Modification by Experience 207 



various devices until the right one is obtained, is but super- 

 ficial. Certainly the behavior of the Paramecium, "trial 

 and error" though it may be, is not learning, and gives no 

 evidence either for or against consciousness as its accompani- 

 ment. If it has a subjective side, the unpleasantness that 

 is most naturally regarded as the accompaniment of a negative 

 reaction would seem to be modified in no way by the repeated 

 performance of the reaction. 



81. Heightened, Reaction as the Result of Previous 



Stimulation 



/ 



But even in the lowest animals the effect of a stimulus is 

 often, as we have seen, altered by the "physiological con- 

 dition" of the animal, and this condition is commonly the 

 result of the stimulation previously received. Sometimes the 

 influence is in the direction of increasing the violence of the 

 response. Thus in the earthworm Jennings points out that 

 various stages of excitability may exist, due to the action of 

 previous stimulation and varying all the way from a state of 

 rest, where a slight stimulus produces no effect, to a condi- 

 tion of violent excitement, where moderate stimulation will 

 cause the animal to "whip around" into a reversed position 

 or wave its head frantically in the air (210). This increased 

 excitability suggests the "nervous irritation" produced in a 

 human being by an accumulation of disagreeable stimuli; 

 but an increased unpleasantness is the only obvious interpre- 

 tation of its psychic aspect. 



82. Cessation of Reaction to a Repeated Stimulus 



While response to a given stimulus may thus be altered by 

 reason of the fact that other stimuli have been acting upon 

 the animal just previously, certain interesting modifications of 

 reaction occur when the same stimulus is repeatedly given. 



