132 THE FITNESS OF THE ENVIRONMENT 



any other substance which, however slightly, 

 can claim to rival water as the milieu 1 of 

 simple organisms, as the milieu interieur of 

 all living things, or in any other of the count- 

 less physiological functions which it performs 

 either automatically or as a result of adapta- 

 tion. 



In truth Darwinian fitness is a perfectly 

 reciprocal relationship. In the world of 

 modern science a fit organism inhabits a fit 

 environment. 



1 Here and elsewhere the word " milieu " has been employed 

 when it is desired to express only that part of the conception 

 of environment which is involved in the literal meaning of 

 the word, leaving to environment the added significance of 

 that which provides food. 



