34 TREE ANCESTORS 



evergreen conifers. Along the coast to the southward the rainfall 

 decreases progressively until in southern CaHfornia land tempera- 

 tures so far exceed ocean temperatures that more or less of the 

 year is without precipitation. The north and south trend of the 

 mountain ranges govern the rainfall of the interior, consequently a 

 large part of the country except the higher chains are entirely or 

 nearly rainless. Woodlands are consequently confined to the flanks 

 and summits of the mountains. The resulting effects upon the 

 vegetation are succinctly summarized in Shreve's (op. cit.) segre- 

 gation of these arid or semi-arid areas into seven units. These are: 

 (1) The California Desert — a region of low and open small leaved 

 shrubs like the creosote bush (Covillea); (2) the Great Basin 

 Desert^a region of stunted shrubs, among which the dominant 

 plant is the sage bush (Artemisia) ; (3) the Arizona Desert — a re- 

 gion of mixed small leaved shrubs such as the creosote bush, and 

 the palo verde (Parkinsonia), succulent stemmed cacti, such as 

 Carnegiea and Opuntia; (4) the Texas Desert of mixed small 

 leaved shrubs and succulent or semi-succulents such as Agave, 

 Dasylirion and Yucca; (5) the Cahfomia semi-desert, a region of 

 great diversity, ranging from open woodland, chiefly evergreen 

 oaks, through chaparral to true desert; (6) the Texas semi-desert, 

 a region of interspersed grassland and scrub made up largely of 

 mesquite (Prosopis) with some succulents; (7) desert to Grassland 

 Transition, a region of grassland with ephemeral or root perennial 

 herbs and more or less succulent or semi-succulent types. 



The forests, as distinct from the vegetation areas, are considered 

 by Sargent' to be separable into three regions which he terms: 

 (1) The Coast forest; (2) the Interior forest; and (3) the Mexican 

 forest. 



The Coast forest is the heaviest, although not the most varied, 

 forest on the continent. It extends along the coast from about 

 latitude 60° to about latitude 40°, ranging eastward at the Inter- 

 national boundary following the eastward extension of heavy rain- 



^ Sargent, C. S., Forest Trees of North America. Tenth Census, vol. 9, 

 1884. This author's fourth category, the Northern forest, has already been 

 discussed as the western part of the sub-Arctic forest. 



