114 Bailey — Tltc American Voles of the Genus Evotomys. 



]tarts of tlie west, they fill the whole Boreal Zone of North 

 America. In [»]aces where they range into a lower zone, local 

 conditions are such as to induce-a strong mixture of boreal species 

 into the flora and fauna. This is most noticeable in the coast 

 regions. 



The material at hand shows the genus to be wonderfully uni- 

 form and the species closely interrelated. No widely divergent 

 forms appear, and several of those now distinguished by names 

 can be traced by every degree of intergradation to the forms 

 from which they have become differentiated. Others in the pro- 

 cess of separation have not gone far enough to warrant recogni- 

 tion b}^ name. By mapping the distribution of the several species 

 the more northern are seen to occupy much larger areas than the 

 southern. Thus the Arctic E. ratllas is common to the polar 

 parts of both continents, and the northern E. gapperi ranges from 

 the Atlantic coast to the Rocky Mountains of British Columbia, 

 while the more southern forms — as carornicnsis, galei, and cal'i- 

 foralcas — are restricted to limited areas. In commenting on these 

 facts nearly ten years ago, Dr. Merriam says: " In high latitudes 

 tlie climatic and general pliysiographic conditions are compara- 

 tively uniform, and boreal localities are not sul)ject to the great 

 and sometimes sudden changes with longitude that are so fre- 

 quent in temperate and tropical regions. Hence it follows that 

 the splitting up of specific types into sul)specific fornis from en- 

 vironmental agencies is much less common within the Arctic 

 circle than farther south, h^tated differently, boreal species are 

 far more stable and i)ersistent than those inhabiting warmer 

 countries. In view of this fact, it is not surprising that the cir- 

 cumpolar PJ. ralilas presents but one phase throughout its entire 

 range (specimens from Scandinavia, Siberia, and Arctic America 

 being practically indistinguishable), while its more southern 

 representatives have become variously modified."* 



Generic chnracters. — -The more important generic characters of 

 Evotomys are cranial and dental, but color alone is sufficient for 

 the recognition of the North American sj^ecieSj'and also of the 

 three Euroi)ean species — rufocanus, ghireolus, and rutilus — that 

 have come into my hands. Except the gray phase in two dichro- 

 matic species, all are characterized by a rufescent or reddish 

 brown dorsal stri})e which extends the whole length of the back. 



*MS notes on the genus Evolomi/a, written in 1888, and placed at my 

 disposal by Dr. C. Hart Merriam. 



