348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
from one another in what may be called differentiation provinces. 
It is our problem to inquire as to what factors among the present 
or recently past conditions have resulted in this isolation and 
consequent differentiation of all these various stocks. 
As will be observed from the map, the most conspicuous gopherless 
areas in California lie in the southeastern desert territory, chiefly on 
the Mohave Desert. Extended explorations of that arid territory 
have been made, with the special object of determining the kinds 
and numbers of rodents and other mammals present. Almost every 
square mile of those deserts, save on such evaporation floors as those 
of “ Searles Lake” and Panamint and Death Valleys (where there is 
a heavy deposit of saline substances, and no chance of plant growth), 
supports a large population of seed-gathering rodents—kangaroo 
rats, pocket mice, and ground squirrels of certain species. Through- 
out all the deserts there are, at times, heavy rains, though they may 
be at intervals of as long as three years; and such rains are followed 
by luxuriant growths of various herbs. These go to seed and thus 
give origin to a nutritious type of food, scattered by the winds 
throughout the drifting sands, to be sought out throughout the long 
dry intervals by the spermophilous mammals just named. But for 
the pocket gopher, specialized for gathering, masticating, and digest- 
ing roots and stems, and not for seed-gathering, the food resources 
of the desert are, over most of its extent, inadequate. Only here and 
there, on mountain tops, where rainfall is more copious and of more 
regular occurrence than in the surrounding territory, and about 
permanent springs, is there produced the proper type of vegetation 
for gopher consumption, in permanent supply. 
As a result of these special conditions of food supply, the general 
distribution of gophers on the deserts is conspicuously discontinuous ; 
the animals exist only in colonies here and there, because surrounded 
by unoccupiable desert; and such colonies are often far isolated 
from one another. The feeble powers of locomotion of the pocket 
gophers mean that they are unable to cross the barren intervals, and 
they are subjected to the same sort of factor, in evolutionary process, 
as land animals sequestered on islands in the sea. As happens under 
this circumstance the world over, we find that greater or less degree 
of inherent, subspecific or, indeed, specific, sets of differences char- 
acterize the more or less isolated stocks. 
As an example of races of pocket gophers which evidently owe 
their origin to the isolation afforded by discontinuity of food sup- 
ply, we may cite the form scapterus on the Panamint Mountains, 
which range rises high enough above the general base level of the 
surrounding desert to enjoy a fairly regular rainfall with a conse- 
quent copious growth of biennial or perennial herbs. 
