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THE AMERICAN NATURALIST [Vol. LI 



yielding in all forty-eight Peromyscus, all belonging to 

 the subspecies ruhidus. Many of these were still in 

 juvenile pelage and such individuals were kept and al- 

 lowed to mature in captivity. 



A hasty comparison of the living Little Eiver animals 

 (as 1 shall call them) with those from the Samoa and 

 Eureka trapping grounds made it plain that, in respect 

 to color, they belonged with the latter group rather than 

 the former. Careful comparisons of series of dead mice 

 and of skins were made later and the bodies were sub- 

 jected to the customary measurements. Owing to numer- 

 ous deaths, however, only twenty-eight individuals were 

 available for these purposes. 



This more critical examination confirmed my earlier 

 belief that the Little Eiver mice agreed pretty closely, in 

 average color, with the redwood stock, but that they dif- 

 fered widely from those taken on the peninsula. It 

 seemed probable, however, that the mean shade was 

 slightly lighter than that of the former animals, making 

 them, to this 'extent, intermediate. 



One conclusion then seemed plain. The peninsula race, 

 exposed to certain modifying conditions, was enabled to 

 differentiate from the mainland stock, owing to the almost 

 insuperable barriers to migration. The Little Eiver 

 stock, exposed to practically the same conditions, have not 

 formed a distinguishable race, because the rate of dif- 

 ferentiation has been far exceeded by the rate of diffusion, 

 or intermingling with the great body of more typical 

 ^'ruhidus," dwelling in the redwood forests which extend 

 back from the coast. We might seem to have, therefore, 

 a particularly clear cut example of the effectiveness of 

 isolation in the formation of a local race. 



Now, T am not yet prepared to admit that 

 elusions would be crroundloss. But here, as so o. 

 pens, a further study of tlio data has shown that the' 

 lem i^ more ('orn|»1o\- t1ian was at first suspected. It is 

 true tlint tlie imro of t1io iiiore northern sand dunes have 

 not formod a distiiu-t vnoo as regards color. But it is 

 none the less certain that they differ from those of the 



