HARMFUL EFFECTS OF CROWDING UPON GROWTH 113 



factors important for different animals. Thus, with Hydra, volume 

 per animal is the controlling factor because of the restriction of food 

 which it conditions. There is no stimulation or depression caused by 

 the crowding of Hydra into a narrow space; and, within reasonable 

 limits, concentration of excretory products are not effective. With 

 Planaria food is again the most important factor, but growth is in- 

 hibited by the concentration of excretion products or of stale food. 

 With the active amphibian larvae, if food is controlled, the major 

 limiting factor is furnished by the more frequent collisions in a dense 

 population or in a restricted area, and the concentration of excretory 

 products plays a wholly secondary role. 



Church (1927) extended these experiments to include the rate of 

 growth of the tropical fish Platypoecilus maculatus rubra in connec- 

 tion with other experiments upon the effect of crowding upon the 

 rate of growth of fishes. Eight liters of water were used in glass 

 aquaria, each of which contained 2, 8, or 16 fish. In each series of 

 experiments, one set of aquaria contained small fish 8-10 mm. long, 

 another set held fish 12-14 mm. long, and the third set was supplied 

 with fish 20-23 i^n^- Adult Platypoecilus range from 30 to 35 mm. 

 The amount of oxygen and the pH of the different aquaria did not dif- 

 fer significantly. The water was left unchanged during the entire ex- 

 periment, which ran in some cases as long as 70 days, except that 

 there were slight additions to replace the small amount lost by evap- 

 oration. The fish were fed the same number of Daphnia per fish 

 per day. 



Under these conditions the large fish always grew less rapidly the 

 more fish there were present in a given container. With the small 

 and medium fish there was some indicatfon of more rapid growth 

 early in the experimental periods among the fish grouped 8 to the 

 aquaria; but as the experiment progressed, the rate of growth was 

 always greatest when the fewest fish were present. Shaw (1929) has 

 repeated these experiments, with similar results. The experience of 

 these two workers demonstrates that when there is sufficient con- 

 centration of waste products the rate of growth is retarded. 



In following out the Goetsch type of experiment, Church placed 

 transparent celluloid containers in the center of each aquarium. 



