Modification by Experience 217 



fractions of the normal routine. It would be decidedly 

 disadvantageous to take the last step while there was any 

 chance that milder measures might prevail. 



In all probability, since the behavior just described has no 

 permanent effect upon the animal, it is physiologically due, as 

 Jennings suggests (208), to the overflow of the nervous energy 

 set free by the stimulus into first one channel and then an- 

 other. In most cases the movements resulting are all adapted 

 to getting rid of the stimulus, though only one of them is 

 successful in so doing ; but we have on record one case where, 

 in a supreme emergency, the stimulus being not only repeated 

 but increased in intensity, every possible outlet is tried, 

 whether it has any fitness to the situation or not. This was 

 observed by Mast, testing the effect of increased temperature 

 on the reactions of planarians. The first influence of such 

 increase from 23 degrees to 26 degrees C., is to produce 

 heightened activity and positive reactions. Then, from 26 

 degrees to 38 degrees, the reactions are negative. From 38 

 degrees to 39 degrees, violent crawling movements set in, and 

 then, curiously enough, the righting reaction is given, per- 

 fectly irrelevant, of course, to the conditions. Finally, the 

 anterior and posterior ends are turned under, the central part 

 is arched upward, and the animal falls over forward on its 

 back (260). 



In all these cases where repetition of the same stimulus 

 produces successively different forms of the negative reaction 

 increasing in violence, it is most natural to think of the psychic 

 accompaniment as an increasing degree of unpleasantness. 

 In our own experience, repeating a stimulus does not alter 

 the quality of the resulting sensation, except where the struc- 

 ture of a special sense organ is a modifying factor, as in the 

 case of visual after-images. Repetition of the stimulus does 

 with us human beings diminish the intensity of the accompany- 



