24:8 YEGETATIOX. SECT. XXVI. 



SECTION XXVI. 



Influence of Temperature on Vegetation Vegetation varies with the Lati- 

 tude and Height above the Sea Geographical Distribution of Land 

 Plants Distribution of Marine Plants Corallines, Shell-fish, Reptiles, 

 Insects, Birds, and Quadrupeds Varieties of Mankind, yet identity of 

 Species. 



THE gradual decrease of temperature in the air and in the earth, 

 from the equator to the poles, is clearly indicated hy its influence 

 on vegetation. In the valleys of the torrid zone, where the mean 

 annual temperature is very high, and where there is abundance 

 of light and moisture, nature adorns the soil with all the luxuri- 

 ance of perpetual summer. The palm, the bombax ceiba, and a 

 variety of magnificent trees, tower to the height of 150 or 200 

 feet above the banana, the bamboo, the arborescent fern, and 

 numberless other tropical productions, so interlaced by creeping 

 and parasitical plants, as often to present an impenetrable barrier. 

 But the richness of vegetation gradually diminishes with the 

 temperature ; the splendour of the tropical forest is succeeded by 

 the regions of the vine and olive ; these again yield to the verdant 

 meadows of more temperate climes ; then follow the birch and 

 the pine, which probably owe their existence in very high lati- 

 tudes more to the warmth of the soil than to that of the air. 

 But even these enduring plants become dwarfish shrubs, till a 

 verdant carpet of mosses and lichens, enamelled with flowers, 

 exhibits the last sign of vegetable life during the short but fervid 

 summers at the polar regions. Such is the effect of cold and 

 diminished light on the vegetable kingdom, that the number of 

 species growing under the equator and in the northern latitudes 

 of 45 and 68 are in the proportion of the numbers 12, 4, and 1. 

 Notwithstanding the remarkable difference between a tropical 

 and polar flora, light and moisture seem to be almost the only 

 requisites for vegetation, since neither heat, cold, nor even com- 

 parative darkness, absolutely destroy the fertility of nature. In 

 salt plains and sandy deserts alone hopeless barrenness prevails. 



