MODIFI ABILITY OF BEHAVIOR 177 



(4) If the stimulus still persists, the animal breaks off its normal 

 activity completely by contracting strongly — devoting itself entirely, 

 as it virere, to getting rid of the stimulation, though retaining the possi- 

 bility of resuming its normal activity in the same place at any moment. 



(5) Finally, if all these reactions remain ineffective, the animal not 

 •only gives up completely its usual activities, but puts in operation 

 another set, having a much more radical effect in separating the animal 

 from the stimulating agent. It abandons its tube, swims away, and 

 forms another one in a situation where the stimulus does not act upon it. 



The behavior of Stentor under the conditions given is evidently a 

 special form of the method of the selection of certain conditions through 

 varied activities, — a form which we have not met before. The organ- 

 ism "tries" one method of action; if this fails, it tries another, till one 

 succeeds. Like other behavior based on this method, it is not a specific 

 reaction to any one stimulus, but is seen whenever analogous conditions 

 are produced in any way. Thus we may use in place of carmine other 

 substances. Chemicals of different kinds produce a similar series of 

 reactions. A decided change in osmotic pressure has a somewhat 

 .similar effect. There are variations in the details of the reaction series 

 under different conditions. Sometimes one step or another is omitted, 

 •or the order of the different steps is varied. But it remains true that 

 under conditions which gradually interfere with the normal activities 

 of the organism, the behavior consists in "trying" successively different 

 reactions, till one is found that affords relief. The production of any 

 given step in. the behavior cannot be explained as a necessary conse- 

 quence of the preceding step. On the contrary, the bringing into opera- 

 tion of any given step depends upon the ineffectiveness of the preceding 

 ones in getting rid of the stimulating condition. The series may cease 

 at any point, as soon as the stimulus disappears. Moreover, it is evi- 

 dent that the succeeding steps are not mere accentuations of the pre- 

 ceding ones, but differ completely in character from them, being based 

 upon different methods of getting rid of the stimulation. 



All our results on Stentor then show clearly that the same organism 

 may react to the same stimulus in various different ways. It may react 

 at first, then cease to react if the stimulus does not interfere with its 

 normal activities; it may react at first by a very pronounced reaction 

 -(contraction), then later by a very slight reaction (bending over to one 

 •side) ; or it may respond, if the stimulus does interfere with its normal 

 functions, by a whole series of different reactions, becoming of a more 

 and more pronounced character. Since in each of these cases the ex- 

 ternal conditions remain throughout the same, the change in reaction 

 must be due to a change in the organism. The organism which reacts 



