234 THE FITNESS OF THE ENVIRONMENT 



be built up into the tissues of the animal, 

 they must now be turned back into such fats, 

 carbohydrates, and proteins as are character- 

 istic of his physical structure ; into glycogen, 

 haemoglobin, fibrinogen, etc. Accordingly, 

 they undergo a process which is the exact 

 reverse of the digestive change, in the simple 

 case: — 



H-C-0-H + H-0-CH 3 = H-C-0-CH 3 + H-0-H 



II II 



o o 



But this is by no means the end of the matter. 

 For example, glycogen thus formed in the 

 liver from the glucose of the portal blood is 

 soon torn down to glucose again. More- 

 over, there are a host of other special cases 

 of the same hydrolytic cleavage, or the re- 

 verse process, in mammalian physiology. For 

 instance, the formation of hippuric acid from 

 benzoic acid and glycocoll, and the formation 

 of urea itself from ammonium carbonate 

 belong to this same class. In fact, such 

 reactions make up a large part of all the 

 chemical changes which take place within 

 the organism. 



It must not be imagined, however, that 

 hydrolytic cleavages are infrequent outside 

 the organism, or that the types of processes 



