ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 297 



reacts positively merely to the localized change, not to the nature of 

 the change. Certain colorless infusoria, and the white Hydra, react 

 to light in such a way as to gather at the lightest side of the vessel 

 containing them. There is no evidence that the light itself is beneficial 

 to them, but their reaction does aid them in obtaining food, since their 

 prey gathers on the lightest side of the vessel. The collecting of Para- 

 mecia in CO 2 can hardly be considered to favor directly the life processes 

 of the animals, but it apparently aids them to obtain food. The sea 

 urchin tends to remain in dark places, and light is apparently injurious 

 to it. Yet it responds to a sudden shadow falling upon it by pointing 

 its spines in the direction from which the shadow comes. This action 

 is defensive, serving to protect it from enemies that in approaching may 

 have cast the shadow. The reaction is produced by the shadow, but it 

 refers, in its biological value, to something behind the shadow. 



In all these cases the reaction to the change cannot be considered 

 due to any direct injurious or beneficial effect of the actual change itself. 

 The actual change merely represents a possible change behind it, which 

 is injurious or beneficial. The organism reacts as if to something else 

 than the change actually occurring; the change has the function of a 

 sign. We may appropriately call stimuli of this sort representative 

 stimuli. 



This reaction to representative stimuli is evidently of thegreatest value, 

 from the biological standpoint. It enables organisms to flee from injury 

 even before the injury occurs, or to go toward a beneficial agent that is 

 at a distance. Such reactions reach an immense development in higher 

 animals ; most of our own reactions, for example, are to such representa- 

 tive stimuli. Only as we react to actual physical pain or pleasure do we 

 share with lower organisms the fundamental reaction to direct injury or 

 benefit. Practically all our reactions to things seen or heard are such 

 reactions to representative stimuli. While such behavior plays a much 

 larger part in higher than in lower organisms, the existence of reactions 

 to representative stimuli even in the low organisms considered in the 

 present work is an evident fact. 



How can we account for such reactions ? It is perhaps worth while 

 to point out that the operation of the law of the resolution of physiologi- 

 cal states, set forth on page 291, would result naturally in the production 

 of such reactions. Let us take as the simplest possible case the reaction 

 of Euglena when its colorless anterior tip is shaded. Since it is only 

 the metabolism of the chlorophyll bodies that is blocked by shade, we 

 cannot suppose that the shading of the colorless tip actually interferes 

 with the life processes. Yet to this change Euglena reacts negatively. 

 We may suppose that the shading of this colorless part induces the indif- 



