THE EVOLUTION OF ANIMAL COMMUNITIES 67 



fermentation which produces acid. The acidity 

 reaches a maximum at the end of about three 

 days and a slow trend toward alkalinity follows; 

 at this time a jelly-like "zooglea" begins to be 

 formed at the surface and gradually increases in 

 thickness until fragments fall to the bottom. 

 As soon as the bacteria have become numerous 

 and their action on the hay "tea" has put a part 

 of it in a form available for animal life, there 

 occurs a great growth of Protozoa, which have 

 been shown time and again to run through approx- 

 imately the following course first reported by 

 Woodruff of Yale: the description of one of his 

 typical series of cultures will furnish the basis 

 for the following account. 



The first animals to appear in numbers are the 

 flagellated protozoans known as monads (Fig. 3). 

 These may come in as early as the second day of 

 the life of the culture and reach a maximum on 

 the third to ninth day, after which they decline 

 rapidly in numbers and frequently become extinct 

 so far as active forms at the surface are concerned 

 although they may live on in reduced numbers 

 for two months. In every case these monads are 

 the first to appear and the first to reach a maxi- 

 mum. This is explained by the fact that these 

 forms feed directly on the bacteria and also upon 

 the food products which the bacteria release into 



