A secondary center of evolution has been the low, hot interior valleys and adjacent foothills of central California where _Dipodomys ingens_, _Dipodomys heermanni_, _Dipodomys venustus_, _Dipodomys agilis_, _Dipodomys elephantinus_ and _Dipodomys nitratoides_ are now found. Although there are as many species as in the principal center of origin, the amount of specialization and adaptive radiation in California is not so great. Probably during the Quaternary, when the process of mountain building was actively under way the animals that had reached central California from the parental center became isolated by the emergence of the Tehachapi Mountains. This mountain range separated the California animals from populations farther south and east. As a result, _D. nitratoides_ was differentiated from _D. merriami_, and _D. heermanni_ underwent an evolution of its own which resulted in animals having either four or five toes on the hind foot. At the same time _Dipodomys ingens_ developed there and has since been undergoing an evolution parallel to that of the large-sized species, _Dipodomys spectabilis_. The two species have paralleled each other not only in large size but to some extent in habits such as building large mounds that are kept free of vegetation and in occupying areas of rather hard clayey soil. Structurally, however, _D. ingens_ has not yet become quite so specialized as _D. spectabilis_, probably because _D. ingens_ has had less time in which to become so. A second species, if it is a full species, _Dipodomys elephantinus_, has also been isolated in central California but has not attained so high a degree of specialization as _D. ingens_. It is interesting to note that in each of the two stocks, two large-sized species have been evolved. In the parental stock the two species are _Dipodomys deserti_ and _Dipodomys spectabilis_; the former is the most specialized species in the genus. In the stock isolated in California, however, even though two large species have been formed they are still below the average in degree of specialization for the genus. As noted elsewhere in this paper, the species from these low, hot valleys, excepting _D. nitratoides_, are all closely related one to another. _Dipodomys venustus_ and _Dipodomys elephantinus_ are either closely related species or possibly only subspecies of one species, _Dipodomys agilis_. It is worthy of note that as the distance away from the center of differentiation increases, the number of species decreases. For example, in the northern Great Basin there are only two species (_Dipodomys ordii_ and _Dipodomys microps_) and farther eastward, on the eastern side of the Rocky Mountains, there is only the one species, _Dipodomys ordii_. In north-central Texas, _Dipodomys elator_, perhaps a relict species, is found occupying an area farther east than that occupied by _Dipodomys ordii_ at that latitude.