ii6 ANIMAL AGGREGATIONS 



range of biological and biochemical relationships, including even the 

 relation worked out by Kennealy (1906) between the racing record 

 for a particular distance and the length of the race. Pearl (1925) 

 finds that essentially the same equation describes the effect of crowd- 

 ing upon the rate of reproduction in Drosophila. 



The problem is obviously complicated by many factors, but it is 

 interesting and probably significant that the relationship can be ex- 

 pressed mathematically in a similar way for such a wide range of 

 phenomena. It is almost an anticlimax to have to record that physi- 

 cal disturbance due to numbers is not the only factor controlKng 

 growth in rapidly moving animals, such as fish, under crowded con- 

 ditions. The careful work of Church and of Shaw, already summa- 

 rized, demonstrates that the accumulation of waste products is also 

 effective with fish, just as a long line of evidence culminating in that 

 given by Goetsch proves that it is effective in the slower-moving 

 planarian worms. 



More recently Wilier and Schnigenberg (1927) and Kawajiri 

 (1928) have independently tested the effect of crowding on the rate 

 of growth of young trout in running water. Both report essentially 

 similar results; the work of the former will be reviewed here, since it 

 is the more comprehensive. These workers used young of the brook 

 trout during their prehatching, yolk-sac, and early feeding stages. 

 In their experiment they tested a wide range of conditions. They 

 used the same number of eggs or of young in different volumes and 

 with different surface areas, and in other tests used different num- 

 bers in the same volumes. All experiments were carried on with 

 water running at a rate of from t^.t^ to 65 cc. per second. 



Their results show that moderate crowding after hatching has no 

 adverse effect upon fish whose prehatching development has been in 

 equally crowded conditions. In fact, under these conditions, one set 

 of experiments show an apparently beneficial effect. On the other 

 hand, crowding the eggs produces definite retardation in length and 

 perhaps also in weight at hatching time. Such retardation is corre- 

 lated with the volume of water rather than with the area of the 

 screen on which the eggs rest. 



Exposure of uncrowded eggs to water that has flowed over a mass 



