racterize populations of a wide geographic region and
distinguish them from _modestus_ named from southern Colorado. Also,
there seemed to be a hiatus of at least 180 miles between the ranges of
_modestus_ in northern Colorado and _modestus_ in eastern Wyoming, and
an even greater distance separating populations of _modestus_ in
northern Colorado from those in western Wyoming. _Microtus
pennsylvanicus_ has not been taken in central or southeastern Wyoming
despite extensive collecting there, which yielded numerous records of
other kinds of _Microtus_ (_M. longicaudus_, _M. montanus_, and _M.
ochrogaster_). Subsequent study revealed a pattern of geographic
variation within the range now ascribed to _modestus_ which, in my
opinion, can be described best by the recognition of three new
subspecies.

MATERIALS, METHODS, AND ACKNOWLEDGMENTS

To study geographic variation in color a method was devised as follows:
A single skin (KU 42407, from 1-1/2 miles east of Buckhorn in Weston
County, Wyoming) was selected as a representative of the paler mice and
arbitrarily given the number 2. A single skin (KU 17491, from 3 miles
east of Moran in Teton County, Wyoming) was selected as a
representative of the darker mice from the western part of Wyoming and
arbitrarily given the number 4. These mice were selected so that they
were respectively paler and darker than the estimated average of the
total variation within the populations to be studied, but the two mice
were not at the extremes of paleness and darkness. Comparisons were
based on visual inspection of the dorsal pelage as a whole. Skins were
compared with these two mice and given whole numbers from one to five.
If paler than the standard for 2, the skin was numbered one; if not
distinguishably paler or darker, it was given the number two; if
intermediate in color to the standards for 2 and 4 and not definitely
more nearly referable to one than the other, it was given the number
three; if it resembled the standard for 4, it was numbered four; and if
darker, it was given the number five. In this manner skins from a given
locality could be evaluated one by one and the results plotted,
averaged, and treated statistically. On Figure 1 the average values for
color of 32 series are mapped to show the geographic variation of
color. The following series of adults are the basis for Figure 1
(abbreviations for collections other than at the University of Kansas
are included in parentheses