ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 295 



an increase. Stentor reacts to an increase of illumination, not to a 

 decrease. Euglena when moderately lighted reacts negatively to a 

 decrease of illumination, not to an increase ; if strongly lighted, it shows 

 the opposite relations. Paramecium reacts at passing into an alkaline 

 solution, but not at passing out ; it reacts at passing out of a weak acid 

 solution, not at passing in. Hydra at 24 degrees reacts to an increase 

 of 2 degrees in temperature, not to an equivalent decrease. Innumerable 

 instances of this fact could be given from the behavior of the lower 

 organisms. 



What decides whether a given change or its opposite shall produce 

 this negative reaction? Examination of the facts brings out the follow- 

 ing relations : The organism generally reacts by a change in its behavior 

 when the change is of such a nature as to lead away from the optimum. 

 By optimum we mean here the conditions most favorable to the life 

 processes of the organism in question. Changes leading toward this 

 optimum produce in many animals no reaction; the organisms simply 

 continue the activity which has brought about this change. Changes 

 leading away from the optimum produce a negative reaction, by which 

 the organism is removed from the operation of this change. There are 

 undoubtedly some limitations and exceptions to this, and with these we 

 shall have to deal later, but, as we have seen for Paramecium, it is un- 

 questionably the rule. Cases where this rule does not hold are striking 

 because exceptional. Reaction in this manner keeps the infusoria in 

 regions of moderate temperature, prevents them from entering injurious 

 chemical substances, brings green organisms such as Euglena into the 

 light, where their metabolic activities are aided, and in general keeps 

 the organisms in regions where the conditions are favorable. In these 

 organisms the chief cause of reaction to a change is its interference with 

 the normal life activities, and the reaction if successful serves to remove 

 the interference. 



C. But in many cases changes which favor the normal activities 

 produce reaction. The response is then of such a character as to retain 

 the organism under the conditions producing the change. Such re- 

 sponses we usually call positive reactions. In many cases it is clear that 

 such reactions are determined by a previously existing unfavorable 

 state of metabolism or of other processes. The Hydra or the sea 

 anemone does not react positively to food substances unless metabolism 

 is in such a state as to require more material; and parallel relations 

 exist in the behavior of many if not all organisms. In unicellular or- 

 ganisms definite positive reactions play a comparatively small part, 

 favorable conditions being secured primarily by a negative reaction to 

 less favorable conditions. It is possible that all positive reactions are 



