63 HEREDITY AND ENVIRONMENT 



substances, such as strong salts or acids, and it 

 collects in non-injurious or beneficial sub- 

 stances, such as weak acids, masses of bacteria 

 upon which it feeds, etc. It avoids extremes 

 of heat and cold and if one end of a dish con- 

 taining Paramecia is heated and the other end 

 is cooled by ice, the Paramecia collect in the 

 region somewhere between these two extremes 

 (Fig. 19). Jennings, by studying carefully 

 the behavior of single individuals, established 

 the fact that this apparently intelligent action 

 is due to differential sensitivity and to the 

 single motor reaction of the animal. If in the 



1 2 



Fig. 21. Diagram of the Avoiding Reaction of Parame- 

 cium. ^ is a solid object or other source of stimulation. 1-6, 

 successive positions occupied by tlie animal. The rotation on 

 the long axis \& not shovi'n. (After Jennings.) 



