1. There are thirty-five recognizable subspecies of the species
_Dipodomys ordii_ of which four are herein, for the first time,
recognized by name. Three subspecies, _D. o. oklahomae_, _D. o.
sennetti_ and _D. o. compactus_, previously were regarded by most
authors as full species.
2. The species _Dipodomys ordii_ is divisible into six complexes,
or groups, of subspecies on both geographic and morphological
bases.
3. _Dipodomys ordii_ is the most generalized Recent species of the
genus.
4. The extremes of geographic variation in _Dipodomys ordii_ are
greater than in any other species of the genus.
5. Color, at least in _Dipodomys ordii_, does not seem to be
correlated with amount of moisture but rather with color of soil.
6. Clinal variation, from north to south, is shown in the
decreasing length of the nasals. This decrease in length of nasals
and resultant decrease in size of the nasal chamber may be
correlated with the decrease of humidity of the environment.
7. Subspeciation has been enhanced by the late Quaternary mountain
building which was prevalent over the western United States.
8. In general, the most primitive kinds of _Dipodomys ordii_ occur
at the periphery of the range of the species.
9. Natural selection plus geographical and ecological isolation
has undoubtedly been operative in speciation and in subspeciation.
10. _Dipodomys deserti_ is found to be the most specialized
species in the genus.
11. Six, rather than nine, groups of species are recognized, on
the basis of morphology, as comprising the genus _Dipodomys_.
12. The center of dispersal for the genus _Dipodomys_ appears to
have been in the southwestern United States and the adjoining part
of Mexico. A secondary center of differentiation is apparent in
the low, hot valleys of central California.
13. Parallel development of species is noted between _Dipodomys_
of the parental center and _Dipodomys_ of the isolated valleys of
central California; _Dipodomys_ in the California center is the
less specialized.
14. The trend of the dipodomyines, as indicated by the fossil
record, has been toward a saltatorial specialization with
consequent morphological changes.
15. The morphological change in the direction of saltatorial
specialization is clearly evident in the compacting and aligning
of the viscera as well as in the lengthening of the distal
segments of the hind legs and the tail, tufting of the tail,
enlargement of the auditory bullae, shortening of the neck and
fusion of the cervical vertebrae for stability and other
modifications of the skeleton.