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 asa 
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 Parameecia 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 
