Modification by Experience 209 



repeated from five to seven times (320). Ants "become 

 used" to the ultra-violet rays which they ordinarily avoid 

 (ng). 



If learning by experience be extended to cover every case 

 where an animal reacts to a stimulus differently because of 

 earlier stimulation, then this is learning by experience. An 

 interesting point suggests itself in regard to the permanency 

 of such learning. In case the animal the next day responds 

 with less vigor to the excitant which it got used to the day 

 before, there would seem some plausibility about the inter- 

 pretation of Nagel, who says with that inclination in favor 

 of the psychic which always characterizes him, that the be- 

 havior of his mollusks "makes the assumption of a certain 

 power of judgment in these animals unavoidable. The 

 animal recognizes that the repeated shadow is not due to the 

 presence of an enemy or other danger" (290). On the other 

 hand, of course, it is perfectly conceivable that an animal 

 might go through such a process of judgment and still be 

 unable to remember it the next day. However, if we find 

 that only very recent stimulation has any effect, the suggestion 

 that this effect is due to some purely physiological alteration 

 in the organism lies near at hand. 



As a matter of fact, the higher the animal the more lasting 

 appears to be the result of "getting used" to a stimulus. 

 For instance Hydra, if it is allowed to reach full expansion 

 after having contracted at a touch, will respond to the 

 seco'nd touch just as it did to the first ; the stimuli, to influence 

 each other, must come in quick succession. The relation 

 of loss of reactive power to the interval between the stimuli 

 was prettily shown by Hargitt for a tube-dwelling marine 

 worm, Hydroides dianthus. Shadows were thrown from a 

 pendulum whose rate could be varied, and it was found that 

 if a full second intervened between the stimuli, the reaction 

 p 



