ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 301 

 B. Negative Reactions 



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In our discussion of the causes of reaction we found that we could 

 classify most stimuli into two groups — those that interfere with the nor- 

 mal life processes, and those that do not. It will be best to consider 

 separately the reactions to these two classes of stimulation, and to take 

 up the reactions to unfavorable stimuli first, since these seem to present 

 the most primitive conditions. 



The simplest reaction to unfavorable stimuli is merely a change in 

 the direction or character of the movement. The organism is moving 

 in a certain direction; when subjected to an unfavorable change, it 

 changes its direction of movement. This is the case in Amoeba, in bac- 

 teria, in infusoria, in rotifera, in the flatworm; indeed, in most free 

 organisms. The mere fact of a change is in itself regulatory or adap- 

 tive. The original behavior has brought on the unfavorable change, 

 hence the best thing to do is to change this behavior. If the unfavor- 

 able condition still persists, the behavior is changed again ; this being 

 continued, the organism is bound to escape from the unfavorable condi- 

 tions if it is possible to do so. The repeated change in behavior under 

 unfavorable stimulation is very striking in Paramecium, in Stentor, in 

 Hydra, in the flatworm, and elsewhere. 



The fundamental principle for this method of reaction is that a 

 change 0} behavior under unfavorable conditions is in itself regulatory. 

 As we have before pointed out, the reactions of organisms are based on 

 the principle, usually correct, that it is the previous behavior of the 

 organism that has brought on the present conditions. Hence if these 

 conditions are unfavorable, a change of behavior is required. 



The developments of this method of behavior found indifferent organ- 

 isms consist in defining, varying, and systematizing the changes that 

 occur. In Amoeba we find perhaps the simplest condition. When this 

 animal in its forward course meets unfavorable conditions it merely goes 

 in some other direction. In what direction it will go cannot be predicted 

 from either the structure of the organism or from the localization of 

 the stimulus, for Amoeba can move with any part in advance. It is evi- 

 dently determined by transient internal conditions. In organisms with 

 definite body axes and other structural relations, the change of motion 

 becomes more definite. In bacteria the organism moves after stimula- 

 tion in the opposite direction. In the free-swimming infusoria, as illus- 

 trated by Paramecium, and in the free Rotifera, there is an elaborate 

 system of movements which make the reaction effective. The animal 

 stops or reverses the movement which has -brought on the unfavorable 

 condition, then swings its anterior end about in a circle as it moves for- 



