288 BEHAVIOR OF THE LOWER ORGANISMS 



stimulus of the carmine grains in the water. The stimulus continues, 

 and after a time the physiological condition changes so that the animal 

 does respond. The change in physiological state can then be due only 

 to the action of the stimulus. In the same way the other changes in the 

 physiological condition of Stentor and the flatworm are evidently due 

 largely, at least, to the continued action of the stimulus. 



(7) The Physiological State may be changed by the Activity of the 



Organism. — This is demonstrated by the spontaneous changes in the 

 behavior of Vorticella or Hydra, of which we have already spoken. At 

 first the animal is in a certain condition which corresponds to extension 

 and activity. It then passes into a condition which results in contrac- 

 tion. But it does not remain contracted ; the contraction itself restores 

 the original condition, so that the animal now again extends and becomes 

 active. Certain of the changes in physiological state seen in Stentor 

 and the flatworm are probably due to the reactions of the organism. 

 Thus, we find that the flatworm, after turning for a long time away from 

 a lateral stimulus, suddenly changes and turns in the opposite direction 

 (p. 253). The change of physiological state conditioning this change of 

 reaction was probably due, not alone to the continuance of the stimu- 

 lus, but to the previous prolonged turning of the flatworm in a certain 

 direction. 



(8) External Agents cause Reaction by changing the Physiological 

 State of the Organism. — We have found that external stimuli cause 

 changes in physiological state, and that changes in physiological state 

 induce changes in behavior, — activities of a definite character. It is 

 evident, then, that external agents must change the behavior of organisms 

 by changing their physiological condition. In other words, in a reaction 

 to an external stimulus the course of events is probably as follows : 

 The stimulus causes first a change in the physiological condition of the 

 organ or organism. This, then, causes a change in behavior, which 

 we call a reaction to the stimulus. What the organism reacts to is 

 the change produced within it by the external agent. Hence, if two 

 different external agents induce the same internal change (as by block- 

 ing certain processes) they will receive the same reaction. 



(9) The Behavior of the Organism at any Moment depends upon its 

 Physiological State at that Moment. — This follows immediately from 

 the principles already developed. We have seen that both in "spon- 

 taneous" movements and in reactions to stimuh the behavior depends 

 on the physiological condition of the animal. The behavior must then 

 depend, secondarily, not only upon the present external stimulus, but 

 upon all the conditions which affect the physiological states. This 

 point will be developed under the two succeeding heads. 



