162 THE FITNESS OF THE ENVIRONMENT 



views may be, the common physiological 

 processes, and the structural conditions which 

 depend for their integrity upon constancy of 

 the concentration of hydrogen and hydroxyl 

 ions are certainly manifold. 



The principal conditions and processes, 

 both inorganic and organic, which rest upon 

 the acid nature of carbonic acid and its char- 

 acteristic distribution between the atmos- 

 phere and aqueous solutions have now been 

 indicated. In their origin at least they are 

 nowise due to the agency of organic evolution. 

 Yet directly, because of the nature of carbon 

 dioxide as a gas, because of its solubility in 

 water, and on account of the precise degree 

 of its weakness as an acid, they possess the 

 highest possible efficiency. This conclusion 

 might be established with rigorous accuracy 

 by means of a mathematical analysis, but the 

 above discussion is sufficient for the present 

 purpose. 1 



In this manner carbonic acid shows itself 

 in its physico-chemical traits variously fitted 

 for the organic mechanism. Less various, to 

 be sure, and less obvious than those of water, 

 such fitnesses as it does possess are quite as 



1 Henderson, American Journal of Physiology, XXI, 173, 

 1908. 



