal pattern but the pig- 

 mented areas yellow, are merely examples of the drop- 

 ping out of the factor for black pigment from the normal 

 combination of the two. Such specimens are of occa- 

 sional occurrence, and examples are in the museum of the 

 Boston Society of Natural History. Similarly are pro- 

 duced red woodchucks or muskrats, or wholly yellow field 



