In view of the foregoing evidence it seems best to estimate the relationships and history of the various species and groups of species only as far back as the early Pleistocene (see Figure 21). Inasmuch as faunas of fossil mammals from the mid-Pleistocene contain few, if any, Recent species (see Hibbard, 1937:193), the living species of _Dipodomys_ have probably had a geologic history no longer than the period of time which has elapsed since the middle Pleistocene, or at the earliest the early Pleistocene. Of the Recent species, only _Dipodomys agilis_ is known as a fossil; it was found in the late Pleistocene tar pits of California. Under the heading "_Dipodomys near ingens_," however, Schultz (1938:206) recorded remains of kangaroo rats from the tar seeps of McKittrick in the San Joaquin Valley of California. DISPERSAL OF THE SEVERAL SPECIES If we assume the region of origin and center of dispersal of a group of animals to be the one in which the greatest numbers of the most specialized species of a given genus are found, then the northern Tableland of Mexico and the adjoining region of the United States in southeastern California and southwestern Nevada is the region of origin and the center of dispersal for the genus _Dipodomys_. _Dipodomys deserti_, _Dipodomys merriami_, _Dipodomys panamintinus_, _Dipodomys microps_, _Dipodomys phillipsii_ and _Dipodomys ordii_ are found in the region mentioned. That the aforementioned region may be the center of differentiation for this genus is further indicated by: First, the finding, in this region, of saline deposits of Cenozoic (Miocene) age, indicating aridity, which is thought to have been one of the essential stimuli for the development of the saltatorial habit in the genus _Dipodomys_; second, the recovery of the advanced heteromyids from the Avawatz and Ricardo of the Clarendonian (Pliocene) of this same region; and third, the present abundance and diversification of kangaroo rats in this same geographic region which has been more or less arid since Miocene time.