THE BEHAVIOR OF INFUSORIA; PARAMECIUM 99 



of other stimuli. The behavior depends not only on the stimulus to 

 which it is primarily reacting, but also upon other external conditions. 

 May the nature of the behavior also depend upon internal conditions? 

 In other words, may the same animal under the same external conditions 

 behave differently at different times? May Paramecium, like higher 

 animals, become modified by the stimuli which it has received, or by 

 its own reactions, so as to react for the future in a manner different from 

 its reactions in the past? 



It is difficult to obtain evidence on this question for Paramecium, 

 because the animal moves about so rapidly that it is hardly possible to 

 follow a given individual and determine whether its reactions do or do 

 not change. Much more is known in regard to this matter, as we shall 

 see later, for the fixed infusorian Stentor. But a number of significant 

 facts have been brought out for Paramecium. 



First we have the fact that the presence of a certain agent or condi- 

 tion may alter the method of reaction to another. Paramecia in 

 heated water react to solids by the avoiding reaction in place of the 

 positive contact reaction; Paramecia in a solution of carbon dioxide, 

 on the other hand, are much more likely to respond by the positive 

 contact reactions. Many conditions heat, cold, chemicals, mechani- 

 cal shock, etc. alter, as we have seen, the reaction to gravity, causing 

 the animals to swim downward instead of upward. Such phenomena 

 indicate that the first agents alter in some way the physiological con- 

 dition of the animals, so that they now react to the second agent in a 

 changed manner. This conclusion is impressed upon the observer 

 by the behavior of the organisms. Specimens in heated water are 

 swimming about violently, so that we should not expect them to come 

 to rest against solids. Those in carbon dioxide move slowly and seem 

 in a condition predisposing to repose, so that coming to rest against 

 solids is the reaction that might be anticipated. The interference 

 between the two stimuli is not purely physical. There is nothing in the 

 physical action of heat or a mechanical jar to make the animals move 

 downward, as happens when the agents reverse the action to gravity. 

 Indeed, in the latter case one can plainly see that the downward move- 

 ment is an active one. The only explanation possible for such cases 

 is that the animals have become changed in some way by the first stimu- 

 lus, so that they now react in an altered manner to the second stimulus. 



Further we find that there are great differences in the reactions of 

 different individual Paramecia, and especially of Paramecia from 

 different cultures. In studying the reactions to chemicals, one often 

 finds that a few individuals swim directly into the given solution, while 

 the majority give the avoiding reaction on coming in contact with it, 



