ITS 



THE AMERICAN NATURALIST 



[Vol. XLVI 



family. Other striking and characteristic forms are Yucca, 

 Agave, Mesquit (Prosopis), the Creosote-bush (Larrea), the 

 Ocotilla (Fouquiera) and several other characteristic American 

 genera. This peculiar xerophytic flora is especially well devel- 

 oped in southern Arizona, and visitors to Tucson have an excel- 

 lent opportunity of seeing it in great perfection. 



In the northern part of the Great Basin region the Cacti are 

 almost entirely absent and the desolate country is covered by 

 Sagebrush, Greasewood, and many scrubby Composite, produc- 

 ing a landscape that is monotonous in the extreme. 



In the northwestern area, through Canada and the northern 

 tier of states, the true desert is practically absent, and the moun- 

 tains are covered with a heavy growth of timber mostly made up 

 of western Conifers. The other plants are largely subarctic and 

 northern types, but mingled with these are a good many western 

 genera, such as Pentstemon, Fritillaria, Gilia, and others. 



On the western slope different conditions prevail and the ame- 

 liorating effects of the west winds from the Pacific make them- 

 selves felt. This is, however, not fully seen until the last of the 

 mountain ranges is reached, the great Cordillera, which stretches 

 practically without a break from Alaska to Patagonia. It is 

 this great barrier, averaging in California some ten thousand feet 

 in height, that largely determines the character of the Pacific 

 Coast climate. Protected entirely from the sudden changes of 

 the great interior continental area, with prevailing westerly 

 winds from the vast Pacific whose temperature scarcely changes 

 from one year's end to the other, the whole western coast enjoys 

 an extraordinarily equable and temperate climate. 



On the Pacific Coast topography is a far more important fac- 

 tor than latitude in determining climate. Close to the coast the 

 climate is uniformly cool, and seldom either hot or cold. San 

 Francisco in latitude 37°, with an annual mean temperature of 

 56° F., shows a range of scarcely ten degrees between the hottest 

 and the coldest months. Sitka, 20° farther north, has a winter 

 temperature about the same as that of New York City. Natur- 

 ally the character of the climate has affected the vegetation pro- 

 foundly, and with the great differences in elevation and precipi- 

 tation found along the Pacific Coast, the variety of the vegeta- 

 tion is much greater than in Atlantic North America. 



The coast of Alaska, as far north as about sixty degrees, has 

 a winter temperature about the same as that of the middle 



