An analysis of external and cranial characters of pygmy mice (see Figure 12) reveals that both species are essentially largest to the north and smallest to the south. There are exceptions to this cline in both species. For example, _B. taylori analogous_ is a large subspecies; it lives allopatrically in the southern part of the range of the species. _B. musculus pallidus_ is not the largest subspecies; it lives allopatrically in the northern part of the range of the species. In west-central Mexico, where the two species are sympatric, _B. taylori_ is smaller than elsewhere and _B. musculus_ is larger than elsewhere. _B. t. analogous_ lives in the mountains of the transverse volcanic zone in central Mexico. Its large size may be a result of the cooler climate in the mountains. _B. t. allex_, the smallest subspecies, lives sympatrically with _B. musculus musculus_ at lower elevations in west-central Mexico. The small size of _allex_ could be a result of the warmer climate of the lower elevations. _B. m. pallidus_, at lower elevations in southern Oaxaca, is smaller than other subspecies of _musculus_ to the south at higher elevations. _B. m. musculus_ lives at low elevations along the coast of west-central Mexico. Unlike _B. m. pallidus_, _B. m. musculus_ is large at lower elevations. It occurs sympatrically with _B. t. allex_. It is my idea that during the period of separation, when the two species were evolving, larger subspecies evolved to the north or at higher altitudes where climates were cooler; smaller subspecies evolved to the south or at lower elevations; the two cognate species, _musculus_ and _taylori_, made contact at lower elevations where individuals of _taylori_ may have been smallest, but individuals of _musculus_ were not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids, if they occurred, were probably inviable, sterile, or ill-suited for occupancy of the habitat of either of the parental stocks. The occurrence of hybrids, therefore, would result in what geneticists call "gamete wastage," and any further divergence in the parental stock, either in external characters (size and shape of body and head), or behavior, useful in recognition of species, would be favored by natural selection (see Dobzhansky, 1951:225; and Koopman, 1950:147). The two species seem to have diverged more in external characters where they occur together than in areas where they live separately (see Figure 12). The two species could be confused if a sample of adults of _taylori_ from 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults of _musculus_ from Tehuantepec, Oaxaca (see Figure 12). No confusion in species identity would arise, however, if a sample of adults was taken from the area where the two species live together (see Figure 12). Brown and Wilson (1956:49) pointed out that where two closely related species