Great Basin Elements. 285 
Pinus Balfouriana, Picea Engelmanni, Abies grandis, form the list of trees 
which occur on the central mountain ranges, associated with which are Cra- 
nothus velutinus (7000—-9000 feet), Ribes cereum, Symphoricarpos montanus, 
Prunus demissa, Ribes irriguum, Sambucus glauca, Cornus pubescens (C. occi- 
dentalis), Crataegus rivularis, Alnıs incana. Among the herbaceous species 
there is a marked predominance of certain orders, genera or section of genera, 
mostly perennials. The number of alpine and subalpine plants is proportion- 
ately very large. 
The alpine and subalpine flora of the higher mountains of the Great Basin 
is largely a derivative one with the admixture of some purely endemic forms 
which have originated as species since the last great ice age, when, as we 
have shown in discussing the origin of the alpine flora of the Sierra Nevada 
Mountains, it was indicated that a communication probably existed across 
Nevada and Utah from the Rocky Mountains to the Sierra Nevada ranges by 
way of the numerous desert mountain ranges. Under their present condition 
of isolation, the migration of these species would be impossible, but when the 
present boreal flora was depressed nearly or quite to the base level of the 
mountains, an excellent route for migration was presented '). The boreal 
element of the mountain flora is clearly recognizable in the lists of alpine and 
subalpine plants which have been presented. The plants of the alpine . 
and of the subalpine region of the higher mountains of the Great Basin wı 
be found in ENGLER’s lists °). 
Great Basin Elements. A critic study of the pla 
has shown it to consist of a southward a of boreal plants on the high 
Mountains with an admixture of southern forms resulting from an Ten or 
migration of representatives of the Sonoran flora some of which, from = 
tesidence in the region, have undergone enough modification t0 be ee 
as distinet sub species, or even species. The arid species of the Een Mexico 
Ower mountain slopes are species which extend northward even = 2% 
through the states of Sonora and Chihuahua. This Be... f 
ment will now be considered after which atten 
the peculiarly endemic Great Basin species, which prob 
region under consideration. As examples of the 
with that of the Great Basin may be mentioned 
ability the Mexican table-land is the original ho 
Yuccas, and such tree forms, as Yucca yeculeana N Fe Bach Koma have 
(Fig. 8), indicate the high degree of development to 
reached. 
nt life of the Great Basin 
* there- 
: s f the Basin flora may, 
The following species of Yucca which constitute an ga Sehottii (Utah), Yucca bac- 
RR be looked upon as originally of southern derivation, Y!+ a N. W. Arizona), Cleistoyucca 
“ata (Utah, Arizona, N. Mexico), Yucca angustissima (S. F- rt 
nn RE 
1) See ante page 250; 256. . damerikas 1902: 92. 
2) EnGter, A.: Die pflanzengeographische Gliederung Nordam 
