IN THE ' BIOLOGIA CENTRALI- AMERICANA.' 543 



necessary here, but as this subject has been dealt with at length in 

 the Introduction, I will only refer to its general characteristics. 

 Northern Mexico consists of a high tableland, the extension of 

 the Arizona plateau ; it is very arid and consequently barren, 

 growing cacti and other such plants. At the spot where the 

 I'ailway crosses the Rio Grande at El Paso, on the borders of 

 Mexico, the plateau descends to only 3700 feet, but soon rises 

 again and has an average altitude of about 8000 feet, till at the 

 end of some 900 miles the City of Mexico is reached. This 

 plateau is bounded on each side by ranges of mountains descend- 

 ing abruptly towards either coast and clothed with forest, which 

 at its summit consists largely of pines and ilex. Both on the 

 Atlantic and Pacific coasts there is a narrow belt of tropical 

 country. About the City of Mexico the plateau is broken by 

 a sei'ies of volcanoes, the highest of which i-eaches 18,000 feet. 

 Southward to Panama the land gradually descends in altitude ; 

 it is, however, very much varied and frequently covered with 

 •forest, alternating with savannas and interspersed with many 

 volcanoes, one of which in Costa Rica attains a height of 

 11,000 feet. At the Isthmus of Panama the land subsides to 

 300 feet. 



The country is divided by the natives, according to altitude, 

 into zones under the names of Tierra caliente, Tierra templada, 

 and Tierra fria, and these zones have an immense influence on 

 the fauna and flora, and are a largely determining factor in the 

 number and diversity of species. The climatic conditions must 

 also be taken into consideration, the rainfall on the Atlantic 

 being far in excess of that on the Pacific, and the vegetation 

 far more luxuriant. 



In Eocene or early Miocene times there was a broad channel 

 separating North and South America, where the Isthmus of 

 Panama now exists, and it seems probable that a series of 

 elevations and subsidences took place, temporarily forming islands 

 before the land became permanently continuous as it now is, thus 

 accounting for the many allied and repi'esentative species found 

 in Chiriqui and Costa Rica. 



When the channel was in existence it must have proved an 

 insuperable baiTier to the migration of land-animals, but when 

 the two continents became united undoubtedly a considerable 

 interchange of animal- and plant-life took place, and there Avas 

 a mingling of northern and southern forms. This, no doubt, 

 accounts for the extraordinary richness iir species of Central 

 America. 



Mr. Pocock, in his remarks on the origin of the Mammalia, 

 says that during Miocene times, when the Panama land-bridge 

 was upheaved, the migration was divided into two categories, one 

 containing the Insectivora, Carnivora, Artiodactyla, Periisodac- 

 tyla, etc., which had been evolved in the northern hemisj^here and 

 inferentially passed from North into South America, while the 

 other comprised the Primates, the Edentates, the Marsupials, 



