2io The Animal Mind 



would always be given ; if the interval was half a second, after 

 the first few stimuli many of the worms failed to react, while 

 if the interval was only a quarter of a second, almost all of 

 them became indifferent (158). Mrs. Yerkes observed that 

 the same annelid would often fail to respond to shadows 

 repeated at intervals of from 5 to 10 seconds, and that 95 

 out of 200 responded when the interval was from one to two 

 minutes (447). On the other hand, the spider experimented 

 on by the Peckhams for some time reacted each day to the 

 sound of the fork by dropping from its web until the sound 

 had been repeated some half dozen times; but after the 

 fifteenth day it would not drop at all (320). There is an 

 adaptive aspect to this difference between Hydroides and the 

 spider. An animal that has little power to discriminate 

 among stimuli could not afford to suspend its negative re- 

 action for any length of time, for another stimulus, indis- 

 tinguishable from the one to which it had become accustomed, 

 might happen along and end its career. But a creature with 

 greater capacity for qualitative discrimination can safely 

 suspend reaction for a considerable period to one out of the 

 many stimuli which it is capable of discriminating. 



Where the effect is temporary, the most obvious suggestion 

 as to its cause is fatigue. In our own experience this word is 

 used chiefly with reference to motor processes; we perceive 

 a certain signal, but are too fatigued to respond. On the 

 sensory side, when a repeated stimulus is no longer perceived, 

 we call the phenomenon one of adaptation. That the failure 

 of Stentor to respond to successive stimuli is not due to motor 

 fatigue appears quite certain to Jennings, since under favor- 

 able conditions he has obtained reactions from the animal 

 for a far longer period than that occupied by the process of 

 getting used to slight mechanical stimulation (203). And in 

 most of the cases cited, the acclimatizing process seems to 



