_Dipodomys ordii_, _Dipodomys phillipsii_ and _Dipodomys merriami_ occupy the southern portion of the range of the genus. Instead of being generalized at this southern part of the periphery of the range as are the kinds found on the other parts of the periphery of the range of the genus, these three southern kinds are notably specialized there in the south. The subspecies _D. o. palmeri_ which occurs in the area, is the most specialized of the species _Dipodomys ordii_; and _Dipodomys phillipsii_ and _Dipodomys merriami_ stand high in the scale of specialization with respect to the other species of the genus. The reason for this is not clear. SUBSPECIATION _Dipodomys ordii_ is, without question, a valid species if one accepts Mayr's (1942:120) definition that "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." _D. ordii_ is not known to hybridize with other species where their geographic ranges are adjacent or overlap. The first part of the definition "actually or potentially interbreeding populations" is substantiated by the 35 recognizable subspecies which can be defined as "a complex of interbreeding and completely fertile individuals which are morphologically identical or vary only within the limits of individual, ecological and seasonal variability. The typical characters of this group of individuals are genetically fixed and no other geographic race of the same species occurs within the same range" (after Rensch, 1934; from Mayr, 1942:106). Thus we find that certain populations of individuals differ from others and that in geographic areas between two of these populations, individuals (intergrades) are found which resemble those of both populations. In another instance, a population may be geographically isolated yet in its characters it may be recognizable as a subspecies without actual intergradation because of slight degrees of difference, or a group may be different from another without being geographically separated and may or may not show intergradation. Subspeciation in _Dipodomys ordii_ almost certainly has been effected, by means of mutations, under the influence of natural selection. Natural selection enhanced by geographic and ~ecologic~ isolation, probably has retained mutations of evolutionary significance, thus permitting the development of the many recognizable subspecies. In the subspecies of _Dipodomys ordii_ the color ranges from pale to dark. The difference in color is as pronounced as that between the full species _D. deserti_ and _D. heermanni_. The lightest-colored subspecies are _Dipodomys ordii celeripes_, _D. o. extractus_ and _D. o. compactus_; the darkest are _D. o. obscurus_ and _D. o. palmeri_.