GROUP BEHAVIOR I37 



similar size were selected for experimentation. Two 

 groups of four each were placed in separate crystal- 

 lizing dishes and eight others were isolated each into 

 a wholly similar dish. The different dishes were sep- 

 arated by black paper so that vision from one to the 

 other was impossible. A known number of the small 

 crustacean, Daphnia, were introduced daily into each 

 dish. These living Daphnia had been screened so as 

 to select the large animals only. As shown by the fig- 

 ure, fish in all groups of four ate decidedly more on 

 the first three days of the experiment. At this time 

 the two lots were shifted. Those that had been 

 grouped were now isolated, and vice versa. There 

 was an immediate shift in the numbers of Daphnia 

 taken, with the newly isolated animals now eating 

 less than the accompanying groups. This indicates 

 that we are dealing with an effect of numbers present 

 rather than with chance differences in individual 

 appetites. This difference kept up steadily until the 

 last three days of observation, when an interesting 

 complication arose. By this time the grouped fish 

 were receiving a total of over six hundred Daphnia 

 daily, including those which were eaten and the 

 extras added to insure an economy of plenty. Each 

 isolated fish was receiving only one-fourth as many. 

 Now six hundred and more large Daphnia, each 

 about an eighth of an inch long, make quite a swarm 



