MODIF1 ABILITY OF BEHAVIOR 173 



It seems improbable that the change of behavior is due to fatigue, 

 since the change occurs after but a single stimulation and a single reac- 

 tion. It could hardly be supposed that these would fatigue the animal 

 to such an extent as to prevent further contractions. And if we use 

 stronger stimuli, we find that the animal continues to contract succes- 

 sively every time the stimulus is applied, for an hour or more. It is 

 evident that the failure to contract after the first stimulation cannot be 

 due to fatigue of the contractile apparatus. 



If we make the stimulation somewhat stronger than in our first ex- 

 periments, as may be done by touching the animal lightly with a capil- 

 lary glass rod, the behavior is a little different. The animal may react 

 the first and second times, then cease to react, or it may react half a 

 dozen times, or more, then cease. If we continue the stimuli, we find a 

 change in the behavior. The animal instead of contracting bends into 

 a new position, and it may do this repeatedly. This shows that the fail- 

 ure, to contract is not due to a failure to perceive the stimulus, in 

 other words, 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. 



Our results thus far show that after responding once or a few times 

 to very weak stimulation, the organism becomes changed, so that it 

 no longer reacts as before, and that this change is not due to fatigue, 

 either of the contractile apparatus or of the perceptive power. The 

 behavior may then be of the same regulatory character as is the similar 

 behavior in higher animals. Indeed, so far as the objective evidence 

 goes, this behavior in Stentor precisely resembles that of higher ani- 

 mals, and is to the same degree in the interest of the organism. 



With still stronger stimulation, produced by touching the animal 

 with the capillary glass rod, another curious phenomenon often shows 

 itself. The animal may react to each of the first half dozen strokes, 

 then cease to react; then after a few more strokes react again, then, 

 cease to react till a large number have been given, and so continue. 

 A typical series, giving the number of strokes before contraction is 

 produced, is the following, obtained from experiments with an individ- 

 ual of Epistylis : 



i 22 10 3 3 i i 22 59 125 (continuous blows 

 for one minute) (| minutes) (i| minutes) (4^ minutes). 



During such experiments the organism, when it does not contract, 

 continually changes its position, as if trying to escape the blows. The 

 reason for the contraction at irregular intervals which become longer 

 as the experiment continues, is not clear. Possibly fatigue may have 

 something to do with this matter. 



