2G.2 INFLUENCE OF HEAT ON VEGETATION. SKCT. XXVII. 



earth are stable, and that their vicissitudes are only 

 periods or oscillations of more or less extent, which van- 

 ish in the mean annual temperature of a sufficient num- 

 ber of years. This constancy of the mean annual temper- 

 ature of the different places on the surface of the globe 

 shows that the same quantity of heat, which is annually 

 received by the earth, is annually radiated into space. 

 Nevertheless a variety of causes may disturb the climate 

 of a place; cultivation may make it warmer; but it is 

 at the expense of some other place, which becomes 

 colder in the same proportion. There may be a suc- 

 cession of cold summers and mild winters, but in some 

 other country the contrary takes place to effect the 

 compensation ; wind, rain, snow, fog, and the other me- 

 teoric phenomena, are the ministers employed to accom- 

 plish the changes. The distribution of heat may vary 

 with a variety of circumstances ; but the absolute quan- 

 tity lost and gained by the whole earth in the course of 

 a year is invariably the same. 



SECTION XXVII. 



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 indi- 

 cated 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 produc- 

 tions, 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 splendor of the tropical forest is succeeded 



