TROPICAL VEGETATION. 



The elastic subterranean forces strive, for the most parf in 

 vain, to break through the snow-clad domes which crown the 

 ridges of the Cordilleras, but even where these forces have 

 actually opened a permanent channel of communication with 

 the outer air, either through crevices or circular craters, they 

 rarely send forth currents of lava, erupting more frequently 

 ignited scoriae, jets of carbonic acid gas, sulphuretted hydrogen, 

 and steam.* 



179. Animals of the tropics. A corresponding variety is found 

 in the animal kingdom in these regions at the level of the sea ; 

 upon the plains which extend over the tropical regions are found 

 the varieties of monkeys, crocodiles, the boa-constrictor, rattle- 

 snake, jaguars, and macaws. Higher up, at a level rising from 

 5000 to 10000 feet, at the base of the Andes, are found the ocelot, 

 the strutheo rea, and the duck. Higher still, the ape, the puma, 

 and the llama, and, in fine, about the snow-line, hawks, vultures, 

 bears, and the condor, which rises upon its vast wings above the 

 lofty summits of Chimborazo and Aconcagua. 



180. The Himalayas are characterised, also, by animals dwell- 

 ing in a succession of stages one above the other. Thus, upon 

 the plains are found the tiger, the peacock, and the Bactrian 

 camel. Higher up the bat, the Cashmere goat, and the pheasant, 

 and just below the snow-line, the sheep, the yak, the pigeon, the 

 robin, &c. 



181. The local character of a climate depends on the mean 

 temperature of the atmosphere and of the surface of the earth. 

 And this temperature itself must depend chiefly on the heat 

 imparted to the atmosphere and the earth by the sun. The solar 

 rays, in passing through the atmosphere to the earth, impart 

 to that very attenuated and transparent fluid an inconsiderable 

 quantity of heat, their chief thermal power being exerted on the 

 surface of the earth, which forms the base of the atmospheric 

 ocean. The earth, like all bodies, absorbs a certain proportion of 

 the heat thus transmitted to it, and what it fails to absorb it 

 reflects exactly as a surface would reflect light. The heat which 

 it reflects, not entering it, does not affect its temperature, and it is 

 warmed exclusively by the portion it absorbs. This portion is so 

 considerable, that if it were uniformly diffused over the entire 

 surface of the earth, it would be sufficient to dissolve annually a 

 shell of ice 100 feet thick covering the entire globe. 



182. Heat received from celestial spaces But besides the 

 heat received from the sun, the earth also receives a considerable 

 portion of heat from the surrounding firmament, in other words, 



* Humboldt's Cosmos. Introduction. 



181 



