1 6 Animal Life and Intelligence. 



as to form carbonic acid gas ; and the atmosphere would 

 be gradually drained of its oxygen and flooded with car- 

 bonic acid gas were it not that plants, through their green 

 colouring matter (chlorophyll), under the influence of light, 

 have the power of decomposing the carbonic acid gas, 

 seizing on the carbon and building it into their tissues, 

 and setting free the oxygen. Thus are animals and green 

 plants complementary elements in the scheme of nature.* 

 The animal eats the carbon elaborated by the plant into 

 organic products (starch and others), and breathes the 

 oxygen which the plant sets free after it has abstracted 

 the carbon. In the animal's body the carbon and 

 oxygen recombine ; its varied activities are thus kept 

 going; and the resultant carbonic acid gas is breathed 

 forth, to be again separated by green, growing plants into 

 carbonaceous food-stuff and vitalizing oxygen. It must 

 be remembered, however, that vegetable protoplasm, like 

 animal protoplasm, respires by the absorption of oxygen 

 and the formation of carbonic acid gas. But in green 

 plants this process is outbalanced by the characteristic 

 action of the chlorophyll, by which carbonic acid gas is 

 decomposed. 



Lastly, we have to consider the relations of animals and 

 plants to energy. Energy is denned as the power of doing 

 work, and it is classified by physicists under two modes 

 potential energy, or energy of position ; and kinetic energy, 

 or energy of motion. The muscles of my arm contain 

 a store of potential energy. Suppose I pull up the weight 



* An interesting problem concerning the atmosphere is suggested by 

 certain geological facts. In our buried coal-seams and other carbonaceous 

 deposits a great quantity of carbon, for the most part abstracted from the 

 atmosphere, has been stored away. Still greater quantities of carbon are 

 imprisoned, in the substance of our limestones, which contain, when pure, 

 44 per cent, of this element. A large quantity of oxygen lias also been taken 

 from the atmosphere to combine with other elements during their oxidation. 

 The question is Was the atmosphere, in the geological past, more richly laden 

 with carbonic acid gas, of which some has entered into combination with lime 

 to form limestone, while some has been decomposed by plants, the carbon 

 being buried as coal, and the oxygen as products of oxidation? Or, has the 

 atmosphere been furnished with continuous fresh supplies of carbonic acid 

 gas? 



