20 VICTOR E. SHELFORD. 



afternoon of a warm sunny day showed less than i degree of 

 difference between the different ponds and the readings \\ere not 

 repeated. 



(/) Excretory Materials in Solution. Dacknowski i'o6) (see 

 Cowles, 'li) found that certain unknown water soluble sub- 

 stances present in bog water are poisonous to plants. Colton 

 ('08), and authors cited by him, found that the excretory prod- 

 ucts of animals are toxic to the producer, and sometimes to 

 other organisms. This is a physiological basis for succession. 

 Knauthe states that the effect of fish on their environments is 

 important, but little of definite character is known concerning it. 



(g) Food. The food of the fishes from these ponds has not 

 been studied, but knowledge of the food habits of the same species 

 was acquired from the study of literature, especially the work of 

 Forbes and Hankinson. The species found in the ponds being 

 known, each pond was inspected with reference to the things 

 eaten by each fish species. Forbes gives the percentage which 

 each item constituted in the individuals which he studied. 



(i) Qualitative. The method of obtaining the results con- 

 sisted in adding Forbes' percentages ['80, p. 38] for the different 

 items of food for each species found in each pond. For example, 

 take the food of lake specimens of the perch. These were found 

 to have eaten fish food existing in pond i as follows: decapod- 

 rated at 14 per cent.; unidentified fish, 50 per cent.; Acan- 

 thopteri, 8 per cent., giving a total of 72 per cent. Pond i 

 contains 72 per cent, of the food of lake perch; Cyprinidae rated 

 at 28 per cent, do not occur (see Table XXII). For the youngest 

 individuals (under one inch) of all the species, all the ponds are 

 qualitatively equal. Hankinson's data on Walnut Lake species 

 show that all our ponds arc about qualitatively equal tor the 

 fish which he considers. 



An inspection of Table XXII, p. 21, shows that in no case an 

 the fish confined to the place where their food is qualitatively best, 

 in fact, as a rule, the fish are in the pond where the food is qualita- 

 tively poorest. The available data on the food of fishes shows that 

 the fish eat food available where they live, rather than that their 

 distribution is due to the presence or absence of certain foodspecies. 

 Excluding students of the food of animals, the idea that food 

 determines distribution is commonly, though erroneously, held. 



