136 THE FITNESS OF THE ENVIRONMENT 



ment and the conditions within the organism 

 alike, that this is necessarily the case. How- 

 ever, the less conspicuous substance is not 

 without its physical fitnesses, and we must 

 now turn to them. 



SOLUBILITY 



The most obvious of the properties of car- 

 bonic acid is its all-pervasiveness. Originally 

 formed in vast quantities by the cosmic pro- 

 cess, and accumulated in the atmosphere, the 

 store has been steadily replenished there by 

 vulcanism. It is probable that of the enor- 

 mous quantities now deposited as limestone 

 in the earth's crust, a quantity sufficient to 

 yield an atmospheric pressure greater ten- 

 fold than the present atmospheric pressure, 

 only a fraction was at any one time actually 

 gaseous. For it happens that the presence 

 of carbonic acid in the atmosphere insures the 

 occurrence of greater, or at least nearly equal, 

 quantities in the ocean and in all the natural 

 waters of the earth. This is due to the solu- 

 bility of carbon dioxide, to the magnitude of 

 its absorption coefficient in water. 



The absorption coefficient is the volume 

 of gas absorbed by one liter of liquid when 



