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THE AMERICAN NATURALIST [Vol. XLIII 



If the white belly of the roof rat turns out to be a 

 character separable from the uniform gray coat, as I 

 have found to be the case with white-bellied gray mice, 

 we may expect to find in the second generation of hybrids 

 some gray rats with a gray belly. 



The chief point of theoretical interest in these results 

 concerns the origin of the black color in the black rat. 

 If it is produced by the loss of the ticking factor, as 

 Castle suggests for other black rodents, it is inexplicable 

 why the black should dominate when crossed with the 

 ticked gray coat of the roof-rat. If, on the other hand, 

 the rat has arisen by the black color spreading over and 

 obscuring the ticking beneath, then we could understand 

 how it dominates in the first generation; also why the 

 result is different from that when the black variety of 

 the Norway rat is crossed with a gray rat. It remains 

 for further work to settle this point. 



The common gray or Norway rat has, as stated, a white 

 belly— the white hairs having a black base. When such 

 a rat is bred to an albino having black (possibly gray 

 also) as a latent character, the first generation contains 

 only gray rats as Crampe and Doncaster have shown; 

 but the color of the belly varies— a point that has hitherto 

 escaped notice In some individuals the ventral hair is 

 a slate-color, in others it is nearly white. In both the 

 base of the hair is blackish. There appears to be an 

 almost, perhaps a complete, gradation between white and 

 slate-colored belly. The extremes may be accounted for 

 as follows : The albino may carry black-bearing and gray- 

 bearing germ cells. When the former unite with a gray- 

 bearing germ cell of the gray Norway rat, the addition 

 of black may cause the black base of the ventral hair 

 to extend farther out towards the tip, making the belly 

 slate-color. When a gray-bearing germ cell (of the 

 albino) meets the gray-bearing germ cell of the albino, 

 little or no change may take place in the ventral region. 



Another interpretation is also possible. The albino 

 may carry a uniform coat, i. e., one that gives gray (or 



