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THE AMERICAN NATURALIST [Vol. XLVITT 



given above (13 in 62) are near this in case of the Muske- 

 get mouse, bnt the matings are of course more promiscu- 

 ous. The case is interesting in connection with the 

 studies of Ramaley (1912) and Pearl (1914), tending to 

 show that in a mixed population the recessives may in- 

 crease so as to exceed the dominants. Although the 

 spotted mice do not, in case of this species, exceed the 

 unspotted individuals, they nevertheless are of far more 

 frequent occurrence than they are in the mainland repre- 

 sentatives of the species. This accords with the fact that 

 island-living mammals are very commonly albinistic, and 

 the cause is doubtless that the population is much more 

 inbred, so that the recessive condition of partial albinism 

 is more likely to be propagated than if successive genera- 

 tions have a wider range over which to spread. It seems 

 probable that heredity will tend to increase the propor- 

 tion of spotted mice of Muskeget, and that if this condi- 

 tion is disadvantageous, a large part of the spotted indi- 

 viduals will be killed off, yet in the course of time they 

 may become adjusted to this condition and will survive 

 in increasing proportion till the white mark becomes 

 characteristic of all the animals. Cory (1912) records 

 the capture of seven muskrats at Hayfield, Iowa, all of 

 which were uniformly marked, having a white ring around 

 the neck and the entire underparts, feet, and end of tail 

 white. I can think of three causes influencing the status 

 of such white markings. These markings may be in- 

 herited in a purely automatic way as unit characters; 

 but if thus inherited they may be (1) increased through 

 selection, natural or sexual; or (2) eliminated by the same 

 agent; or (3) they may be, at first, of no influence at all 

 in the economy of the animal and persist or not, accord- 

 ing as they are heritable. 



I have mentioned that island mammals tend to be more 

 albinistic than their mainland representatives. Other 

 cases may be mentioned, as the common squirrel (Sciurus 

 vulgaris leucurus) of Great Britain, which differs nota- 

 bly from that of the continent in having frequently a 



