THE PECULIAR FACULTIES 269 



fluid breathed to the ammal's essential fluid, appears to be oxygen. . 

 It is liberated from the respired fluid, combines with the essential fluid 

 of the animal, and restores to the latter quahties which it had lost. 



There are, as we know, two different respiratory fluids which pro- 

 vide oxygen for breathing. These fluids are air and water ; in general 

 they are the media in which Uving bodies are immersed, or by which 

 they are surrounded. 



Water indeed is the respiratory fluid of many animals which live 

 permanently in its depths. It is beUeved that this fluid does not de- 

 compose when giving up oxygen ; but that it always has a certain 

 amoimt of air mixed up with it, and that it is this air which is decom- 

 posed in the act of breathing, and thus provides oxygen for the essential 

 fluid of the animal. This is the way in which fishes and many aquatic 

 animals breathe ; but this respiration is less active, and yields its 

 restorative principles more slowly, than that which takes place in free 

 air. 



Free atmospheric air is the second respiratory fluid, and that which 

 is breathed by a large number of animals which hve permanently 

 in it or within reach of it : it is promptly decomposed in the act of 

 breathing, and thereupon 3delds up its oxygen to the essential fluid of 

 the animal. This kind of breathing, which is characteristic of the 

 most perfect animals and many others, is the most active : and its 

 activity is proportional to the development of the organ in which it is 

 carried out. 



It is not enough to discuss the existence of a special organ for breath- 

 ing ; we must pay attention to the character of this organ, in order to 

 judge of the height of the animal's development, by means of the faster 

 or slower recurrence of the necessity for restoring its essential fluid. 



In proportion as the essential fluid of animals becomes more complex 

 and animalised, the degradations which it suffers during hfe are greater 

 and more rapid, and the restorations required gradually develop in 

 proportion to the changes experienced. 



In the simplest and most imperfect animals, such as the infusorians 

 and polyps, the essential fluid is so elementary, so little animaUsed, 

 and becomes so slowly degraded, that the restorations of the food 

 are sufficient. But soon afterwards, nature begins to require a new 

 method for preserving the essential fluid of animals in a proper con- 

 dition. It is then that she creates respiration ; but at first she only 

 sets up a very weak and inactive respiratory system, — that namely 

 furnished by water, which has itself to convey its influence to every 

 part of the animal. 



Nature subsequently varies the type of respiration in accordance 

 with the progressive increase of the requirement. She makes this 



