umbrinus_ are as
great as, or greater than, between many species of chipmunks, such as
between _E. minimus_ and _E. amoenus_, and between _E. quadrivittatus_
and _E. cinereicollis_.

Although I know of no ecological differences between _E. umbrinus_
and _E. ruficaudus_, the morphological differences, as for example,
differences in the structure of the baculum, and differences in color
pattern, lead me to maintain _E. ruficaudus_ and _E. umbrinus_ as
separate species.

The present distribution of these two species is attributable to the
uplift of the Rocky Mountains in the Pleistocene. That the uplift of
the Rocky Mountains and the erosion which produced the present-day
relief took place in Pleistocene times is supported by the evidence
found by several geologists such as Hunt and Sokoloff (1950:109-123).

The present geographic distribution of _E. umbrinus_ and _E.
quadrivittatus_ conceivably came about as follows: _E. umbrinus_-like
chipmunks were present, before the uplift of the major chains of
mountains, on isolated, low mountain ranges that were not covered with
glaciers (such as the laccolithic mountains that occur in Utah) in
Pleistocene time, while _E. quadrivittatus_-like chipmunks were
present in the central parts of New Mexico, Colorado, and southern
Wyoming. With the advent of uplift, the habitats in the central
parts of these states were changed from a plains-like habitat to
a habitat that resembled the forest habitats that exist today.
_E. umbrinus_-like chipmunks then invaded this newly formed habitat
and displaced any _E. quadrivittatus_-like chipmunks that were less
well adapted to live there. The Colorado River probably served
as a barrier that kept the _E. umbrinus_-like chipmunks and _E.
quadrivittatus_-like chipmunks separated up to this time. Invasion of
the new forest-niche by _E. umbrinus_-like chipmunks may have taken
place through the Uinta Mountains of northeastern Utah, after the
glaciers disappeared from these mountains, since the Colorado River
probably prevented any eastward migration farther south.

TABLE 1

Average and Extreme Measurements in Millimeters of Adult
_Eutamias quadrivittatus_ and _E. umbrinus_

KEY
A: Greatest length of skull
B: Zygomatic breadth
C: Cranial breadth
D: Length of nasals
E: Total length
F: Length of tail
G: Length of lower tooth-row
H: Condylo-alveolar length of mandible

====