20 The Study of Animal Life part i 



energy of the complex stuffs which animals eat or which 

 we use as fuel. But animals feed on plants or on creatures 

 like themselves, and are thus saved the expense of build- 

 ing up food -stuffs from crude materials. Their most 

 characteristic transformation of energy is that by which the 

 power of complex chemical substances is used in locomotion 

 and work. In so working, and eventually 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 the living matter of both is oxi- 

 dised, like the fat of a burning candle ; in plant, in animal, 

 in candle, oxygen passes in, as a condition of life or com- 

 bustion, and carbonic acid gas passes out as a waste-pro- 

 duct. Herein there is no difference except in degree between 

 plant and animal. Each lives, and must therefore breathe. 

 But the hving of plants is less intense, therefore the breath- 

 ing 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 retained, 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 are absorbing oxygen and liberating car- 

 bonic 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 



