The morphological difference between the extinct _B. minimus_ and the living _B. musculus_ is not great, and musculus seems to be the product of the _B. sawrockensis-B. minimus_ line of development. Morphological characters of the parental stock of the two living species, _musculus_ and _taylori_, may have been intermediate between those of _B. minimus_ and those of _B. musculus_. The principal part of the range of _Baiomys_ today is in Mexico, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. Therefore, populations of pygmy mice occurring to the south in central Mexico might be expected to maintain a relatively high degree of heterozygosity in morphological and behavioral characters. The occurrence of any physical or biotic barrier that would have separated this homogeneous group would be conducive to speciation. There is evidence that a barrier occurred in the Pleistocene in central Mexico sufficient to separate the supposed interbreeding, relatively homogeneous populations of pygmy mice. According to Sears (1955:529) and De Terra _et al_. (1949:51), parts of the higher regions in the Valley of Mexico, and the transverse volcanic zone in central Mexico were glaciated. On the mountain Ixtaccihuatl, De Terra (_op. cit._:52) found evidence of four marked advances of ice, from oldest to youngest, as follows: Salto, ice advanced to 3100 meters; Xopano, ice at 3200-3300 meters; Trancas, ice to 3400 meters; Ayolotepito, ice to 4350 meters. The Salto advance is correlated by De Terra (_loc. cit._) with the Iowan glacial period. The advance of ice down the mountain sides in the transverse volcanic zone was accompanied by cool moist climates or pluvial periods. Such climates probably altered habitat formerly suitable for _Baiomys_. There is no record of _Baiomys_ known to me exceeding 8000 feet in elevation, although the lower edge of the ice on Ixtaccihuatl is at approximately 15,300 feet (4600 meters, Sears, _loc. cit._). Presumably, the advance of ice down the mountains forced the pygmy mice to move to lower altitudes. Pluvial conditions possibly rendered the habitat even at lower altitudes uninhabitable for the mice, with the result that none continued to live in the transverse volcanic zone, but only north and south thereof. Long-continued separation of these northern and southern segments allowed species formation to occur. As climatic and habitat conditions became more favorable in central Mexico, the two species moved back toward each other, and eventually their geographic ranges overlapped.