220 THE FITNESS OF THE ENVIRONMENT 



That the very elements which make up water 

 and carbonic acid, and apparently they alone, 

 should possess this wonderful property is, 

 when taken together with the physical prop- 

 erties of water and carbonic acid and their 

 place in cosmic evolution as constituents of 

 the atmosphere, a fact which cannot lightly 

 be set aside. 



Not less valuable for the organism than the 

 multiplicity of organic substances, and the 

 diversity of their properties, are the great 

 variety of chemical changes which they can 

 undergo, and that characteristic instability 

 which renders such great complexity of chem- 

 ical behavior easily attainable. In short, 

 organic substances are uniquely fitted not 

 only to provide complexity of structure to the 

 organism, but also, through their instability 

 and manifold transformations, to endow it 

 with diverse chemical activities, with com- 

 plexity of physiological function. 



One factor in determining the complexity 

 of chemical changes which organic chemical 

 substances manifest is the enormous number 

 of simple structural relationships which every 

 substance bears to others. This may be 

 readily illustrated by the formulas of some of 

 the derivatives of propane which possess 

 biological importance : — 



