OF PLANTS AND ANIMALS. 157 



covered exclusively "by that species, of which the individuals 

 form a true society and give a peculiar aspect to that re- 

 gion. This congregating together of numerous individuals 

 of the same species, constituting what Humboldt calls social 

 plants, always indicates great uniformity in the nature of 

 the soil. It is thus that the Sphagnum, or bog-moss, 

 covers the soil to a considerable extent in humid and ex- 

 posed parts of forests ; that sedges, heaths, rhododendrons, 

 and firs, occupy immense spaces on the surface of the earth, 

 to the exclusion of all other species, which find themselves 

 smothered out by the social plants, these regions being 

 their especial domain. 



The knowledge of the choice or predilection of a species 

 for this or that situation, is current amongst all engaged 

 in practical horticulture, and is called into requisition 

 every day, in the formation of groups and beds of flowers 

 in parks and gardens. 



It is well known that many animals are equally social 

 in their habits. Birds migrate in flocks; sheep congre- 

 gate in pastures; and the prairies of the far West are 

 sometimes covered with herds of buffaloes. 



Temperature. If the earth were throughout homoge- 

 neous, if its surface were not formed of land and sea, of 

 islands and continents, of mountains and plains, the tem- 

 perature of a determinate point of the globe would be 

 given by its latitude, and the isothermal lines, or lines of 

 equal temperature, would be parallel to themselves and to 

 the equator. But the surface of the earth is not homo- 

 geneous. Elevation has the same effect on temperature as 

 an increase of distance from the equator, even under the 

 same parallels of latitude. Now in proportion as countries 

 are elevated, in the same ratio is their temperature reduced. 

 This remark applies not only to those mountain chains 

 14 



