314 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
above the sea; a large portion of the drainage of the Colorado River 
and its tributaries, the Green, the Grand, and the San Juan; and the 
valley of the Rio Grande in southern Colorado. This whole area 
impresses the traveler by its apparent botanical uniformity. It is 
throughout a country naturally arid; the gray dull growth of the 
sagebrush (Artemisia) is over the higher land, varied in the lowland 
with the somewhat livelier hues of the greasewood (Sarcobatus). 
Between the scattered plants bare earth is always visible, for sage- 
brush can not be said to ‘‘cover” or ‘‘clothe”’ the ground, nor does 
the greasewood hide the whiteness of the alkali in which it grows. 
Westward and southwestward the aridity increases; through much 
of central and southern and over enormous areas of western Utah 
the land is even without sagebrush—a desert almost or quite devoid 
of vegetation. 
Through a land seemingly so uniform one would expect a uniform 
flora, but in Penstemon, the genus with which my studies have 
made me most familiar, precisely the opposite is the case. The 
high plains of the east have their distinctive species; others enter 
the valleys of northern Wyoming from the plains of Montana; 
different species occur on the ‘‘Red Desert,” the valley of the Green 
River in southwestern Wyoming; in the North or Middle Park of 
northern Colorado; in the Grand or the Gunnison valleys of western 
Colorado; in the San Juan and Dolores valleys of southwestern 
Colorado; in the western drainage of the Colorado River from the 
base of the Uintas southward to northern Arizona ; In the valley of 
the Virgin; in the Salt Lake Valley; and through the valley of the 
Snake River. No species of the lowland occurs over more than two 
or three of these regions. TIll-defined divisions within larger areas 
may be noted, as the valley of the Arkansas River from Pueblo to 
Las Animas counties, Colorado, and the valley of the Duchesne 
River in northeastern Utah. Evidently the factor controlling the 
development of species has been not diversity of valleys one from 
another, but simply the geographic isolation of each. 
Throughout this dry country permanent watercourses are few, and 
their isolation when upon different river systems would seem far 
greater than that of the upland plains. Yet the few aquatic or wet- 
land Scrophulariaceae are wide-ranging species, and occur in streams 
draining to the Atlantic and to the Pacific. Such are several species 
of Mimulus and Veronica. Doubtless the explanation of this seem- 
ing anomaly is that these species owe their distribution to the trans- 
porting agency of birds. 
The surface of this plateau was summarized as level or rolling, and 
such also are the geologic strata; but eastward, and much more so 
westward, this surface is broken by steep escarpments, buttes, and 
bluffs which mark the edge of geologic formations, many of them 
