ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 309 

 G. Positive Reactions 



We have thus far dealt primarily with reactions to environmental 

 conditions that interfere with the normal hfe processes. We find that 

 these induce changes in behavior, subjecting the organism to new con- 

 ditions, the more favorable one of which is selected. This gives us a 

 basis for the understanding of reactions toward conditions which favor 

 the normal life processes, — that is, positive reactions. 



In conditions that are completely favorable — so that all the life 

 processes are taking place without lack or hindrance — there is of 

 course no need for a change in behavior, for definite reactions of any 

 sort. The most natural behavior on reaching such conditions, and that 

 which is actually found as a general rule among lower organisms, is a 

 continuation of the activities already in progress. These activities have 

 resulted in favorable conditions, hence it is natural to keep them up; 

 there is no cause for a change. This we find strikingly exemplified in 

 bacteria, infusoria, rotifers, and many . other organisms under most 

 classes of stimuli. A change in behavior takes place only when the ac- 

 tivities tend to remove the organism from the favorable conditions. 

 Unfavorable conditions cause a change in behavior ; favorable condi- 

 tions cause none. It is perhaps a general rule in organisms, high or 

 low, that continued completely favorable conditions do not lead to defi- 

 nite reactions. Of course while the external conditions remain the same, 

 the internal processes may change in such a way that these conditions 

 are no longer favorable, and now the behavior may change. 



But when the organism is not completely enveloped by favorable 

 conditions, but is on the boundary, if we may so express it, between favor- 

 able and unfavorable ones, then there is often a definite change in the 

 behavior leading toward the favorable conditions, — a positive reaction. 

 To understand such reactions, we may start from the fact that unfavor- 

 able internal conditions (as well as external ones) cause a change of 

 behavior. The Hydra or sea anemone whose metaboUc processes are 

 interfered with by lack of material, exchanges its usual behavior for 

 activities of a totally different character, setting forth on a tour of ex- 

 ploration. It is a general fact that the hungry animal sets in operation 

 trains of activity differing from the usual ones. Interference with respi- 

 ration or with other internal processes has similar effects. An increase 

 of temperature above that favorable for the physiological processes like- 

 wise starts violent activities. Indeed, it is a general rule that changes 

 of internal condition unfavorable to the physiological processes set in 

 operation marked changes in behavior. 



But the activities thus induced are in themselves undirected, save 



