144 MODIFICATIONS OF BEHAVIOR 



is much more susceptible to the effects of fatigue than the 

 afferent or efferent nerves. It is not a fatal objection to the 

 theory of fatigue that the response falls off very quickly. 

 An extreme sensitiveness may result from a certain con- 

 dition of balance which a slight chemical change might 

 overthrow without rendering the organism insensitive to 

 stronger stimuli. A very high degree of sensibility is as a 

 rule very easily affected. In ourselves the ability to detect 

 a faint odor or taste is exhausted by a very few trials 

 and in lower organisms where sensitivity is often exceed- 

 ingly acute, there are, as we might expect, much greater 

 fluctuations. It is not improbable that in many cases 

 something analogous to fatigue may take place in the 

 central apparatus in the pathway between the afferent and 

 the efferent impulses. The animal might thus, without hav- 

 ing either its receptors or its motor apparatus appreciably 

 affected, fail to respond in the same way, if at all, to stimuli 

 which at first brought about a reaction. 



At times the response to a given stimulus may be increased 

 instead of diminished with repeated application. Statke- 

 witsch found that Paramoecia would often fail to respond to 

 weak induction shocks, but where several were given the re- 

 sults were cumulative and the animals swam toward the 

 cathode. Planaria after a period of rest are apparently in a 

 condition of lowered tonus and fail to react in the usual 

 manner to stimuli but if the stimuli are repeated the re- 

 actions appear and for a time may increase in vigor. An 

 analogous phenomenon is presented in the responses to light 

 of Ranatra and fiddler crabs which become more and more 

 energetic with longer exposure to the stimulus. An inter- 

 esting instance is afforded in the reaction of the tentacle 

 of Cerianthus to repeated tactile stimuli. Bohn found 

 that the tentacles when stimulated became flexed toward 



