158 ANIMALS AND PLANTS. 



sary that the chemical composition of the animal body 

 should be more complicated than that of the plant ; and 

 it is so, inasmuch as an additional substance, nitrogen, 

 enters into it as an essential element ; while, in plants, 

 nitrogen is only accidentally joined with the three other 

 fundamental constituents of organic beings carbon, hy- 

 drogen, and oxygen. Indeed, he afterwards affirms that 

 nitrogen is peculiar to animals ; and herein he places 

 the third distinction between the animal and the plant. 

 The soil and the atmosphere supply plants with water, 

 composed of hydrogen and oxygen; air, consisting of 

 nitrogen and oxygen; and carbonic acid, containing 

 carbon and oxygen. They retain the hydrogen and the 

 carbon, exhale the superfluous oxygen, and absorb little 

 or no nitrogen. The essential character of vegetable life 

 is the exhalation of oxygen, which is effected through 

 the agency of light. Animals, on the contrary,' derive 

 their nourishment either directly or indirectly from 

 plants. They get rid of the superfluous hydrogen and 

 carbon, and accumulate nitrogen. The relations gf plants 

 and animals to the atmosphere are therefore inverse. 

 The plant withdraws water and carbonic acid from the 

 atmosphere, the animal contributes both to it. Respira- 

 tion that is, the absorption of oxygen and the exhala- 

 tion of carbonic acid is the specially animal function 

 of animals, and constitutes their fourth distinctive char- 

 acter. 



Thus wrote Cuvier in 1828. But, in the fourth and 

 fifth decades of this century, the greatest and most rapid 



