occur together, characters (morphological, ecological, physiological, or behavioral) of each species are easily distinguished. However, where the two species are allopatric, the two closely related species so resemble one another that the species are not easily distinguished. This phenomenon has been called "character displacement" by Brown and Wilson (_loc. cit._). In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of both _B. musculus_ and _B. taylori_ in a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained one _taylori_ and 33 _musculus_; another sample from La Resolana, Jalisco, had a ratio of 25 _taylori_ to 6 _musculus_. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. Possibly this disparity in numbers is a result of interspecific competition. Hooper (_op. cit._:90) pointed out that where the range of _B. musculus_ (typical of arid tropical lowlands) meets that of _B. taylori_ (typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range of _musculus_ extending into the highlands and parts of the range of _taylori_ extending into the lowlands. In the lowlands, _musculus_ may be better adapted to environmental conditions and, therefore, more successful in competition with _taylori_ for available habitat. The reverse situation may exist in the highlands. Also, the fact that _musculus_ is more of a diurnal animal than is _taylori_ may account for the difference in numbers of individuals of the two species taken in trap lines. Many collectors set their traps in late afternoon or evening and retrieve them in early morning. Such a schedule might not yield many _musculus_. If interspecific competition does occur in the area where the two species occur, any change in habits or microhabitat by either species that would reduce this competition would be favored by natural selection (see Mayr, 1949:518; Lack, 1944:262-263; and Brown, 1958:154-155). Brown (_op. cit._:154), as I understand him, pointed out (taking account of Gause's principle) that when two species having similar ecological valences move into the same niche in the same locality, one of three things must eventually happen: (_a_) the two species occupy different geographic ranges; (_b_) they compete and one is eventually eliminated; (_c_) the two species, because of differentiation or specialization, exploit different aspects of the niche. In _Baiomys_, (_c_) seems to apply. Natural selection probably would favor a continuation of diurnal activity in _musculus_ and nocturnal activity in _taylori_, thereby preventing frequent meeting of the two species. AREAS OF PRESENT DIFFERENTIATION