240 THE FITNESS OF THE ENVIRONMENT 



account of the variety of chemical processes 

 into which they can enter. They make up 

 the active agents of inorganic chemistry, and 

 it is safe to assume that their activity depends 

 in great part upon the properties of oxygen 

 and hydrogen. 



The importance of oxygen and hydrogen 

 in inorganic chemistry possesses a double 

 significance in the present inquiry. In the 

 first place it provides further confirmation 

 of the view that the elements which make 

 up water and carbon dioxide are unique. For 

 the data of inorganic chemistry prove that 

 hydrogen and oxygen are likely to confer 

 great chemical activity wherever they are, 

 and that they are quite unrivaled in this 

 respect. Secondly, the occurrence of hydrogen 

 and oxygen as primary factors of the metabolic 

 process and as the chief constituents of the 

 environment and of the living organism enables 

 the latter to make use of other elements at 

 need. Without hydrogen and oxygen, op- 

 portunities for the introduction of such other 

 elements into the physiological processes would 

 be necessarily much restricted, and in many 

 cases the physiological utility of compounds 

 containing the elements of inorganic chemistry 

 is very great. 



Chlorophyll, for example, contains mag- 



