Jennings, Behavior of Paramcciuui. 463 



Again, the reaction at times keeps the organism in the 

 closest possible touch with the environment, continuing as long 

 as certain conditions continue, increasing in effectiveness as the 

 conditions causing it increase in intensity, and ceasing when the 

 conditions causing it cease, maintaining the organism through- 

 out in certain relations with the source of stimulation. Alto- 

 gether, I believe that the following admission must be made. 

 If we consider the reaction of Paramecium a reflex, it is because 

 we are convinced beforehand that such an organism can show 

 only reflexes. If the actions of Paramecium did belong to some 

 higher type ot behavior, there could be little objective evidence of 

 this, beyond what we already have. 



In Paramecium the reaction has not been shown to be 

 modifiable by previous experience, so that from this criterion 

 the behavior retains the characteristics of a reflex. But in a 

 close relative, Stentor, such modification by experience has 

 been demonstrated (Jennings, 1902), so that it may be presumed 

 that technical difficulties alone have thus far prevented our ob- 

 serving it in Paramecium. 



The effectiveness of the method of reacting by "trial and 

 error" that we have described above for Paramecium depends 

 upon the power of discrimination of the reacting organism. 

 By "discrimination" of stimuli we mean, in an objective study 

 of behavior, that the organism reacts differently to the different 

 stimuli in question. In this sense Paramecium discriminates 

 acids from alkalies and salts, and these again from sugar. Fur- 

 thermore, it discriminates different strengths of solution, react- 

 ing differently, for example, with relation to weak and to strong 

 acids. On tlie other hand, it does not effectively discriminate 

 different acid subs'.ances, save in so far as one is stronger than 

 another. Thus it swims into weak carbonic acid, which is harm- 

 less, and likewise into weak sulphuric acid and copper sulphate, 

 which kill it. It does not markedly discriminate a ten per cent 

 sugar solution from water, hence it swims readily into such a 

 sugar solution and is killed by the osmotic action.^ Thus in re- 



^ Details as to the facts cited are given in my previous papers on Paramecium 

 liere we are concerned only witli the interpretation of these facts. 



