THE CIRCULATION OF FOOD MATERIALS 297 



SECTION 51. The Circulation of Food Materials in the 



Organic World. 



In the entire organic world destruction and reconstruction proceed 

 simultaneously and without cessation, and in order to supply the energy 

 necessary for the maintenance of life upon the earth, enormous masses of 

 organic material must be daily decomposed into carbon dioxide and water. 

 Living organisms aid in the destruction of dead ones, and such exceedingly 

 minute forms as bacteria are especially adapted for this purpose, owing to 

 their rapid powers of multiplication and their intense metabolic activity. 

 Processes of oxidation also play an important part in the decomposition of 

 organic remains, and though these and the vitalistic aids to disorganization 

 often seem of trifling importance, nevertheless, owing to the continuity of 

 their action, much more may be accomplished by them than by sudden 

 and violent, but local, conflagrations, as, for example, the burning of a forest. 

 Slow decomposition ultimately results, after many changes, in the production 

 of the same final products as those of ordinary combustion, namely, carbon 

 dioxide, water, inorganic salts of nitrogen, and residual ash. 



Unless certain plants had the power of regenerating organic substance 

 from these ultimate products of decomposition, the supply of organic material 

 would gradually be consumed, and all terrestrial life would cease. This 

 power is practically restricted to green plants, so that both animals and 

 plants derive their energy either directly or indirectly from the sun's rays, 

 which are converted into chemical energy by the agency of the chlorophyll- 

 apparatus. When a piece of coal is burnt, a portion of the sun's radiant 

 energy is liberated which was stored up in the form of potential energy 

 many years ago, so that every steam-engine is directly or indirectly driven 

 by the sun. 



The absolute amount of energy thus liberated or stored up cannot be 

 precisely calculated, since the processes of decomposition and reconstruction 

 in the living organism are frequently interrupted by converse changes ; 

 moreover, both construction and destruction occur simultaneously in every 

 meadow, forest, or pond. The fact that life has persisted for many ages 

 shows, however, that in spite of local or temporary variations, an approxi- 

 mate balance has, on the whole, been maintained, for had the processes of 

 destruction been the least degree more active than those of reconstruc- 

 tion, the total amount of organic material must ultimately have gradually 

 decreased, until all further continuance of life became impossible. This is 

 a logical conclusion of more decisive value than any observations made 

 during the limited period allotted to man. The latter, however, do actually 

 show that the average percentage amounts of oxygen and carbon dioxide 

 present in the air remain approximately constant. 



