Production of Heritable Variations in Protozoa 99 



temperature by only two degrees each month. At 73 

 many died, but as the temperature remained at this point 

 for two months, the remainder recovered their vigor. At 

 78 a critical point was reached; as many of the animals 

 died, the temperature was lowered to 77 till there was 

 recovery, and by repeatedly alternating the temperature 

 between these two points, in eight months the animals lived 

 healthily at 78. They now underwent a most interesting 

 visible change ; the protoplasm became filled with small 

 vacuoles. These continued for a month or two, then the 

 vacuoles disappeared. Now the temperature could be far- 

 ther increased; in three months it was raised to 80 F. 



By a continuation of this slow and painful process the 

 animals were finally brought to live vigorously at a tem- 

 perature of 158 F. (70 C.). There were repeated critical 

 points, at which the animals had to be kept for months 

 before further advance could be made. In several of these 

 there was a renewed formation of vacuoles in the protoplasm, 

 the vacuoles finally disappearing. After these periods the 

 raising of the temperature could continue more rapidly. 

 To bring the animals to 158 degrees, seven years were re- 

 quired. The experiment was then most unfortunately ended 

 by an accident. 



No such long continued experiment has ever been carried 

 through since this work of Dallinger. It is clear that not 

 only had the organisms of a given generation been changed, 

 but they transmitted the change to their offspring. For at 

 the end of any period of 24 hours a totally new generation 

 was present. At the beginning of the experiment all the 

 individuals were destroyed by a temperature of 78 ; while 

 at the end they lived and flourished at a temperature above 

 150. 



Yet it is to be remembered that even at the beginning 



