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 México.
7. The two species are now sympatric in west central México, 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.
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.