240 COMPARATIVE ZOOLOGY 



3. Organs and their Functions. Animals, like plants, 

 grow, feel, and move ; these three are the capital facts 

 of every organism. Besides these there may be some 

 peculiar phenomena, as motion and will. 



Life is manifested in certain special operations, 

 called functions, performed by certain special parts, 

 called organs. Thus, the stomach is an organ, whose 

 function is digestion. A single organ may manifest 

 vitality, but it does not (save in the very lowest forms) 

 show forth the whole life of the animal. For, in being 

 set apart for a special purpose, an organ takes upon 

 itself, so to speak, to do something for the benefit of 

 the whole animal, in return for which it is absolved 

 from doing many things. The stomach is not called 

 upon to circulate or purify the blood. 



There may be functions without special organs, as 

 the amoeba digests, respires, moves, and reproduces by 

 its general mass. But, as we ascend the scale of animal 

 life, we pass from the simple to the complex : groups 

 of cells or tissues, instead of being repetitions of each 

 other, take on a difference, and become distinguished 

 as special parts with specific duties. The higher the 

 rank of the animal, the more complicated the organs. 

 The more elaborated the structure, the more compli- 

 cated the functions. But in all animals, the functions 

 are performed under conditions essentially the same. 

 Thus, respiration in the sponge, the fish, and in man 

 has one object and one means, though the methods 

 differ. A function, therefore, is a group of similar 

 phenomena effected by analogous structures. 



The life of an animal consists in the accumulation 

 and expenditure of force. The tissues are storehouses 

 of power, which, as waste goes on, is given off in various 

 forms. Thus, the nervous tissue generates nerve force; 

 the muscles, motion. If we contemplate the phenomena 



