INTEGRATION OF AGGREGATIONS 85 



wading and diving birds, and the large beetles, dragon-fly nymphs, 

 and giant water bugs feed on the young bass at every opportunity. 



An illustration of remote and unsuspected rivalries is found in the 

 relation of the black bass to the bladderwort {U tricularia) , which fills 

 many acres of the northern Illinois lakes. Upon the leaves of this 

 plant are small bladders, several hundred to the plant, which are 

 tiny traps for the capture of entomostracans and other minute ani- 

 mals. The plant usually has no roots and lives largely on the animals 

 taken through these bladders. Ten of these sacs, taken at random, 

 upon examination gave 93 animals of 26 different species, of the 

 Entomostraca and insect larvae. Hence, the bladderwort competes 

 with the fishes for food and, by destroying large amounts, helps keep 

 down the number of black bass in an otherwise favorable lake; and 

 they have an especial advantage since, when the Entomostraca be- 

 come scarce, they may grow roots and live as other plants. 



These simple instances sufhce to illustrate the intimate way in 

 which the living forms of a lake are united. 



A different phase of the story is shown by the study of fluviatile 

 prairie lakes which are appendages of river systems and form in 

 oxbow cut-offs or bayous, or in other regions where the usual deposi- 

 tion of materials has been retarded. Normally they are connected 

 with each other during the rainy period and for a longer or shorter 

 time during the summer. The amount and variation of animal Ufe 

 in them is dependent chiefly upon the frequency, extent, and dura- 

 tion of the overflows. In them we may see illustrated the method by 

 which the flexible system of the animal community adjusts itself to 

 widely and rapidly fluctuating conditions. 



Whenever the waters of a river remain for a long time outside its 

 banks, the breeding grounds of the fishes and other animals are cor- 

 respondingly extended. The slow and stagnant waters of such an 

 overflow, frequently enriched by sewage to a limited extent, form 

 the best possible place for the growth of myriads of algae and Pro- 

 tozoa. This development allows a similarly great development of 

 Entomostraca. These animals increase with tremendous rapidity due 

 to the pace at which their life-circle is run and to their high rate of 

 reproduction. The sudden development of food resources allows a 



