20 THE STUDY OF ANIMAL LIFE CHAP. 



we use as fuel. But animals feed on plants or on crea- 

 tures like themselves, and are thus saved the expense of 

 building up food-stuffs from crude materials. Their 

 most characteristic transformation of energy is that by 



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which the power of complex chemical substances is used 

 in locomotion and work. In so working, and eventually 



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in dying, they form waste-products water and carbonic 

 acid, ammonia and nitrates, and so on which may be 

 again utilised by plants. 



How often is the inaccurate statement repeated " that 

 animals take in oxygen and give out carbonic acid, 

 whereas plants take in carbonic acid and give out oxygen " ! 

 This is most misleading. It contrasts two entirely dis- 

 tinct processes a breathing process in the animal with 

 a feeding process in the plant. The edge is at once 

 taken off the contrast when the student realises that 

 plants and animals being both (though not equally) alive, 

 must alike breathe. As they live there is in both an 

 oxidation of complex organic substances, just as in the 

 burning of a candle ; in plant, in animal, in candle, 

 oxygen passes in, as a condition of life or combustion, 

 and carbonic acid gas passes out as a w r aste-product. 

 Herein there is no difference except in degree between 

 plant and animal. Each lives, and must therefore 

 breathe. But the living of plants is less intense, there- 

 fore the breathing process is less marked. Moreover, in 

 sunlight the respiration is disguised by an exactly reverse 

 process peculiar to plants the feeding already noticed, 

 by which carbonic acid gas is absorbed, its carbon re- 

 tained, and part of its oxygen liberated. 



There is an old-fashioned experiment which illustrates 

 the " balance of nature." In a glass globe, half -filled 

 with water, are placed some minute water-plants and 

 water-animals. The vessel is then sealed. As both the 

 plants and the animals arc absorbing oxygen and liber- 

 ating carbonic acid gas, it seems as if the little living 

 world enclosed in the globe would soon end in death. 

 But, as we have seen, the plants are able in sunlight to 

 absorb carbonic acid and liberate oxygen, and if present 

 in sufficient numbers will compensate both for their own 



