Of the Changes which Life has experienced on the Globe. 299 



bit at the present day, according to the various climates or si- 

 tuations in which they occur, liave been gradually established 

 under the predominating influence of a small number of natural 

 causes, and constitute at length the order of distribution which 

 life now presents at the surface of the earth. 



Originally life extended from one pole to the other, and ani- 

 mated the whole of this surface. The frozen regions of the North, 

 and the snow-clad summits of the Alps^ were covered with the 

 same verdure ; and the forms of the pristine animals and vege- 

 tables presented either extraordinary types of which we have 

 now no example, or species which belonged to families and genera 

 still existing, but in most cases only between the Tropics. As 

 we approach nearer to the present times, we find in all places re- 

 mains more and more resembling those of the plants and animals 

 which now live in the same country. At a later period, the ori- 

 ginal races of animals and vegetables were gradually expelled 

 from the north toward the south, from the summits to the plains, 

 in proportion as the uniform mean temperature of the earth's 

 surface yielded to more powerful causes, which brought about 

 the establishment of climates. These gradual variations in the 

 temperature, the lowering of the general level of the seas, the 

 equally successive and gradual diminution of the energy of vol- 

 canic phenomena arising from the original igneous state of the 

 earth, as well as of the strength and power of atmospheric phe- 

 nomena, and of the tides — ^such were the regular, general, and 

 continued natural causes of the modifications which life has un- 

 dergone, and of almost all the changes that have been produced 

 upon the earth's surface. The results of these first causes, such 

 as the estabhshment of local influences over the temperature of 

 the same climate, the formation of a multitude of particular 

 basins, some containing salt, others fresh water ; the pouring 

 out of these lakes into one another, and into the great basin of 

 the sea ; the partial debacles which thence resulted ; the ravages 

 of the sea on the low parts of the continents at first, and then 

 the formation of vast lagoons in the same places ; lastly, the esta- 

 blishment of the general system of draining and watering, or of 

 the hydrographic reticulation which covers the globe — such 

 were the irregular, and more or less violent and perturbing se- 

 condary causes of the partial vicissitudes experienced by animal 



