VIL] ANIMALS AND PLANTS. 151 



was necessary 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, hydrogen, 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 con- 

 trary, derive their nourishment either directly or 

 indirectly from plants. They get rid of the super- 

 fluous hydrogen and carbon, and accumulate nitrogen. 

 The relations of plants and animals to the atmo- 

 sphere are therefore inverse. The plant withdraws 

 water and carbonic acid from the atmosphere, the 

 animal contributes both to it. Kespiration that is, 

 the absorption of oxygen and the exhalation of car- 

 bonic acid is the specially animal function of 

 animals, and constitutes their fourth distinctive 

 character. 



Thus wrote Cuvier in 1828. But, in the fourth 

 and fifth decades of this century, the greatest and 



