NO LIMIT TO THE DURATION OF ORGANIC NATURE. 289 



tion, the soil and vegetation together have expanded to their 

 present extent at the expense of the carbonic acid of the 

 atmosphere. 



The carbonic acid of the atmosphere is but limited in quan- 

 tity ; nevertheless, so vast is the atmosphere, that the with- 

 drawal therefrom of so large a quantity as to supply all the 

 carbon at present contained in the soil of the earth and in the 

 whole animal and vegetable kingdoms, makes but a small 

 difference on that proportion. The other simple bodies which 

 enter into the food of plants oxygen, hydrogen, nitrogen, and 

 the rest are inexhaustible in nature, so that there is no limit 

 to the increase of the organic world but the amount of carbonic 

 acid in the atmosphere. The animal kingdom is continually 

 engaged, by the function of respiration, in restoring to mineral 

 nature the carbon, in the shape of carbonic acid, which the 

 vegetable kingdom has worked up into the organic form. Thus 

 a single horse gives to the atmosphere in a year about a ton of 

 carbon combined with oxygen. 



Writers on the functions of plants and animals have specu- 

 lated on the period during which organic life can maintain 

 itself on the surface of the earth. To this there is no limit but 

 the will of the Author of Nature. Since there is manifest 

 proof that from time to time the land is elevated above the 

 present level of the ocean, there is no reason to predict thafc 

 the dry land will be gradually, in the long course of time, 

 washed down into the ocean. Neither is there any ground for 

 the belief, which some distinguished philosophers have enter- 

 tained, that the respiration of animals will at length so poison 

 the air as to render it impossible for animals to live therein. 

 Those who reason thus forget that animal life, being ex- 

 clusively supported by the vegetable kingdom, cannot outstep 

 it ; and that the vegetable kingdom continually destroys the 

 poisonous carbonic acid produced by the animal kingdom. 



T 



