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 

 Linncea, 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 Calif ornian 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, i.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- 



