Modification by Experience 251 



capable of doing. We judge that they are so by the simple ' 

 fact that on being subjected after an interval to the same 

 presumably disagreeable stimulus, they do not at once 

 maEfe the reaction that was previously successful in getting •: 

 rid of it. A human being, recalling that reaction in idea, 

 would be able to do so. We shall see in the next chapter \ 

 that many animals, while they do not learn the successful^ 

 reaction from a single experience, do gradually diminish/ 

 the number of unsuccessful ones made in a series of ex-' 

 periences. It is quite possible that this will prove to be 

 true of all animals, as experimental evidence accumulates. 



§ 72. Modification Due to Essentially Temporary Physio- 

 logical States: (6) Cessation of Reaction to a Repeated Slight 

 Stimulus. 



The type of modified response just described occurs^ 

 when the stimulus is strong, and presumably injurious. 

 When it is of moderate intensity only, the organism tends 

 to respond less and less violently as the stimulus is re- 

 peated at short intervals, until finally the response lapses 

 entirely. The Ciliata Vorticella and Stentor, which spend 

 a part of their time attached to solids by a contractile 

 stem, contract at the first application of a moderately in- 

 tense mechanical stimulus, but fail to react at all when the 

 stimulus is several times repeated (370). Hydra responds 

 to mechanical stimulation by contraction, but gets used to 

 the process when repeated and gives no further reaction 

 (751 a). The sea-anemone Aiptasia reacts by a sharp con- 

 traction to a drop of water falling on it ; later it ceases its 

 response to this stimulus. If exposed to light, it contracts 

 and remains in this state for some hours, but afterwards 

 expands again (374). The annelid Bispira voluticornis 



