36 TREE ANCESTORS 



the immediate river bottoms are practically treeless. These for- 

 ests are destitute of variety and for the most part stunted. They 

 are characterized by cottonwood (Populus), nut pine {Pinus cdulis), 

 white iir {Ahics concolor), lodgepole pine {Pinus murrayana), 

 various spruces {Picea Engelmanni and parryana), etc. 



The greater part of Mexico, including Baja or Southern Cali- 

 fornia, is occupied by vegetational areas which are the southward 

 continuation of those of the southwestern United States. Nowhere 

 down to latitude 22° can the term forest be apphed to the plant 

 cover except in the higher elevations of the Eastern and Western 

 Sierra Madre that bound the Anahuac Desert Plateau. Above 

 7000 feet there are open pine forests {Pinus arizonica, chihuahuana, 

 micro phylla,cemhroides) and clumps of scrub oak {Quercus gisea etc.), 

 with aspens and a few broad leaved temperate types. At about 

 latitude 23° the broken transverse volcanic chains of high moun- 

 tains strike at almost right angles to the trends of the Sierra Madre. 

 This is the region termed volcanic by Thayer* and United Cordil- 

 leran by Harshberger. The higher elevations are covered with 

 open pine forests and the subordinate slopes by oak forests with a 

 mixture of other broad leaved temperate types and some stragglers 

 from the tropical forests to the southward. The Gulf Coastal 

 Plain, more or less covered with chaparral in its northern extent 

 passes gradually into a typical tropical forest toward the south, 

 while on the West Coast the Jaliscan Coastal Plain furnishes wild 

 figs and Taxodium in the bottoms and chaparral on the divides, 

 with open growths of oak and pine at higher elevations. 



CENTRAL AMERICAN ANTILLEAN REGION 



This is somewhat of a composite of insular and continental 

 areas with typical tropical lowland forests and with a few repre- 

 sentatives of temperate types in the higher uplands. It cannot be 

 properly described without going into more detail than is warranted 

 in the present connection, and while it is a region of surpassing 



^Thayer, Physiographic Provinces of Mexico. Jour. Geol., vol. 24, pp. 

 61-94, 1915. Harshberger, J. W., Phytogeographic Survey of North America, 

 1911. 



