346 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
roots and thickened underground stems, which tide over the dry 
season, than in the remainder of North America, where there is no 
long dry season. In other words, the ancestral pocket gophers of 
the remote past made the fortunate discovery of an oncoming type 
of food source correlated with the increasing aridity of what came to 
be a marked climatic and vegetational province. 
Restricting our attention now to Zhomomys as the genus occurs in 
California, I will revert to the fact of its well-nigh universal distri- 
bution within the State. How can the fact of this wide distribution 
be harmonized with the restriction in the animal’s mode of existence 
which we have just pointed out in some detail? Examination of the 
territory wherever pocket gophers thrive, from one end of the State 
to the other, does show most emphatically close concordance of occur- 
rence with those very, and special, conditions—of suitable food, and 
of consistency of soil which permits of digging. In other words, 
these two critical factors are widespread, and wherever they extend 
pocket gophers have gone. 
The hindrances, locally, to the spread of pocket gophers are 
comprised in, not altitude, not cold, not heat, but in discontinuity 
of ground wherein the pocket gopher can extend its burrows; in 
discontinuity of ground in which sufficient food of the kind the 
pocket gopher can use is available throughout the year; and of 
course, in impassable bodies or streams of water. In other words, 
we find operating as outright barriers to their distribution only 
ground such as lava flows which can not be penetrated by gophers, 
or ground which is too dry or too alkaline to support adequate plant 
growth for the gophers’ food throughout the year, or permanent 
streams or bodies of water which gophers can not cross. 
In this latter connection, the pocket gopher can thrive, we know, 
without ever drinking; in many parts of the State the only water it 
can get for long periods is contained in the plant tissues which it 
uses for food. On the other hand, the animal can live healthily in 
soil that is saturated with water. Yet it is forced out of the ground 
when the land is flooded, as during very heavy rains or when under 
irrigation. Not only a river itself, but the adjacent bottomland 
subject to overflow at high water, may thus be effective in limiting 
the spread of gophers locally. A gopher can swim short distances 
when forced to; but it does not take to water voluntarily. These facts 
bear on the problem of geographic differentiation of races now to be 
discussed. 
I have pointed out that, despite the pocket gopher’s extreme 
specializations in structure and habits, despite its restriction to a 
very narrow range of living conditions, yet the fact that these special 
living conditions are widespread has permitted of the very wide 
