§4J ACCLIMATIZATION TO EXTREME TEMPERATURES 253 



dying, but soon ceased, and after two months, the temperature 

 was raised half a degree more, and eventually to 25.5°. Here 

 the organisms began to succumb again, and it was necessary 

 repeatedly to lower the temperature slightly, and then to 

 advance it to 25.5°, until, after several weeks, unfavorable 

 appearances ceased. For eight months, the temperature could 

 not be raised from this stationary point a quarter of a degree 

 without unfavorable appearances. During several years, pro- 

 ceeding by slow stages, Dallinger succeeded in rearing the 

 organisms up to a temperature of 70° C, at which the experi- 

 ment was ended by an accident. 



In this case it is plain that the high temperature acted upon 

 the same protoplasm at the end of the experiment as it did 

 at the beginning. But while the protoplasm at the beginning 

 of the experiment was killed at 23° C, at the end it withstood 

 70°. It will be seen that, by gradual elevation of the tempera- 

 ture, Flagellata may become acclimated to a temperature of 

 water far above that which they can withstand when taken 

 directly from out of doors, and approaching that of the hottest 

 springs containing life. 



A series of experiments, less extensive than that of Dal- 

 linger, was carried on by Dr. Castle and myself ('95, pp. 

 236-240) upon the tadpoles of our common toad, Bufo lentigi- 

 nosus. Recently laid eggs were divided into two lots : one 

 lot was kept in a warm oven at a constant temperature of 2-4° 

 to 25°, others at about 15° C. Both lots developed normally, 

 but the former much the more rapidly. At the end of 4 

 weeks the point of heat-rigor was ascertained for each lot, by 

 gradually heating (in from 5 to 10 minutes) the water contain- 

 ing them, until the tadpoles showed no response to stimulus, 

 but, upon cooling, regained activity. The result was that the 

 toad tadpoles had gained an increased capacity to heat. For 

 when they were reared at a temperature of about 15° C, every 

 tadpole went into heat-rigor at 41° C, or below; whereas, 

 when they were reared at 24° to 25°, a temperature 10° 

 higher, no tadpole die3 under 43°, the average increase of 

 resistance being 3.2°. This increased capacity of resistance 

 was not produced by the dying off of the less resistant indi- 

 viduals, for no deaths occurred in these experiments during 



