70 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



NICOTIANA GLAUCA Graham. 

 CONETON; TRONADORA: TOBACCO. 



A South American tree, naturalized and frequently cultivated on 

 the Mexican Border, from Texas to California. " Natives of Buenos 

 Ayres; naturalized in Mexico; formerly cultivated; now wild" 

 (Botany of California). 



Several introduced species of palm grow in semicultivation on 

 the Mexican Border, about towns. 



LIFE AREAS OF THE MEXICAN BOUNDARY REGION. 



The land areas of North America comprise three primary biologic 

 regions: a Boreal Region, an Austral Region, and a Tropical Region, 

 each forming a broad belt that extends transversely across the con- 

 tinent. In general, the Boreal Region corresponds to British Amer- 

 ica, also including Greenland and Alaska; the Austral Region to the 

 United States; and the Tropical Region to Mexico, the AVest Indies, 

 and Central America. Owing to the differences of temperature as a 

 controlling factor, the boundaries of these primary regions are, how- 

 ever, very irregular, conforming to certain isothermal lines, rather 

 than to parallels of latitude. Thus, over limited and detached areas 

 in which differences in altitude give similar temperatures, the 

 Boreal belt is pushed south through the whole breadth of the Aus- 

 tral, and extends into Mexico, or portions of the Austral belt are 

 crowded across the Tropical, as in the high table-land of Mex- 

 ico. It is not strange, therefore, that while the border region of 

 Mexico and the United States lies in greater part within the low- 

 est division of the Austral Region (Lower Austral Life Zone), a line 

 drawn from either end of the Boundary Line to the summit of San 

 Francisco Mountains, in central Arizona, crosses the same primary 

 life areas that are bisected by a line drawn from the equator to the 

 north pole through the continent of North America. The mountain 

 ranges crossed by or in close proximity to the Mexican Boundary Line 

 are not nearly so high as San Francisco Mountains, which is the 

 highest land in Arizona ; but we have, nevertheless, portions of each 

 of the three primary biologic regions in the boundary strip, begin- 

 ning on the east with the Tamaulipan or highest division of the 

 Tropical Region, rising through the three subdivisions of the Aus- 

 tral Region, penetrating the lowest zone (Canadian) of the Boreal 

 in the highest mountains of the interior, and again descending 

 through the Austral to the Lower Californian division of the Tropical 

 Region on the west. 



Although the Tropical and Boreal areas of the Mexican Boundary 

 strip are so small as to be geographically insignificant, the fact that 



