Produced by Chris Curnow, Simon Gardner, Joseph Cooper and the Online Distributed Proofreading Team at http://www.pgdp.net UNIVERSITY OF KANSAS PUBLICATIONS MUSEUM OF NATURAL HISTORY Volume 7, No. 7, pp. 489-506, 2 figures in text July 23, 1954 Subspeciation in the Meadow Mouse, Microtus montanus, in Wyoming and Colorado BY SYDNEY ANDERSON UNIVERSITY OF KANSAS LAWRENCE 1954 UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson Volume 7, No. 7, pp. 489-506, 2 figures in text Published July 23, 1954 UNIVERSITY OF KANSAS Lawrence, Kansas PRINTED BY FERD VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1954 25-3560 Subspeciation in the Meadow Mouse, Microtus montanus, in Wyoming and Colorado BY SYDNEY ANDERSON _Microtus montanus_ reaches the eastern limits of its geographic distribution in Wyoming and Colorado. There the mountains, but in general not the lowlands, are occupied by this species. A certain minimum of moisture may be of direct importance to the mouse and certainly is indirectly important, because certain hydrophytic or mesophytic grasses used by the mouse for food, for protection from enemies, and for shelter from the elements are dependent on the moisture. Areas suitable for _Microtus montanus_ are separated by deserts that are dominated by sagebrush and other xerophytic plants or by forests or rocky exposures at higher altitudes. A relatively small percentage, probably less than ten per cent, of the total area even in the more favorable parts of the range of the species is suitable for occupancy. In these mice, as in other microtines (Elton, 1942; Piper, 1909), there are seasonal, and irregularly multiannual fluctuations in population density, which sometimes are extreme. Consequently the mice at some times seem to be absent from suitable habitats, and at some other times occur there in amazingly large numbers. Because the species is broken up into partly isolated, or at times completely isolated, colonies or local populations it may be supposed that various evolutionary forces such as selection and random genetic drift operate to foster variation. The degree to which racial distinction is attained may depend upon these forces and the time available. In _Microtus montanus_ in the eastern Rocky Mountains the degree of subspecific distinction is not great.