208 ERNEST WARREN. 



about ten minutes) heating the water in which the Daphnia 

 were placed. 



The ends of the temperature curves in the neighbourhood 

 of 33° C. and 35° C. give no indication of being influenced 

 in any way by the approaching point of heat -rigor. Clearly 

 the curves would end practically abruptly against the verti- 

 cal through this point. The temperature at which heat- 

 rigor occurs introduces a new order of things^ which is 

 probably distinct from the influence of temperature in 

 accelerating the time of salt-rigor. 



/^ 3. The Effect of the Physiological Condition of 

 the Animal on the Rate of Killing. 



It was noticed by repeated trials that out of a number of 

 Daphnia taken from the tank and placed into "2 per cent. 

 NaCl some 75 per cent, died in 4 days. The individuals 

 which survived were removed and placed separately in tum- 

 blers of fresh water, and broods were obtained. When the 

 offspring had grown up they were put into '2 per cent. NaCl, 

 and it was found that all the individuals survived 4 days' 

 immersion. These individuals had been bred in a fairly 

 constant temperature of 20° C. The question now to answer 

 was, whether this strong resisting power was inherited from 

 the parents, or whether it was due to the change in the 

 conditions of life. That is the change from living in the large 

 tank to living in clean tumblers, and so under different con- 

 ditions with regard to food, light, temperature, etc. The 

 following observations show that the latter alternative is quite 

 sufficient to account for the result. 



The tank in which the Daphnia were living had become 

 choked with the luxuriant growth of Yallisneria, and it was 

 noticed that the Daphnia were dwindling in numbers. On 

 the morning of November 23rd most of the Vallisneria Avas 

 removed. In a few days the Daphnia were visibly increasing 

 in numbers, and in about three weeks there Avas quite a swarm. 

 Soiue of these Daphnia were now put into "2 per cent. NaCl 



