160 THE FITNESS OF THE ENVIRONMENT 



the concentration of hydrogen ions. Indeed, 

 these ions are the effective agents in the 

 process. 



Reactions of this type, in which carbohy- 

 drates, fats, proteins, and other substances 

 take part, make up a very large, if not the 

 largest fraction of all the processes of metabo- 

 lism, and there can be no doubt that for their 

 regulation very accurate adjustment of acidity 

 and alkalinity is essential. In the body, to 

 be sure, such reactions are under the control of 

 enzymes, but the concentration of hydrogen 

 and hydroxyl ions is not less, rather more 

 important for that reason. Beside retaining 

 their direct influence upon the reaction, the 

 ions also exert an influence upon the enzymes 

 themselves. Moreover, not only enzymes, but 

 almost all colloidal bodies, especially such 

 unstable structures as the colloids of proto- 

 plasm, are profoundly affected by changes of 

 reaction, and for the preservation of their 

 stability they also are absolutely dependent 

 upon constancy of acidity and alkalinity. 



Finally, it is to be noted that glucose, which 

 is the principal source of energy in physiolog- 

 ical processes, is very unstable in even faintly 

 alkaline solutions, and that its stability varies 

 in most marked degree with the slightest 

 change in the concentration of hydroxyl 



