126 ANIMAL AGGREGATIONS 



both efifects may be attributed to the electrical charge carried by the 

 paraffin. 



Warren (1900), in his work on the effect of crowding on Daphnia, 

 had previously found that media in which excretory products are 

 allowed to accumulate cause a decrease in the number of genera- 

 tions and the number of offspring in a brood, and that reproduction 

 ceases long before the animals die. Such water is injurious, though 

 not usually fatal to fresh Daphnia; and the reproductive power of 

 the newly introduced Daphnia is soon reduced. The injurious nature 

 of the water seems to pass off after a sufficiently long period. 



Our experience in growing Daphnia in quantity for fish food in a 

 considerable volume of water, of perhaps 10-100 liters, accords with 

 the experimental results of Warren. Events run as follows: A month 

 or 6 weeks after having stocked such an aquarium with a few Daph- 

 nia, conditions being favorable, several hundreds of animals may 

 be living in good condition and reproducing. Then suddenly a change 

 begins. The greater number die, young and old alike. Perhaps from 

 I to 3 per liter survive, and these will live for months without pro- 

 ducing eggs. After a very considerable time eggs are formed and 

 Daphnia may become fairly plentiful again, but the second swarm 

 is never as numerous as the first. During the time when the Daphnia 

 have ceased to reproduce and have, for the most part, died off, the 

 water may be teeming with other entomostracans, ostracods or cope- 

 pods.- This indicates a certain specificity in the effect of the Daphnia 

 metabolic wastes. The duration of the period of depression of repro- 

 duction is greatly shortened by keeping the food value of the medium 

 at a high level. 



EFFECT OF CROWDING ON RATE OF EGG-LAYING OF HENS 



The effect of density of population upon rate of reproduction in a 

 different medium and with animals far removed in habits and in the 

 evolutionary scale from Protozoa or Entomostraca was reported by 

 Pearl and Surface (1909) from the experiments of Professor Go well 

 of the Maine Agriculture Station. These men report the result of 

 investigations concerning egg production extending over several 

 years. The chickens studied were kept in pens containing 50, 100, 



