PREFACE vii 



lation of neutrality (the concentration of ion- 

 ized hydrogen and hydroxyl at the neutral 

 point). 



One does not like to accept a fact of such 

 far-reaching importance as mere chance, and 

 yet no other explanation was at hand. For, 

 after the briefest consideration, it was obvious 

 that here, at least, natural selection could not 

 be involved. But it was also certain that tin's 

 is no unique instance of a property of a simple 

 substance automatically serving a very useful 

 purpose in the processes of life. Like every 

 one who has received a chemical training, I 

 was vaguely conscious of numerous other simi- 

 lar cases ; like every one who has any acquaint- 

 ance with the general properties of matter, I 

 knew that the remarkable thermal properties 

 of water are of great importance to living or- 

 ganisms. However, in spite of the fact inat 

 I had been brought face to face with a definite 

 problem whose solution now appears to he 

 perfectly patent, so great is the natural inertia 

 of the mind, and so firmly established was thf 

 belief that natural selection is, on the whole, 

 quite adequate to account for biological fit- 

 ness, that for a number of years I made no 

 further progress. 



Then, finally, after a long period of uncer- 

 tainty, came the realization of the reciprocal 



