442 THE ZOOLOGIST. 
naturalists. Indeed, some apology must be made for the in- 
clusion of fragments of the merest elements of science; but I 
think the best plan is to pass rapidly over this side of the 
subject, and not run even the very slight risk attached to the 
step of leaving it altogether to the intelligence of the reader. 
Immaterial exceptions, such as plant-eating fungi, animals 
possessing chlorophyll, &c., have been carefully considered, and 
will not be referred to again. I do not know of any biological 
fact that serves as an obstacle in the way of the acceptance of 
the theory. 
Animals live entirely upon plants. There is no exception to 
this great law, but often it happens that the dependence is in- 
direct, as when a man feeds on the flesh of an herbivorous 
animal. Sometimes the nourishment passes through a long 
series of animal forms, but in every case it can be demonstrated 
that every atom of it comes from the plant, and never from 
inorganic matters, with the exception of water. 
Plants are not to so great an extent dependent upon animals. 
Their food is inorganic, and most of it is derived from the air. 
It isa popular error to hold that a plant invariably takes its 
food mainly from the soil. Very little beyond water is drawn 
through the roots, and the bulk of the plant is built up from the 
air breathed in through the leaves. The constituent used is 
the carbon dioxide (CO,) present in the atmosphere in the pro- 
portion of about °04 per cent. This is extracted from the air, 
and the plant returns to the atmosphere the oxygen which is a 
waste product of growing vegetation. The animal, of course, 
breathes in oxygen and breathes out carbon dioxide, and in this 
way each lives on the waste product of the other. Although 
carbon dioxide may be evolved by volcanoes, or by certain sapro- 
phytic plants, or in other ways, the green plant is practically 
dependent upon animals for the bulk of its nourishment. 
Animals, therefore, can only live in direct association with 
plants. The latter organisms are not, however, bound down to 
the neighbourhood of animals, for the carbon is carried to them 
by atmospheric currents ; but soil (except in the case of aquatic 
plants), water, and sunlight are also necessary for the growth of 
plants. The last is extremely important, and a knowledge of 
this fact is the very heart and foundation of a knowledge of 
