5. The two living species, _B. musculus_ and _B. taylori_, are seemingly derived from a common ancestor that in morphological structure was intermediate between _B. minimus_ and _B. musculus_. 6. The living species of pygmy mice resulted from a geographic separation, perhaps occurring in the Iowan glacial period (See De Terra, 1949:51) in the transverse volcanic zone of central Mexico. 7. The two species are now sympatric in west central Mexico, where morphological characters (size and shape of body and length of skull) differ most. Where the two species are allopatric, these same morphological characters differ least. 8. This is a documented instance of character displacement in mammals. 9. On the basis of internal morphological characters studied (auditory ossicles, hyoid apparatus, and baculum), _Baiomys_ seems to be more closely related to a South American hesperomine, perhaps _Calomys_, than to any North American cricetine. 10. Pygmy mice were more widely distributed in the past than they are at present. Part of the ancestral stock of the pygmy mice may have emigrated from North America into South America in a brief period in the Pliocene; if so, it is easy to understand why certain South American hesperomines resemble _Baiomys_. 11. The combination of morphological and behavioral characters in the living pygmy mice warrants generic status for them. If _Baiomys_ were treated as a subgenus of the genus _Peromyscus_, there would be adequate justification for including in the genus _Peromyscus_ a number of other genera, some of them occurring in South America. Such lumping of genera would reduce our understanding of the natural relationships among this group of cricetine rodents. LITERATURE CITED ALLEN, J. A. 1903. List of mammals collected by Mr. J. H. Batty in New Mexico and Durango, with descriptions of new species and subspecies. Bull. Amer. Mus. Nat. Hist., 19:587-612, November 12. ALLEN, J. A., and CHAPMAN, F. M. 1897. On a collection of mammals from Jalapa and Las Vigas, state of Veracruz. Bull. Amer. Mus. Nat. Hist., 9:197-208, June 16. AXELROD, D. I. 1950. Evolution of the desert vegetation in western North America. Carnegie Inst. Washington (Contrib. Paleo.), Publ. 590(6):215-306, 4 figs., 3 pls., 1 table, December 27. BAILEY, V. 1905. Biological survey of Texas. N. Amer. Fauna, 25:1-222, 16 pls., 24 figs., October 24. BAKER, R. H. 1951. Mammals from Tamaulipas Mexico. Univ. Kansas Publ., Mus. Nat. Hist., 5:207-218, December 15. BLAIR, W. F. 1941. Observations on the life history of _Baiomys taylori subater_. Jour. Mamm., 22:378-383, 1 fig., November 14. 1942. Systematic relationships of _Peromyscus_ and several related genera as shown by the baculum. Jour. Mamm., 23:196-204, 2 figs., 1 table, May 14.