This beautiful squirrel has a distinctive appearance and an uncertain specific rank. It is included here because of all the mammals discussed in this booklet it best exemplifies the effects of isolationism. There is little doubt that the ancestors of both _aberti_ and _kaibabensis_ were of one common stock. How the progenitors of the Kaibab squirrel came to be marooned on the North Rim of the Grand Canyon is of little moment. Perhaps they were already there when the Colorado plateau was young and the river was just beginning its mighty task. Possibly they emigrated later when the gorge was not as deep as it is now. At any rate, it can be assumed that they have lived on the North Rim for thousands of years, isolated from their cousins on the South Rim by only 20 miles of thin air horizontally, but a trip on foot that involves a descent of a mile through two life zones (Upper Sonoran and Lower Sonoran), a crossing of a wide and turbulent river, and an ascent to the South Rim through the same two desert zones. Surely this is an undertaking for a squirrel of the cool forests that would be too hazardous to be successful, even if attempted. The factors that have changed this squirrel’s coloration are not definitely known, but climatic conditions are probably at least partially responsible. The North Rim is approximately a thousand feet higher than the South Rim and is considerably colder. At this higher elevation much of the Kaibab squirrel’s habitat falls within the Canadian Life Zone. This in turn makes certain vegetable food available which is rare or unknown on the South Rim. Thus diet also may have something to do with its unusual appearance. At various times the Kaibab squirrel has been known as a distinct species, _Sciurus kaibabensis_; at others, it has been considered merely a subspecies of _Sciurus aberti_. The latter is its standing at this time. Regardless of specific rank, it is a form that should be stringently protected. The population is small and goes through the same fluctuations as _Sciurus aberti_. During the summer of 1946 only one individual was known in the area around Grand Canyon Lodge, where they usually were found in some numbers. At such times the heedless destruction of only a few squirrels could conceivably result in the extermination of this rare and beautiful animal. Arizona gray squirrel _Sciurus arizonensis_ (Latin: shade-tail ... of Arizona) Range: Central to southeastern Arizona and adjacent parts of western New Mexico in the Upper Sonoran and Transition Life Zones. [Illustration: Habitat map] Habitat: Associated with the native black walnuts of canyons, or often found among the pines on canyon rims.