718 INHERITANCE 



In a famous investigation by Dallinger (1887), published fifty-two 

 years ago, three species of flagellates were acclimatized, in seven years, 

 to a high degree of heat. At first the organisims could not tolerate a 

 temperature higher than 26'^' C. If subjected to this temperature for some 

 time, death occurred. By raising the temperature a half degree at a time, 

 at the average rate of two degrees a month, the tolerance was in the 

 seven years raised to 70° C. 



When the temperature was raised at any period, usually many of the 

 individuals died. Others lived and multiplied, replacing those that died. 

 There was thus a selective action of the heat; the individuals that did 

 not become acclimatized died. 



At certain periods, the cytoplasm of the organisms became filled with 

 small vacuoles, which lasted for a month or more, then disappeared. I'he 

 temperature could not be raised further until the vacuoles had disap- 

 peared. It has been suggested that this vacuolization is a process of getting 

 rid of water, since it is known that protoplasm containing little water 

 can tolerate higher temperatures than protoplasms containing much 

 water. 



In cases of acclimatization to high temperatures, it is obvious that all 

 parts of the organism must be altered. If any part — nucleus, chromo- 

 somes, cytoplasm — failed to acquire the increased resistance to heat, the 

 organisms would die. 



Extensive experimental studies in this field were carried on for many 

 years by Jollos (1913, 1920, 1921). He investigated the acclimatization 

 of Paramecium candatum to arsenic (solutions of arsenious acid), to 

 certain other chemicals, and to high temperatures. 



The most extensive work was on acclimatization to solutions of arsenic. 

 It was found that P. aurelia could not be acclimatized to arsenic, and that 

 some biotypes of P. caudatum were likewise refractory. But in other 

 biotypes of P. caudatum acclimatization was successfully produced. 



In the successful experiments, a method combining selective action 

 with subjection to gradually increasing concentrations of arsenic was 

 employed. Using as a standard concentration a one-tenth normal solution 

 of arsenious acid, the animals were first placed in a very weak concen- 

 tration, the maximum tolerance at the beginning being about one per- 

 cent of the standard solution. The organisms were left for a long time 

 (days or months) in a solution too weak to destroy them. Then the con- 



