POCKET GOPHERS 
GRINNELL 351 
skull and teeth that individual variation does not bridge over the 
structural interval between it and its near relative, perpes. Hence 
systematists call it a species, rather than a subspecies, the only cri- 
terion for the latter systematic rank being intergradation or blending 
of its characters with those of another race. Operarius is restricted 
to the vicinity of the permanent springs which occur around the 
eastern side of Owens Lake in the vicinity of Keeler. There, under 
conditions of moisture obtaining very locally, there is a luxurious 
and permanent growth of salt grass, upon which almost exclusively 
this species of gopher depends for food. 
An entirely different motif, as one may say, of differentiation is 
provided on the tops of isolated mountain peaks or ranges. In 
general, the 33 forms of the genus 7homomys in California fall into 
five “ groups ” of major systematic significance. These groups prob- 
ably represent much older periods of differentiation, and the sub- 
sidiary forms have budded from each of these major stems. The 
composition by races and the distribution of each of the five groups 
is illustrated by the differential shading of the areas of four of them 
on the accompanying map. The area for the bottae group is left 
unshaded. 
It will be observed that three of the groups, which may be called 
the botiae, perpallidus, and quadratus groups, respectively, are essen- 
tially lowland groups in that they occupy territory of relatively low 
altitude; while two, the monticola and alpinus groups, are high- 
mountain dwelling. In other words, it would appear that some con- 
dition associated with altitude has had critical effect in checking the 
unlimited spread of certain types upward and of certain other types 
downward. 
Relatively thoroughgoing trapping of gophers along a typical 
section, transversely of the Sierra Nevada, in the Yosemite region 
shows a sequence of forms by groups from west to east as follows: 
Pascalis, of the San Joaquin Valley floor, and mewa, of the some- 
what higher foothill, digger pine belt, belong to the bottae group; 
awahnee, of the alpinus group, occupies middle altitudes on the west- 
ern slope; monticola, of the monticola group, occupies the whole 
upper country from about the 6,000-foot level to timber line and 
through the passes and down onto the upper eastern slopes; and at 
the eastern base of the Sierra Nevada is fisheri, of the qguadratus 
group. (See accompanying profile, showing correlation with “life 
zones.”) It would appear from this sequence of forms that tem- 
perature does, after all, in some measure constitute a factor bearing 
critically upon the successful existence of, and hence determining the 
geographical limitation of, these several species and subspecies of 
pocket gophers. It might be supposed that relatively uniform tem- 
