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