84 THE MECHANISM OF LIFE 



that occurs. CO2 and H2O are combined together, and the energy 

 necessary to bring about the combination is taken from the 

 sunlight. A chemist could cause the combination to take place 

 at a very high temperature and in quite special conditions, but it 

 occurs in the green plant of itself, and at ordinary temperatures. 

 All the above is very important, and may not be neglected 

 when we are considering the nature of the living process. We 

 return to the matter in a later chapter. 



The Balance o£ Life. — We see now in what way the energy 

 degraded by animal life becomes rebuilt up again. The animal 

 takes proteids, fats, and carbohydrates into its body, disin- 

 tegrates and oxidises these compounds, makes use of their con- 

 tained energy to obtain heat and do mechanical work, and then 

 excretes the products of disintegration and oxidation in the 

 form of water, carbonic acid, and urea. 



The bacteria convert the urea (and other nitrogenous residues) 

 into nitrate. 



The green plants take energy from solar radiation, and rebuild 

 water, carbonic acid, and nitrate into proteid, fat, and carbo- 

 hydrate, when the cycle of operations recommences. 



Now it is easy to see that all animal life depends upon plant 

 life. Animals are carnivorous (feeding upon flesh), or her- 

 bivorous (feeding upon vegetable substances), or omnivorous 

 (eating both flesh and vegetable substance). There are also 

 some animals which are called saprozoic, with regard to their 

 manner of nutrition, and these can utilise as food liquids or 

 debris containing broken-down organic matter. Here we need 

 not consider the saprozoic organisms, and it is enough for our 

 present purpose to regard animals as either carnivorous or 

 herbivorous, or both. Since the carnivores eat other carnivores 

 or herbivores, or both, and since the herbivores eat vegetable 

 tissues, it is easy to see that all animal life on the earth ultimately 

 depends upon vegetable life. 



On the other hand, vegetable life depends for its continuance 

 on a supply of water, carbonic acid, and nitrate. The quantity of 

 water on the earth is (for our purpose) unlimited, but not so the 

 quantity of carbonic acid and nitrate, and upon the supplies of the 

 latter substances the abundance of vegetable life hangs. What, 

 then, are the sources of carbonic acid and nitrate ? Some 

 quantity of each substance comes from the earth in the exhala- 

 tions from volcanoes, and possibly as the results of the disintegra- 

 tion of certain mineral substances. There is probably far more 



