158 THE FITNESS OF THE ENVIRONMENT 



bonates the carbonic acid which is liberated 

 escapes through the lungs into the air. This 

 action is analogous to the simple heterogeneous 

 equilibrium explained above, and calls for no 

 special discussion. 



The importance of this physiological equi- 

 librium is to be sought in part in one of 

 the primary characteristics of the metabolic 

 process, — the chiefly oxidative formation of 

 acid. In the main the foodstuffs are neutral 

 substances, but their principal end products, 

 except water, are almost exclusively acid 

 compounds, — carbonic acid, phosphoric acid, 

 sulphuric acid, uric acid. Moreover, there 

 is a well marked tendency, at least in man and 

 the vertebrates, under very many pathological 

 conditions, to form other acids, such as lactic 

 acid, /3-oxybutyric acid, acetoacetic acid, etc. 

 This tendency toward acidity of reaction and 

 the accumulation of acid in the body is one 

 of the inevitable characteristics of metabo- 

 lism ; the constant resistance of the organism 

 one of the fundamental regulatory processes. 

 Now it comes about through the carbonate 

 equilibrium that the stronger acids, as soon as 

 they are formed, and wherever they are 

 formed, normally find an ample supply of 

 bicarbonates at their disposal, and accordingly 

 react as follows : — 



