THE ARGUMENT 271 



be of biological importance 

 or which appear to be related 

 to complexity, regulation, and 

 metabolism. 



IX. There are no other compounds which 

 share more than a small part 

 of the qualities of fitness of 

 water and carbonic acid ; no 

 other elements which share 

 those of carbon, hydrogen, and 

 oxygen. 

 X. None of the characteristics of these 

 substances is known to be 

 unfit, or seriously inferior to 

 the same characteristic in any 

 other substance. 



XI. Therefore the fitness of the environ- 

 ment is both real and unique. 



In drawing this final conclusion I mean to 

 assert the following propositions : — 



I. The fitness of the environment is one 

 part of a reciprocal relationship of which the 

 fitness of the organism is the other. This 

 relationship is completely and perfectly recip- 

 rocal ; 1 the one fitness is not less important 



1 This is not to be understood as an assertion that the rela- 

 tionship is symmetrical. The fact is that each organism fits 

 its particular environment, while the environment in its most 

 general and universal characteristics fits the most general and 

 universal characteristics of the organic mechanism. 



