242 THE FITNESS OF THE ENVIRONMENT 



CH 2 OH 



J 



CHOH 



I 



°\ /O-H 



O-P^O 



X)-H, 



a compound of phosphorus, hydrogen, and 

 oxygen with glycerine. 



Sulphur is a constituent of the proteins, 

 and occurs in many other important com- 

 pounds. In the metabolic process of the 

 animal sulphur is converted from a derivative 

 of hydrogen sulphide, H 2 S, by oxidation, into 

 sulphuric acid, H 2 S0 4 , and in the plant the 

 process is reversed. 



Iodine occurs in the thyroid and in many 

 marine organisms. The availability of this 

 element depends upon its existence in nature 

 as iodides, that is to say, upon its capacity 

 to unite with hydrogen to form hydriodic 

 acid, HI. Finally, it is the analogous com- 

 pound of chlorine with hydrogen, hydro- 

 chloric acid, which contributes acidity to the 

 gastric juice. 



This list might be much further extended, 

 but I think that the nature of the case is now 

 established. The conclusion seems inevitable 



