COMPARISON OF ANIMAL AND PLANT BIOLOGY 127 



of which fuels are of organic origin). Thus there is a never- 

 ending cycle of carbon dioxide being taken up from the air, 

 built up into plant substance, which in turn is oxidized or be- 

 comes food for animals, then is oxidized to form excretions 

 or built into animal substance, which ultimately dies and 

 decays and is again taken up by the plant. In short, the 

 foods made in plants are by animals changed back into ex- 

 cretions which plants use over again in making new foods. 



Without chlorophyll and sunlight much of the carbon 

 dioxide in the world now stored in the organic compounds of 

 plants and animals would ultimately become free in the air, 

 and be so abundant as to poison all life. Certainly all ani- 

 mals would perish because of the lack of the foods which plants 

 with chlorophyll must make primarily for themselves, and 

 which secondarily are used by other plants and also by ail 

 animals. The cycle of carbon, then, is as follows : Car- 

 bon from the carbon dioxide in the air enters into the 

 composition of plant substances. This may be oxidized (in 

 the living plant) or decayed (in the dead plant) directly 

 back to carbon dioxide, which gets free in the air ; or the plant 

 substance may be used by animals and thence oxidized (in 

 living animals) or decayed (after death of the animal's body) 

 back to free carbon dioxide. 



Since animals depend upon plants in this food relation, 

 philosophical biologists have concluded that plants with 

 chlorophyll must have existed on this earth before animals 

 did. In fact, with the exception of a few of the lowest bacteria 

 (a group of microscopic plants), all animals and plants are 

 dependent upon the plants which are able to use light and 

 carbon dioxide in making foods. Some of the simplest 

 bacteria can make food materials without light and from 

 purely inorganic materials. Obviously such organisms could 

 have lived on this earth before all other living things which 

 we know and at a time when there were conditions which 

 were not adapted to plants with chlorophyll. 



