ng, are relatively paler, have a relatively longer tail and longer
hindfeet, lesser condylobasilar length, and wider braincase. Most of
these variations are of questionable significance; they may be chance
variations owing to errors in sampling.

Much of the south-central part of the state is relatively low and
relatively arid. This area includes the arid basin of the Green River
and its major tributaries and the arid Red Desert along the continental
divide in Sweetwater County. This area might have acted as a barrier to
the mice; gene flow might have been prevented between the populations of
the western part of the state and those farther east in the Medicine Bow
Mountains and Laramie Mountains. Nevertheless geographic variations of
subspecific worth have not taken place. The barrier has either not been
of as long duration, or has not been so complete and effective, as the
other barriers in the state, namely the Absaroka Range, the Big Horn
Basin, the Shoshone Basin, and the valley of the North Platte River.
These four barriers presumably have led to the differentiation of the
two subspecies that are newly named beyond. Each of the two areas which
is set apart by these barriers and in which one of the newly named
subspecies has evolved is small; therefore there is a lesser amount of
suitable habitat available for each of the newly named mice than there
is for _M. m. nanus_. It is conceivable, therefore, that in periods of
adverse conditions in each of the small areas the size of the effective
breeding population may have been so small that random genetic drift
could have operated effectively, or that selection was more critical
than in a larger, more stable population. It is difficult to test these
possibilities because the selective value of the taxonomic characters is
unknown. The observed pattern of variation and facts of distribution
are, however, not contradictory to the above possibilities.

_Specimens examined._--Total, 993, distributed as follows: All
specimens unless otherwise indicated are in the University of
Kansas Museum of Natural History. Specimens in other museums are
labeled as follows: Chicago Natural History Museum (Chi);
University of Michigan, Museum of Zoology (Mich); American Museum
of Natural History (AMNH); United States National Museum (USNM);
Biological Surveys Collection (USBS). Localities that are not
represented in Fig. 1 because overlapping or crowd