THE ANIMAL COMMUNITY 11 



of river systems and form in oxbow cut-offs or 

 bayous, or in other regions where the normal 

 deposition of materials has been retarded. Nor- 

 mally they are connected with each other during 

 the rainy season. The amount and variation 

 of animal life in them depends chiefly on the fre- 

 quency, extent and duration of the overflows. 

 They illustrate how a flexible animal community 

 adjusts itself to widely and rapidly fluctuating 

 conditions. 



Whenever the waters of a river remain long 

 outside its banks, the breeding grounds of fishes 

 and other animals are correspondingly extended. 

 The slow and stagnant waters of such an overflow, 

 frequently enriched by sewage to a limited extent, 

 furnish the best possible place for the growth of 

 myriads of algae and of the one-celled animals 

 called Protozoa. This development allows a 

 similarly great increase in the numbers of small 

 crustaceans. These animals reproduce asexually 

 and increase with tremendous rapidity under 

 favorable conditions. The sudden development 

 of food resources allows a corresponding increase 

 in the rapidly breeding fishes which feed upon 

 them; and at last the game fishes, which derive 

 their food mainly from other fishes, also increase 

 in numbers, both on account of the greater extent 

 of the breeding grounds and because of the greater 



