340 Vegetation in Nevada and Arizona. [June, 
only of a single species (Cerocarpus ledifolius), at others of a 
genus (Pinus), but more frequently of a variety of genera or 
even of families. In the northern portion of Nevada a single 
genus of plants is often found occupying a large extent of rolling 
country, when, as we reach the limit of distribution, a few yards 
farther on will find us in another and a distinct group. At some 
points, again, the line of demarcation between the desert flora 
(Artemisia tridentata) and that of the foot-hills (Sarcobatus ver- 
miculatus) is so sharply drawn as to be visible from any elevated 
point of observation, this being apparent on account of the differ- 
ence of the color of foliage. 
Again, we find areas over which the vegetation is as yet an 
indiscriminate mingling of genera and families, but over which 
the Composite appear to have the ** balance of power.” 
This peculiar distribution is apparent over all that portion of 
Nevada and Arizona before named, but as we go southward we 
find each genus or family of plants or trees gradually rising in 
élevation, and if we do not discover all the preceding individuals, 
we find representatives belonging to the same genera or families 
replacing them at those altitudes, which in turn give rise to other 
forms, to other types, or a- new belt, occupying that space caused 
by the elevation of the belt above. This succession is visible in 
following the elevated mountain regions of the American conti- 
nent, but is modified in the vicinity of seas, as at Panama. As 
the vegetation is thickest and most luxuriant in the tropics, it 
forms a covering which decreases in quantity and growth to 
wards either pole. The equatorial zone is the home of ferns 
and palms, which gradually lose predominance in advancing to- 
wards the temperate zones as they do in ascending the moun- 
tains situated in the equatorial regions. - That'at great altitudes 
the belts of vegetation and timber are again met with, which as 
cend from both the northern and southern parallels of latitude, 18 
verified by Von Humboldt,! who says, ‘* The great elevation at- 
tained in several tropical countries, not only by single mountains 
but even extensive districts, enables the inhabitants of the torrid 
zone to behold also those vegetable forms which, demanding @ 
cooler temperature, would seem to belong to other zones. Ele- 
vation above the level of the sea gives this cooler temperature 
even in the hottest parts of the earth, and cypresses, pines, oaks, 
berberries, and alders nearly allied to our own cover the mount- 
1 Aspects of Nature, ete., Alex. von Humboldt. ‘Trans. by Mrs. Sabine, Philadel- 
phia, 1849, pages 245, 246 
