28 COMPARATIVE ZOOLOGY. 



cartilage ; but the ternary compounds help in repairing 

 waste, while both produce heat. When oxidized, whether 

 for work or warmth, these complex compounds break up 

 into the simple compounds water, carbon dioxide, and 

 (ultimately) ammonia, and as such are returned to earth 

 and air from the animal. Both plant and animal end 

 their life by going back to the mineral world: and thus 

 the circle is complete from dust to dust. Carbonate of 

 ammonia and water, a blade of grass and a horse, are but 

 the same elements differently combined and arranged. 

 Plants compress the forces of inorganic nature into chem- 

 ical compounds ; animals liberate them. Plants produce ; 

 animals consume. The work of plants is synthesis, a 

 building-up ; the work of animals is analysis, or destruc- 

 tion. The tendency in plants is deoxidation; the tenden- 

 cy in animals is oxidation. Without plants, animals would 

 perish; without animals, plants had no need to be. There 

 is no plant which may not serve as food to some animal. 



CHAPTEK IY. 



LIFE. 



ALL forces are known by the phenomena which they 

 cause. So long as the animal and plant were supposed to 

 exist in opposition to ordinary physical forces or indepen- 

 dently of them, a vital force or principle was postulated 

 by which the work of the body was performed. It is now 

 known that most, if not all, of the phenomena manifested 

 by a living body are due to one or more of the ordinary 

 physical forces heat, chemical affinity, electricity, etc. 

 There is no work done which demands a vital force. 



The common modern view is that vitality is simply a 



