300 INFLUENCE OF HEAT ON VEGETATION. [SECT, xxvii. 



SECTION XXVII. 



Influence of Temperature on Vegetation Vegetation varies with the Latitude 

 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 by 

 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 luxuriance 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 pro- 

 ductions, so interlaced by creeping and parasitical plants 

 as often to present an impenetrable barrier. But the 

 richness of vegetation gradually diminishes with the tem- 

 perature ; the splendour of the tropical forest is succeeded 

 by the regions of the olive and vine ; 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 latitudes more to the warmth of the soil than 

 to that of the air. But even these enduring plants become 

 dwarfish stunted shrubs, till a verdant carpet of mosses and 

 lichens, enamelled with flowers, exhibits the last sign of 

 vegetable life during the short but fervent 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 line, and in the northern latitudes of 45 and 68, 

 are in the proportion of the numbers 12, 4, and 1. Not- 



