380 ANIMAL RESPIRATION 



the plant. We may therefore say that animals, by constantly 

 breathing out carbonic acid gas, keep up the supply of a substance 

 green plants require as a part of their food. Green plants, on the 

 other hand, in the course of their feeding, constantly give out large 

 quantities of oxygen gas, and keep up the supply of that element 

 necessary for breathing purposes. And even if animals could dis- 

 pense with green plants as food, the absence of such plants would 

 probably soon result in such a diminution of the oxygen in the air 

 and such an increase in its carbonic acid gas, that ordinary animal 

 life would become impossible. There is thus a constant, though, 

 of course, quite unconscious, exchange of good offices between 

 animals and green plants, and the composition of the air is kept 

 uniform for indefinitely long periods of time. It is, however, quite 

 possible, as some have maintained, that in very remote geological 

 periods the air contained a much larger percentage of carbonic 

 acid gas than it does now, and this was possibly one of the causes 

 that led to the luxuriant growth of plants during the geological 

 period (Carboniferous) from the deposits of which most of our 

 coal is obtained. 



It is now clear why the remarks already made (see p. 379) about 

 the difficulty which aquatic animals have in breathing, when kept in 

 a deep aquarium, need qualification. For if there is an abundant 

 plant-growth in such an aquarium, a large amount of oxygen is 

 given off which can be breathed by the animals present. When 

 bright sunlight is allowed to fall upon the plants in such an aqua- 

 rium, small bubbles of gas may often be seen to collect on them, 

 and this, when tested, proves to be oxygen. 



In the succeeding chapters of this section it will be convenient 

 first of all to consider the typical breathers in water, since they 

 represent what must have been the original state of things. 

 Typical breathers in air will next be discussed, and afterwards 

 forms which are in process of transition from breathing in water 

 to breathing in air, while mention will also have to be made of 

 air-breathing forms which have reverted to breathing in water for 

 part, at least, of their existence. 



