164 ANIMALS AND PLANTS vi 



were no means by which the motion of its fluids 

 could be produced by internal causes. Hence 

 arose the second great distinctive character of 

 animals, or the circulatory system, which is less 

 important than the digestive, since it was un- 

 necessary, and therefore is absent, in the more 

 simple animals. 



Animals further needed muscles for loco- 

 motion and nerves for sensibility. Hence, says 

 Cuvier, it was necessary that the chemical compo- 

 sition of the animal body should be more compli- 

 cated 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 after- 

 wards 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 



