84 THE BEHAVIOR OF PROTOZOA 
of the organism. For the physiological state A there is one 
reaction, for state B another, and so on. As in a higher 
animal the behavior of Stentor depends upon its previous 
history. In a sense the organism may be said to profit by 
experience, although it cannot be said to learn, because 
there is no formation of new associations such as occurs 
in the learning of higher forms. 
What are these internal physiological states to which the 
adaptive changes of behavior in Stentor are due? We 
have here two kinds of modification, as Jennings has pointed 
out, the failure to respond to a stimulus which at first evoked 
a reaction, and the replacement of one reaction by another. 
In regard to the first modification Jennings remarks that 
“Tt seems improbable that the change of behavior is due 
to fatigue, since the change occurs after but a single stimula- 
tion and a single contraction. It could hardly be supposed 
that these would fatigue the animal to such an extent as to 
prevent further contraction. And if we use stronger 
stimuli, we find that the animal continues to contract suc- 
cessively every time the stimulus is applied, for an hour or 
more.” Even after the animal has ceased to contract 
strongly it may respond to a stimulus by bending to one side, 
thus showing, according to Jennings, that the failure to 
contract is not due to “a fatigue of the perceptive power, 
for the bending into a new position shows that the stimulus 
is perceived, though the reaction differs from the first one.” 
Fatigue is usually associated in our minds with a condition 
indicative of exhaustion, and of such Stentor certainly gives 
no evidence after a few stimulations, but that there may be 
a slight degree of essentially the same state which when carried 
further we designate as fatigue, seems to me a possibility 
not ruled out by the experiments. It is possible, however, 
that the change in question may be due to something simi- 
