VALIDITY OF THE YELLOW-NECKED MOUSE. 247 



In many ways this can be accounted for geologically. Another 

 interesting fact is that these island races in the case of Mice 

 and Voles are often of a larger size than those of the mainland. 

 Within quite recent years we find a Vole on the Orkneys related 

 more or less to the Common Field Vole, a Vole on Guernsey 

 also of this type, two Bank Voles, one from Skomer Island and 

 the other on Jersey. All these exceed the mainland forms in 

 size. As mentioned above, Mice of the sylvaticus type are fre- 

 quently found in houses, and are also often found in granaries. 

 It is therefore not difficult to see how easily they could be im- 

 ported in sacks of corn or bales of hay to islands, where, in the 

 course of time, they would soon found colonies, and possibly 

 under different climatic influences the descendants diverge from 

 the type, forming at first a variety and then a species. 



