548 



animal substance in the water, including mollusks and decomposing 

 organic matter. The insects taken by the bass, themselves take other in- 

 sects and small Crustacea. The crawfishes are nearly omnivorous, 

 and of! the other crustaceans some eat Entomostraca and some algae 

 and Protoza. At only the second step, therefore, we find our bass 

 brought into dependence upon nearly every class of animals in the 

 water. 



And now, if we search for its competitors we shall find these also 

 extremely numerous. In the first place, I have found that all our young 

 fishes except the Catostomidae feed at first almost wholly on Entomos- 

 traca, so that the little bass finds himself at the very beginning of his 

 life engaged in a scramble for food with all the other little fishes in the 

 lake. In fact, not only young fishes but a multitude of other animals 

 as well, especially insects and the larger Crustacea, feed upon these 

 Entomostraca, so that the competitors of the bass are not confined to 

 members of its own class. Even mollusks, while they do not directly 

 compete with it do so indirectly, for they appropriate myriads of the 

 microscopic forms upon which the Entomostraca largely depend for 

 food. But the enemies of the bass do not all attack it by appropriating 

 its food supplies, for many devour the little fish itself. A great variety 

 of predaceous fishes, turtles, water-snakes, wading and diving birds, 

 and even bugs of gigantic dimensions destroy it on the slightest oppor- 

 tunity. It is in fact hardly too much to say that fishes which reach ma- 

 turity are relatively as rare as centenarians among human kind. 



As an illustration of the remote and unsuspected rivalries which re- 

 veal themselves on a careful study of such a situation, we may take the 

 relations of fishes to the bladderwort'- — a flowering plant which fills 

 many acres of the water in the shallow lakes of northern Illinois. Upon 

 the leaves of this species are found little bladders — several hundred to 

 each plant — which when closely examined are seen to be tiny traps for 

 the capture of Entomostraca and other minute animals. The plant usual- 

 ly has no roots, but lives entirely upon the animal food obtained through 

 these little bladders. Ten of these sacs which I took at random from 

 a mature plant contained no less than ninety-three animals (more than 

 nine to a bladder), belonging to twenty-eight different species. Seventy- 

 six of these were Entomostraca, and eight others were minute insect lar- 

 vae. When we estimate the myriads of small insects and Crustacea 

 which these plants must appropriate during a year to their own support, 

 and consider the fact that these are of the kinds most useful as food for 

 young fishes of nearly all descriptions, we must conclude that the blad- 

 derworts compete with fishes for food, and tend to keep down their 

 number by diminishing the food resources of the young. The plants 

 even have a certain advantage in this competition, since they are not strict- 

 ly dependent on Entomostraca, as the fishes are, but sometimes take 

 root, developing then but very few leaves and bladders. This probably 

 happens under conditions unfavorable to their support by the other 



'Utricularia. 



