v] THE SOURCES OF FOOD 123 



others (the anaerobic bacteria) can act in the absence 

 of oxygen except that which is contained in the sub- 

 stance which they are attacking. When putrefaction 

 takes place in the absence of oxygen the breakdown 

 is much more complex and evil-smelling compounds 

 are produced : in the presence of oxygen the process 

 of putrefaction is more rapid. The bacteria of putre- 

 faction and fermentation have a mode of nutrition 

 which is very similar to that of the saprozoic animals, 

 but they are living among abundance of food and 

 their metabolism is very wasteful. That is to say 

 they break down a large quantity of proteid or 

 carbohydrate in order to obtain a small quantity of 

 energy; and the total amount of bacterial life in a 

 fermenting or putrefying mixture is far less than 

 could be sustained if the latter were used to the 

 greatest advantage. Now we cannot say with con- 

 fidence that the bacteria are either plants or animals 

 so we hedge, and speak of their mode of nutrition 

 as metatrophic. 



In the long run the products of bacterial action 

 on organic matter are mainly ammonia, carbonic acid, 

 water, sulphuretted and phosphoretted hydrogen. 

 Now the urea excreted by animals undergoes change 

 to ammonia and carbonic acid, either by means of an 

 enzyme, or by bacterial action, so that the excretory 

 products formed by an animal during its life are in 

 the long run the same as those which are formed 



