464 Journal of Comparative Neurology and Psychology. 



gard to powerful acid substances and to sugar solution it makes 

 what we would call in ourselves a "mistake." In higher ani- 

 mals we recognize that the power of accurate discrimination is 

 one of the "higher" powers, becoming more secure as develop- 

 ment progresses. We cannot, therefore, be surprised that it 

 should not be perfect in so low an organism, nor that such or- 

 ganisms, through lack of discrimination of injurious and non- 

 injurious agents, often react in a way that leads to their de- 

 struction. Any organism reacting by the method of "trial and 

 error" is subject to the possibility of destruction in some of the 

 "trials." 



This method of "trial and error," based on the "avoiding 

 reaction" above described, plays a large part in the behavior of 

 Paramecium. Through it are produced the "negative" reac- 

 tions to agents of all sorts, as well as the collections formed in 

 certain chemicals, in regions of optimum temperature, and the 

 like. On the other hand, there exist certain reactions in which 

 the final relation to the environment is brought about in a more 

 direcc way — notably "positive thigmotaxis" and certain features 

 of the reaction to the electric current. These reactions will be 

 taken up later. 



II. Nature of Stimulation. 



Just what is the nature of the stimulation which produces 

 this reaction by "trial and error" in Paramecium? An exam- 

 ination of the facts shows that as a general rule the effective 

 stimuli consist of some change in the conditions, or, what is the 

 same thing to the organism, of some change in the relation of 

 the organism to the conditions. Change is the essential feature 

 in producing the chief reactions of Paramecium. 



This statement requires of course some qualification in de- 

 tail. A change may be nearly instantaneous, while the conse- 

 quent reaction of the animal of course requires time, and must, 

 therefore, continue for a certain period after the change has 

 been completed. If the animal is suddenly subjected to a one- 

 fourth per cent solution of common salt, it continues to react 

 for a short time after the instant of the change, though if the 



