GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 239 
its flora. The longitudinal ranges of mountains travers- 
ing the state form part of a continuous series of lofty 
mountains extending from Alaska to Mexico, and serve 
as a highway for the migration of many northern types, 
which are thus enabled to extend their range far beyond 
their usual southern limit. Thus in the mountains of 
northern California are found northern genera like 
Linnea, and violets and trilliums like those of the north- 
eastern states. With these occur also some northern 
Asiatic genera, like Fritillaria, which are entirely absent 
from eastern North America. In the northern part of 
the state, and in Oregon and Washington, many of the 
plants common to the sub-polar zone occur in the val- 
leys, but further south they ascend the mountains and 
finally disappear entirely. 
The most remarkable feature of the Californian flora 
is the extraordinary number and immense size of the 
coniferous trees. Evidently the conditions upon the 
mountain slopes of the Pacific -coast are especially 
favorable for the growth of these ancient trees. A very 
even temperature, with heavy precipitation, has appar- 
ently been the reason for the survival of the last of the 
great Sequoias, as well as other Conifers not found 
elsewhere. 
Southward, the rainfall in California diminishes very 
rapidly, and the conditions are unsuited to plants re- 
quiring much moisture. Consequently we find the 
northern plants giving way to immigrants from the dry 
regions south, ¢.e. from Mexico and South America. To 
the European or eastern American, the aspect of the 
great valleys of central and southern California is very 
unfamiliar, and entirely different from that of the moun- 
