INHERITANCE 719 



centration was slightly increased, until many were killed. The few that 

 survived were restored to a weak solution and allowed to multiply. 

 Again there was an increase in concentration until most died; then a 

 restoration to a weak solution. As this continued, it was found that the 

 animals became able to tolerate higher concentrations. In three or four 

 months, the toleration was thus increased from about one percent to 

 2.5 percent of the standard solution. In other cases the tolerance was 

 raised in several months to 5 or 6 percent. 



When the organisms that had thus acquired a higher resistance to 

 arsenic were restored to water containing no arsenic, the increased resist- 

 ance was for long periods not lost. Tests at intervals showed that they 

 still retained the higher tolerance to arsenic. The acquired higher resist- 

 ance lasted in some cases for eight months or more. As the animals were 

 reproducing at about the rate of one fission daily, the acquired resistance 

 was inherited for about 250 generations. 



But during this time in water the acquired resistance to arsenic gradu- 

 ally decreased. The rate of decrease was very slow, so that in such a 

 case as that mentioned above, the tolerance had not returned to its 

 original low level until after a period of eight months. 



The course of events may be illustrated from the history of JoUos's 

 clone A of P. caudatum. In this clone the original maximum tolerance 

 to arsenic was to 1.1 percent of the standard solution. By cultivation in 

 gradually increasing concentrations for four months, the maximum toler- 

 ance was raised to 5 percent. Upon restoration to water containing no 

 arsenic, the tolerance was, in tests for successive periods after the restora- 

 tion, as follows: 6 days, 5 percent; 22 days, 5 percent; 46 days, 4.5 

 percent; 53 days, 4.7 percent; 60 days, 4.7 percent; 75 days, 4.7 percent; 

 130 days, 4.5 percent; 144 days, 4.2 percent; 151 days, 4.0 percent; 

 166 days, 3.0 percent; 183 days, 2.5 percent; 198 .days, 1.25 percent; 

 255 days, 1.0 percent. 



Thus the acquired resistance persisted in a very marked degree for 

 more than five months, but had been entirely lost at the end of eight 

 and a half months. 



Jollos acclimatized Paramecium also to high temperatures, continuing 

 the experimental cultures in some cases as long as two and a half years. 

 The tolerance to high temperatures was increased, and the increased toler- 

 ance lasted in some cases to six months after removal from the high 



