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THE AMERICAN NATURALIST [Vol. XLVIII 



Mustela hiberniea of Ireland and Microtus orcadensis of the 

 Orkney Islands. It is at leasl suggestive, in this connection, that 

 so many of the Scottish islands yield animals differing from 

 those of the mainland. In the majority of cases, however, the 

 peculiar British mammals are closely related to those of the con- 

 tinent, and might well be of very recent origin. There is a 

 decided tendency to darker colors, such as has been noted also 

 among British moths. In spite of this tendency, however, some 

 forms are lighter than their relatives, the most conspicuous case 

 being the light-tailed British squirrel. In several cases the differ- 

 ence noted has in part to do with particular phases; thus the 

 squirrel has no dark phase, and the ermine does not turn so white 

 in winter. The British red grouse, it will be remembered, is 

 peculiar in lacking a white winter phase. Some of these differ- 

 ences may be due to the direct effect of the mild and moist 

 British climate, and would perhaps disappear in the descendants 

 of British animals taken elsewhere. The experiments on birds by 

 Beebe are very suggestive in this connection. In other cases, the 

 distinctions are such as might readily result from changes in one 

 or two "units," such as are observed in experimental breeding. 

 When we have a variable type, subject to losses and new combi- 

 nations of unit characters, it is perhaps to be expected that 

 different groups of individuals, isolated from one another, will 

 after a time produce different homozygous combinations. That 

 is to say, the result comes from a long series of "accidents," 

 which will probably not be duplicated in two different places. In 

 this way mere isolation may be an adequate cause of mollification, 

 providing always that through variation degrees of hetero- 

 zygosity have arisen. 



In the common house mouse, Mus musculus, Hagedoorn 1 has 

 isolated and figured a great number of color varieties, for nearly 

 all of which he has constructed zygotic formulae. Little 2 has 

 also described and figured a similar series of varieties, appar- 

 ently in ignorance of Hagedoorn 's paper, which he does not cite. 

 He gives zygotic formulae for thirty-two different varieties, but 

 not all of them are visibly different. Albino varieties, resulting 

 from the dropping out of a particular determiner, may be pro- 

 duced, corresponding in other respects to each of the thirty-two 

 colored forms, although they all look alike, and will only show 

 their true characters on crossing. Several of the varieties show 



iZeit. f. ind. Abst. Ver., 1912. 



2 "Experimental Studies of the Inheritance of Color in Mice," 1913. 



