THE EVOLUTION OF ANIMAL COMMUNITIES 73 



extinct in from eighteen to eighty-seven days. 

 At the surface of the cultures they run their cycle 

 after the Paramecia have completed theirs and 

 before the Vorticella are through. 



The following summary of this sequence of 

 protozoans in hay infusions gives the essential 

 facts : 



Relatively few protozoans are to be found at 

 any time at the center of the cultures. Aside 

 from Amoeba, the animals we have been con- 

 sidering are essentially surface forms. After 

 standing for some months the culture becomes 

 almost balanced, that is, it reaches a climax con- 

 dition where life is fairly stable. Green filamen- 

 tous algae appear at the bottom and at the end 

 of three years almost all these animals mentioned 

 may be found in small numbers, chiefly at the 

 bottom among the algae. Often, however, all 

 of these protozoans disappear completely from 

 the cultures as active forms, and only rotifers 

 and small crustaceans are left. These many- 



