Modification by Experience 253 



enon one of adaptation. In true sensory adaptation, the 

 sense organ becomes incapable of responding to the stimu- 

 lus ; for example, a person who has been for some time sub- 

 jected to a certain odor is unable to smell it any more, 

 however much he tries. Closely related to this phenom- 

 enon and yet different from it, is the lapse of attention to a 

 repeated stimulus: we no longer notice the ticking of a 

 clock, although the sense organ is unaffected by its con- 

 tinuance, and we can quite well hear it if our attention is 

 attracted in that direction. 



That the failure of Stentor to respond to successive stimuli 

 is not due to motor fatigue appears quite certain to Jen- 

 nings, since under favorable conditions he has obtained re- 

 actions from the animal for a period far longer than that 

 occupied by the process of getting used to slight mechanical 

 stimulation (370). And in most of the cases cited, the 

 acclimatizing process seems to occur too rapidly to make 

 fatigue of the motor apparatus probable. In the lower 

 animal forms, sensory adaptation offers the most natural 

 explanation for the phenomenon ; in the higher animals, 

 lapse of attention is very likely also involved. The modi- 

 fication of consciousness in both cases would be the loss 

 of the sensation; where adaptation occurs, the sensation 

 would be for the time irrecoverably lost; where there is 

 merely lapse of attention, it could be regained by a proper 

 direction of attention. 



A much discussed case of the cessation of response to a 

 repeated stimulus is found in connection with the food- 

 taking reaction. One would expect the dominant condition 

 here to be loss of hunger, and as a matter of fact, observ- 

 ers of the feeding processes in many lower animals have 

 found that such reactions cease or turn into negative 

 responses when the animal is satiated ; although Pieron 



