THE WAY OF THE WILD 57 



characteristic and essential elements. Now the world's supply 

 of these important elements is in the form of exceedingly raw 

 material floating in the air. Oxygen can be taken in by the leaves 

 of plants and the lungs of animals and used at once and directly 

 by the organism. Carbon and nitrogen, however, exist in the 

 air in a condition useless for the direct needs of either plants 

 or animals. 



The great problem of subsistence is therefore, primarily, to get 

 carbon and nitrogen, which all animals and plants alike, whether 

 large or small, high or low, must secure in large and constant 

 quantities in order to maintain life and its activities. 



Now carbon exists in combination with oxygen as C0 2 . This 

 is a very simple but a very stable compound, and in this form no 

 animal can use it. Only the green chlorophyll of leaves, and that 

 in the presence of sunlight, can break this compact with oxygen, 

 and thus the pioneer labor of securing carbon and bringing it 

 into more complex compounds, especially those including hydro- 

 gen, is, and must be, performed by the higher plants ; and on 

 these and their remains must all animals depend for their carbon 

 supply, as must also the nonchlorophyll plants like bacteria. 



Of course many animals live on other animals and thus short- 

 circuit the carbon problem, just as many bacteria are directly 

 parasitic on living plants and even animals. In general, plants 

 and animals both take their oxygen direct from the air, but a 

 few bacteria and other low forms of plant life depend upon 

 getting oxygen as they do carbon, — by taking it from its combi- 

 nations, even in a living plant or animal. Such parasites are, of 

 course, dangerous to life, and they lie at the base of some of our 

 most troublesome plant and animal diseases. 1 



Nitrogen is still more difficult than carbon to bring into the 

 combined state. It is a lazy element, and the immense stock in 



1 It would be a mistake to assume that all diseases, even those of a germ 

 character, are due to vegetable parasites. It is now generally held that the 

 germ of smallpox, for example, is a protozoon, that is, animal rather than 

 vegetable, though at this level of life we are down where plants and animals 

 shade into each other by almost imperceptible differences. 



