76 THE BEHAVIOR OF PROTOZOA 
to its life. While we commonly speak of the positive reac- 
tions of the organism, strictly speaking there are, with the 
exception of its thigmotaxis, no positive reactions. If 
Parameecia collect in weak acid it is not because the acid 
attracts them in any way; they are not stimulated even 
when they accidentally enter the acid; they react only 
when they pass from the acid into the water. It is not 
their positive reaction to the acid but their negative reaction 
to the water that causes what appears to be a positive 
chemotaxis. The same is of course true of their reactions 
to various other stimuli. Their life is one of continual 
avoidance. Only when conditions are favorable is there 
cessation of movement. If we do not wish to attribute to 
such creatures the power of choice we must admit that the 
method of behavior secures it the same advantages that 
choice affords. 
The behavior of Paramecium, like that of every other 
organism, is modified by changes of its internal condition. 
Strong induction shocks render Paramcecium insensitive to 
weaker shocks and if individuals are kept for a time at 
a temperature higher than normal a higher temperature is 
required to cause the avoiding reaction. Individuals that 
have been kept without food become restless while well fed 
ones are more sluggish and more apt to come to rest against 
solid objects. Reactions to gravity are influenced by food 
and other conditions and thigmotaxis is markedly affected by 
temperature. Behavior may be modified by repetition of the 
same contact stimulus as described in the following quota- 
tion from Jennings: “If a bit of filter paper is placed in a 
preparation of Parameecia, the following behavior may often 
be observed. An individual swims against it, gives the avoid- 
ing reaction in aslightly marked way, swimming backward a 
little; then it swims forward again, jerks back a shorter 
