246 The Animal Mind 



in the following pages the various ways in which an organ- 

 ism's past experience may modify its behavior. For each 

 type of modification we shall try to find a parallel in human 

 experience, and thus to interpret, so far as possible, the 

 conscious aspect of the learning process. To begin with, 

 we shall distinguish between those modifications which 

 depend on some comparatively lasting alteration in the 

 organism (in its nervous system if it has one), that is, the 

 kind of modification which is ordinarily understood by 

 the term "learning" ; and modifications which are due to 

 a change essentially temporary in its character, in the 

 physiological state of the organism. Even in the lowest 

 animals the effect of a stimulus depends on the organism's 

 physiological condition, and this condition is often the re- 

 sult of stimulation recently received. 



71. Modifications Due to Essentially Temporary Physio- 

 logical States : (a) Heightened Reaction as the Result of Pre- 

 vious Stimulation. 



Sometimes the effect of the stimuli which the organism 

 has just received increases the violence of its response to 

 a given stimulus. 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 condition 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 

 (377). This increased excitability suggests the "nervous 

 irritation" produced in a human being by an accumu- 

 lation of disagreeable stimuli ; an increased feeling of 

 unpleasantness accompanied by more diffused organic 



