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 CO,. 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.} 
Nitrogen is still more difficult than carbon to bring into the 
combined state. It is a lazy element, and the immense stock in 
1Jt 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 protozoén, 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. 
