THE NEOTROPICAL REGIONS 



263 



The American tropics, except for the West Indies and Galapagos 

 Islands, constitute a single continental area. 



Mexico 



The northern part of Mexico, both geographically and biologic- 

 ally, is part of the region which includes much of the states of 

 Texas, New Mexico, Arizona and California. 



The southern plateau region of the United States is continued 

 southward into Mexico, where it becomes much higher, reaching 



Fig. 75. — Sub-tropical vegetation, Mexico. Branches covered with epiphytes, 

 largely Bromeliads. Photo., Mr. G. N. Collins. 



7,000 feet in the vicinity of Mexico City. The greater part of 

 Mexico is occupied by this central plateau, to the west of which 

 the mountains form a continuation of the California Sierra. Part 

 of this system forms the central range of the long peninsula of 

 Lower California. The plateau of the Mexican mainland is sepa- 

 rated from the Gulf of California by a high mountain range, which 

 extends southward along the whole coastal region which forms a 

 narrow strip between the mountains and the Bea. Eastward the 

 plateau descends more gradually to the Gulf of Mexico. 



Much of Mexico is volcanic, and the great volcanic cones, one 



