Production of Heritable Variations in Protozoa 101 



in the inherited effects. Now when both sets were placed at 

 intermediate temperatures, the set that had been .kept at the 

 higher temperature divided less rapidly than the set that 

 had been kept at low temperature. This difference per- 

 sisted for as long a period as the stocks were retained,' 

 about two months. 



Other evidence showed that the high temperature grad- 

 ually injured the stock, so that in the course of time the 

 high temperature set came to divide less rapidly even while 

 subjected to high temperature. At the end of six months 

 those kept at the high temperature all died out, while the 

 other set was still vigorous. The persistent low fission rate 

 of the high temperature set when restored to normal tem- 

 perature was apparently a manifestation of this injury. 

 The latter, whatever its nature, was inherited in the vege- 

 tative reproduction. 



Dallinger did not determine how long the acquired re- 

 sistance to heat would have lasted if his animals had been 

 restored gradually to lower temperatures ; and no study for 

 long periods of the permanence of the effects observed by 

 him was made by Middleton. But this matter has been 

 studied particularly in Paramecium, by Jollos (1913 a, 

 1914). He attempted to accustom Paramecium caudatum 

 to higher temperatures, and to increased concentration of 

 certain compounds of arsenic. Some races resisted acclima- 

 tization completely. In others after long periods the ani- 

 mals could stand somewhat higher temperatures or higher 

 concentrations than before. But when they were returned to 

 the normal conditions, they lost their acquired immunity al- 

 most at once. 



In other cases the animals acquired a resistance to poisons 

 which was retained by their descendants for many genera- 

 tions. Thus, in a certain race B the animals were killed 



