156 THE FITNESS OF THE ENVIRONMENT 



librium. The process is complicated by the 

 intervention of all the other acids and bases 

 of the body. Of these, however, only phos- 

 phates, and in lesser degree proteins, are im- 

 portant. Thus it is certain that in the one 

 universal chemical equilibrium of protoplasm 

 which has thus far been defined and quantita- 

 tively described the carbonates take a prin- 

 cipal part. 



It is not possible to explain the significance 

 of carbonic acid in this physiological process 

 as chiefly an adaptation ; for natural selection 

 can have nothing to do with the occurrence of 

 carbonic acid in the living organism, or, pre- 

 sumably, with the nature of the original 

 living things upon the earth. 1 The presence 

 of carbonic acid is inevitable, and whatever 

 the first forms of terrestrial life may have 

 been, certain it is that carbonic acid was one 

 of the constituent substances. From that day 

 to this it has steadily fulfilled the function of 

 regulating the reaction of protoplasm, and of 

 body tissues and fluids. 



The recent studies of Hasselbach and Lunds- 

 gaard 2 indicate that the hydrogen ion concen- 

 tration of normal blood at body temperature 



1 It was this obvious fact which originally led me to a re- 

 consideration of fitness. 



2 " Biochemische Zeitschrift, Vol. 38, p. 77, 1912. 



