It8 THE BEHAVIOR OF LOWER ORGANISMS. 



as we found in Stentor and the flatworm. Thorough investigation of 

 any of these organisms from this point of view would doubtless bring 

 to light a variety of physiological conditions on which the reactions 

 depend. 



CHANGES IN THE SENSE OF REACTIONS WITH CHANGES 

 IN THE INTENSITY OF THE STIMULUS. 



Must we not bring under the same point of view the well-known 

 phenomenon of a change in the sense of the reaction with a change in 

 the intensity of the stimulus ? As a simplest case of this we may take the 

 reaction of Stentor to mechanical stimuli. As shown in the ninth of 

 my studies (Jennings, 1902), Stentor reacts to a very weak mechani- 

 cal stimulus on one side of the disk by bending toward the source of 

 stimulus; to a stronger but otherwise similar stimulus it responds by 

 contracting into the tube, or (later) by bending in another direction. 

 In the same way the flatworm reacts positively to a weak mechanical 

 or chemical stimulus, negatively to a stronger one. How can we ex- 

 plain these opposite reactions to stimuli of the same quality, differing 

 only in intensity? 



We have here, it seems to me, the same phenomenon shown in the 

 production of a change in physiological condition by a stimulus. We 

 know that even a single stimulus may produce a changed physiological 

 condition, as when after a single stimulus the organism no longer 

 reacts as before. We know also that the nature of the physiological 

 condition determines the reaction. In the present case we must con- 

 clude that a light stimulus throws the organism into a certain physio- 

 logical condition, whose concomitant reaction is turning toward the 

 point stimulated. A more intense stimulus induces a different 

 physiological condition, whose concomitant reaction is a contraction 

 into the tube (Stentor), or a turning in the opposite direction (flatworm). 

 The action of the stimulus, as we have seen in the foregoing paper 

 devoted to the theory of tropisms, cannot in most cases be directly on 

 the motor organs, so that from this point of view also we are almost 

 forced to the conclusion that the primary action of the stimulus is 

 to change the physiological condition of the organism. In any reac- 

 tion to stimulus we would have, therefore, the following steps : The 

 stimulus acting on the organism changes its physiological condition ; 

 this physiological condition induces a certain type of reaction. In 

 determining what physiological condition shall be produced, the inten- 

 sity of the stimulus is fully as important as its quality. 



We have a similar reversal of the reaction as the intensity changes 

 in reactions to light. Many organisms are positive to weak light ; 

 negative to strong light. The cause of this reversal of the reaction as 



