The probable geographic range of _P. boylii_ is based on trapping data (see Fig. 1). The brush mouse is confined to systems of wooded cliffs in Kansas. The two subspecies seem to be separated by more than 80 miles of grasslands. Blair (1959) has postulated that in the northeastern part of its range _P. b. attwateri_ is represented by disjunct, relict populations formed by diminishing montane or cool, moist environmental conditions. He has implied that the critical climatic change occurred during post-Wisconsin times, and that the isolation of these populations occurred so recently that no morphological differentiation has resulted in them. Inasmuch as the species is widely distributed in Mexico, the southwestern United States, and in California, and has been recorded from the Pleistocene of California (Hay, 1927:323), it is reasonable to suppose that the species immigrated into Kansas from the southwest and that the immigration was in a generally northward or eastward direction. If long tail and large eyes are specializations for a scansorial mode of life (discussed below), then _P. b. cansensis_ must be considered more primitive than _P. b. attwateri_ for the eyes are less protuberant and the tail is shorter in _P. b. cansensis_ than in the latter. I suggest that _P. b. cansensis_ occurred in what is now known as Kansas before _P. b. attwateri_ entered this area by way of the Ozark Mountains. The occurrence of a mouse of "the _truei_ or _boylei_ group" (Hibbard, 1955:213) in southwestern Kansas in the Jinglebob interglacial fauna of the Pleistocene adds little to support the thesis outlined above, but is not inconsistent with the thesis. Incidentally, the geographic distribution of _P. boylii_ may differ somewhat from that shown by Blair (1959:fig. 5); whereas he has mapped the distribution of _P. boylii_ to show disjunctivity in _P. b. attwateri_ and homogeneity in the distribution of other subspecies of the brush mouse to the westward and southward, disjunctivity actually occurs frequently also in the western and southern subspecies. _Ecology_