140 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a state of affiiirs, however, seems improbable, since, if it existed, it woukl 

 be possible to obtain from two pure gray animals offspring some of which 

 would show merely the black, the yellow, or the chocolate pigment 

 characters. No case of this sort is known to the writer. Further and 

 more extensive tests will be necessary to a full understanding of the 

 processes here taking place, but it seems for the present that pigment 

 segregations occur chiefly, if not exclusively, under the influence of cross- 

 breeding with other color varieties or with albinos. 



Among rats, the black pigment character appears to behave much as 

 in the house mouse. Black rats breed true, as observed by Crampe 

 ('85) and others. Crampe also found that black or black-white rats, if 

 they contained albinism recessive, produced only black pigmented young, 

 in addition to the Mendelian proportion of albinos. 



In the case of rabbits, however, black seems to be of a somewhat 

 different nature. Woods (: 03) finds that on interbreeding black rab- 

 bits a small proportion come gray or yellow. His figures are : 105 

 black, 9 gray, 8 yellow. This may indicate simply an impure black, or 

 the yellow element may be latent or recessive in certain individuals. 

 In the case of the albino 112, both of ivhose parents were black, it was 

 found that, with three exceptions, only black and albino young were 

 produced when it was mated with a black heterozygote. The three 

 exceptions were all gray young, and came of a black animal whose 

 parents and four grandparents were black. However, since black ani- 

 mals bred inter se may produce a small proportion of gray young, the 

 result from breeding black to black is practically the same as that 

 obtained by breeding black to albino of black parentage, as has been 

 found true of mice. 



3. T/ie chocolate type. The chocolate mice used in these experiments 

 were derived from the gray hetei'ozygotes of the cross between a black- 

 white and an albino stock. According to Bateson ( : 03''), his pupil, 

 Miss Durham, finds the hairs of chocolate mice to contain only the 

 chocolate pigment, and this observation the writer has been able to con- 

 firm by a microscopical examination of hairs from his chocolate mice. 



Of the mice of this type obtained from the gray heterozygotes above 

 mentioned, some were found to have the albino character recessive, 

 while others were pure with respect to albinism. Mice of the latter 

 sort, when interbred, produced only chocolate offspring. A similar 

 result was obtained by mating a pure chocolate animal to one having 

 albinism recessive. In this case the gametic unions (formula 5, p. 72) 

 are of two sorts : 



