254 HEAT AND PROTOPLASM [Cn. VIII 



the gradual elevation of the temperatures in the cultures. The 

 increased resistance was due, therefore, to a change in the 

 protoplasm' of the individuals. 



The question now arose : In how far is this change in the 

 protoplasm permanent? Will a return of the individuals to 

 cool water cause a return to the old point of heat-rigor ? We 

 made a few experiments on this subject which showed that 

 tadpoles which during 33 days in warm water have acquired 

 an increased resistance of 3.2 lose part of that acquired resist- 

 ance during 17 days' sojourn in cooler water. But the loss 

 is a very slow one. The effect of the high temperature on the 

 tadpoles is not, therefore, transitory, but persists we have 

 not been able to determine how long after the cause has been 

 removed. 



So we may conclude : Individual organisms have the capacity 

 of becoming adapted to a high degree of temperature, so that a 

 temperature which normally is fatal may be withstood. This 

 adaptation of the individual accompanies the subjection of 

 organisms to temperatures higher than those to which they 

 have already become accustomed. This capacity exists among 

 both Protozoa and Metazoa. The effect of the elevated tem- 

 perature persists (though in diminished degree) a considerable 

 time after the individual has been restored to a lower tempera- 

 ture. 



Acclimatization may show itself not only in the change of 

 the maximum temperature, but also in the elevation of the 

 optimum. This is shown by the following experiments of 

 MENDELSSOHN ('95, p. 19). When Paramecia are placed in a 

 trough whose temperature is 24 to 28 at one end and 36 to 

 38 at the other, they are found to collect at the cooler end, 

 which indicates that the temperature of that end lies nearer 

 their optimum. If, however, the Paramecia, while uniformly 

 distributed in the trough, are subjected to a uniform tempera- 

 ture of 36 to 38 for from 4 to 6 hours, and then, in the same 

 trough, to a temperature varying from 24 at one end to 36 at 

 the other, they no longer collect at the usual optimum of 24 to 

 28, but at 30 to 36. Thus in 4 to 6 hours, by the action of 

 a temperature of 36 to 38, the optimum has been raised 6 

 to 8. 



