356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
more or less salt or alkaline, have no outlets, and derive their sup- 
port chiefly from springs or short mountain streams. 
Heat and lack of moisture, which are the dominant features of the 
climate, increase markedly toward the south, and reach their ex- 
treme in parts of western Arizona and southeastern California, 
where a summer temperature of 100° to 120° in the shade is of almost 
daily occurrence, where the average aridity of the atmosphere is 
more than three times as great as in the eastern United States, and 
the annual rainfall, confined chiefly to the winter months, is only 
3 to 9 inches. 
Little of all this dreary and forbidding region lacks vegetation 
entirely, for only the mirage-haunted alkali plains and the barest 
rocky slopes of the seared desert ranges are shunned by the hardy 
desert shrubs. The bottoms of the valleys, the sloping or nearly level 
mesas, the sides of the hills and mountains are all clothed with a 
growth, sometimes scanty, sometimes wonderfully varied, of mes- 
quite, sagebrush, greasewood, cactuses, yuccas, or other similarly 
characteristic forms. The only trees worthy the name, except on the 
mountains, which rise partly beyond the arid influence of the valleys 
and support in places forests of pines, are the cottonwoods, and 
these are found only at springs or along streams. 
An environment apparently more uninviting to every form of 
animal life it would be hard io find; for the bare rocks, the reaches 
of sand, the pebble-strewn mesas, and the clay fiats incrusted with 
salt and alkali offer seemingly no protection or concealment; while 
the fiery heat, the desiccating air, and above all the lack of water 
appear hostile alike to all kinds of living creatures. Yet life there 
is, and relatively much; lizards of brilliant hues scamper about over 
the sand or lie on the rocks to bask in the sun; coyotes roam the 
plains by day and bark from the hills at night; rock squirrels and 
wood rats inhabit the cliffs; the little pocket mice and the singular 
kangaroo rats live in holes on the gravelly slopes or among the sand 
dunes; and many birds of many kinds are conspicuous almost every- 
where, as well in the summer as when during the seasons of migra- 
tion their numbers in species and individuals are greatly augmented. 
Only the bare and barren expanses of salt and allxali in the valleys 
are uninhabited, and even here at times some bird of strong flight 
may be seen soaring on lofty pinion above the inhospital region. 
The lakes of the region form the great attraction for most of the 
water birds and those that are usually termed waders, and furnish, 
too, along their sometimes marshy shores, a home for various other 
species. 
The American avocet, in its becoming attire of black, white, and 
cinnamon, is a conspicuous and characteristic figure about these 
