256 University of California Publications in Zoology [Vol. 21 



GENERAL CONSIDERATIONS 

 Our present information relative to the Microtus montanns group 

 points to the conclusion that ecological segregation is a factor as potent 

 as any other in limiting the dispersal of the group as a whole and at 

 the same time favoring its multiplication in particular areas. Microtus 

 montanus is often absent from areas which apparently are favorable 

 to its presence, and in certain localities, where conditions are suitable, 

 it is present in rather large colonies. This, coupled with its wide 

 distribution, would naturally result in slightly different, geographic 

 variations becoming more or less characteristic of each colony. Any 

 such variation that did arise would have a better chance of surviving, 

 thus isolated, than if there was free intercrossing with the other 

 colonies which comprise any given subspecies. Each marshy area 

 becomes a center for differentiation. What one is really dealing with 

 is a set of colonies, the individuals of each more or less variable, 

 though usually with less individual variation in such a community 

 than is observed among the individuals of the whole subspecies. The 

 differences between the colonies are so slight that one cannot treat 

 each aggregation as a separate subspecies. To recognize any one of 

 these colonies by a subspecific name would result in more confusion 

 than convenience. On the other hand, the placing of any particular 

 set of these colonies within one subspecies to which fairly definite 

 limits of variability can be given does not really indicate their true 

 importance, though for the sake of convenience it is necessary to make 

 such a disposition of the material. 



METHODS USED IN STUDYING SPECIMENS 

 The methods used for studying skins and measuring skulls 

 (Thomas, 1905, pp. 191-196) are described in a previous paper of 

 the writer on the Microtus calif ornicus group (1918, p. 2). The color 

 terms are taken from Ridgway (1912). 



ACKNOWLEDGMENTS 



All the specimens of the Microtus montanus group and the data 

 pertaining thereto in the Museum of Vertebrate Zoology were placed 

 at the writer's disposal. For the loan of important, additional material 

 the writer is indebted to the following: Dr. E. W. Nelson, chief of 

 the Bureau of Biological Survey of the United States Department of 

 Agriculture; Mr. Gerrit S. Miller, Jr., curator of the Division of 



