138 
JOURNAL OF MAMMALOGY 
The three most important conditions necessary for the successful 
life of any species of animal, as pointed out by Grinnell in his Colorado 
River Report, 1914, are adequate food supply, safe breeding dens and 
places for temporary refuge in extremity. A sufficient supply of the 
proper sort of food is the first requirement. Unless safe breeding dens 
are availabe the future generations of a species are placed in jeopardy. 
To a kangaroo rat that is being closely pursued by a hungry kit fox 
the nearness of a protecting burrow may be a question of life or death. 
With these esssentials of rodent existence in mind, let us note the effect 
of the recent radical changes in Imperial Valley on the various species 
involved. 
Field work carried on by the writer in March and April, 1921, in 
the Imperial region on both sides of the International Boundary, 
showed that the rodent population of that region could be readily divided 
into two classes. The first class consists of rodents that tend to be 
driven out by reclamation. Belonging to this category are the round- 
tailed ground squirrel, Colorado Valley woodrat, kangaroo rat and 
pocket mouse. 
The other class is composed of species that have benefited by, and 
increased as a result of, the reclamation of desert lands. The harvest 
mouse, cotton rat, beaver, muskrat and gopher belong to this second 
class. The species that are driven out by reclamation are those that 
are most abundant in the sand dune areas. The true desert rodents 
are able to live without water other than that obtained from the food 
that they eat; as a matter of fact, irrigation drives them out. With 
the one exception of the gopher, the rodents that have increased under 
reclamation are all water or stream-side dwellers. 
Reclamation in Lower California has not been carried on as rapidly 
or as extensively as it has on the American side, and as a result large 
areas still remain where wholly natural conditions prevail. An excellent 
chance was thus afforded for observing original conditions and for seeing 
reclamation forces in actual operation. The immediate results of 
different stages of reclamation and irrigation were also observed. In 
one instance, an area one mile long and one-half mile wide lying just 
south of the border and eleven miles east of Mexicali was studied in 
detail. One half of this selected area was covered by original mesquite 
crowned sand dunes surrounded by patches of arrow-weed and quail 
brush. The rest of the tract had recently been or was being cleared, 
leveled, and planted to cotton or alfalfa. The ground squirrels, pocket 
mice, kangaroo rats and woodrats that had formerly inhabited the 
