Jennings, Behavior of Paraviecuim. 465 



conditions now remain constant, it soon ceases to react. The 

 length of time the reaction may continue after the change is 

 completed varies with different agents, becoming longer as the 

 agent is more powerful. The phenomena may be expressed in 

 the following somewhat indefinite way : the animal reacts to the 

 change as long as its effect as a change continues. In the limit- 

 ing case of a stimulus so powerful as to be destructive, the re- 

 action may continue for a considerable period, till death inter- 

 venes. In such cases we have then a continued reaction to a 

 condition that remains constant for some time. But with de- 

 structive agents, the action of the agent seems progressive, so 

 that there is really a continual change in the relation of the or- 

 ganism to the agent, till the progressive series of changes ends 

 in death. Whatever the explanation in these rare cases of de- 

 structive conditions, change is elsewhere the fundamental fea- 

 ture of the stimuli producing the chief reactions in Paramecium. 

 This is the result which stands out clearly from all my work on 

 stimulation in Paramecium. 



A change from one condition to another produces a reac- 

 tion when neither the preceding nor the following condition, 

 acting continuously, produces any such effect. Thus, Parame- 

 cia may live and behave normally in water at 20° or at 30°, yet 

 a change from one to the other, or a very much less marked 

 change, produces the avoiding reaction. Paramecia may live 

 without reaction in tap water or in water containing one-tenth 

 per cent sodium chloride, but the change from the former to 

 the latter produces the avoiding reaction. This relation could 

 be illustrated by innumerable cases, taken from my earlier pa- 

 pers on Paramecium. 



In all cases of course a certain amount of change is required 

 in order to produce reaction ; in other words, there is a certain 

 necessary threshold of stimulation. Since the change itself is 

 the real cause of the reaction, it is probable that the amount of 

 change necessary will bear some definite relation to the inten- 

 sity of action of the agent in question before the change. In 

 other words, it is probable that the reactions are subject to 

 Weber's law, as they are known to be in bacteria (Pfeffer, 



