246 THE FITNESS OF THE ENVIRONMENT 



yields upon oxidation; carbon is surpassed 

 by but one other element, boron. Although 

 necessarily a good deal of this heat cannot be 

 stored in the compounds of the two elements 

 which still contain some oxygen, yet enough 

 remains to make the common constituents 

 of the organism greater reservoirs of energy 

 than most of the other elements themselves, 

 far greater than compounds of any other 

 elements. Thus the heat of combustion of 

 carbohydrates ranges from about 3.7 calories 

 to about 4.2 calories per gram, that of the 

 proteins from about 5 calories to about 6 

 calories, that of the fats from about 9.2 calo- 

 ries to about 9.5 calories. On account of the 

 small quantity of oxygen and the large quan- 

 tity of hydrogen which they contain, the fats 

 are a richer source of energy than carbon 

 itself, or than any other element except hydro- 

 gen and boron. 



There remains one other equally important 

 consideration to be dealt with: the very great 

 energy change which is involved in processes 

 of oxidation and reduction compared with 

 other chemical processes. The following 

 table, showing the amount of heat liberated 

 in the process of formation of certain binary 

 compounds from their several elements, illus- 

 trates the case: — 



