Buck, Tolman, and Tolman (1925) showed the balancing function of the tail in _Mus musculus_. Climbers (for example, squirrels) often possess long, well-haired tails. It is reasonable to suggest (as did Hall, 1955:134) that the long, tufted tail is an adaptation for a scansorial existence. Little observation is necessary to observe how such a tail is used in balancing. Furthermore, it is used as a prop when the mouse is climbing a vertical surface. Dalquest (1955:144) mentioned tree-climbing in _P. boylii_ from San Luis Potosi, Mexico. It may occur in _P. b. attwateri_ or in _P. b. cansensis_ also, but there is no evidence as yet to prove it. The brush mouse can seldom be induced to jump from heights of two feet or more. Rather it tends to scamper downward or to remain in place. It often swings itself over an edge, holding to it by its hind feet, and sometimes to it lightly with its tail, and reduces a short jump by almost the length of its body. Such caution seems to be an adaptation in a mouse that lives as a climber. Many animals of cavernous habitats have small eyes (see Dobzhansky, 1951:284). Some nocturnal animals (for example, owls) have large eyes. The brush mouse has large, protuberant eyes; it lives in the deep crevices and fissures of the cliffs on which it is found, but it is not strictly a cave-dwelling animal. Perhaps large eyes aid the brush mouse in performing activities in the partial darkness of a deep crevice or hole in a cliff. Brush mice experimentally placed in what appeared to be total darkness fed, built houses of cotton, and ran and climbed in the usual manner. On several occasions the captive brush mice hid surplus seeds and on other occasions hid acorns by burying them and sometimes by placing them in a small jar. The mice never carried the surplus food into their house. Black (1937:195) has claimed that the brush mouse builds a nest similar to that of the nest of the pack rat, _Neotoma floridana_. Hall (1955:134) doubts this to be the case. Dalquest (1953:144) described a nest of _P. boylii_ from San Luis Potosi as seven inches in diameter, made of leaves, and found in a hollow tree. Drake (1958:110) noted that _P. b. rowleyi_ lives in holes and crevices in rocky bluffs in Durango, Mexico. I have found this to be the case for _P. b. attwateri_, as did A. Metcalf (unpublished) for _P. b. cansensis_. Nests of sticks and leaves were taken apart by Metcalf, and all sign indicated only the presence of the pack rat. I have observed that there are no such houses on the cliffs along Shoal Creek, Cherokee County, and that no pack rats have been obtained from there (pack rats have not been reported from Cherokee County). Blair (1938) found two brush mice (_P. b. attwateri_) in the house of a pack rat in Oklahoma. Nests of the brush mice that occur in Kansas have not been found.