Harper, Behavior of Corethra. 451 



THE APPLICATION OF THE TRIAL AND ERROR THEORY TO 



OTHER ANIMALS 



As the trial and error hypothesis has received attention elsewhere 

 in this paper, it may be well to indicate its application in other 

 cases as understood by the writer, to avoid ambiguity. It is here 

 used in the sense given by Holmes ('06), implying varied move- 

 ments under stimulation with accidental orientation. If used 

 loosely as seems to be often the case, implying only failure to move 

 in a direct line toward or away from a source of stimulus, it could 

 very well be applied to Corethra in the case of some reactions 

 (though the food reaction at least is very precise) inasmuch as its 

 spasmodic and unsymmetrical movements do not carry it in a 

 straight path. 



In Paramcecium the application of the trial and error interpre- 

 tation to its behavior under stimulation hinges on the question 

 whether its avoiding reaction of backing and then turning always 

 to the aboral side when stimulated is to be interpreted as a move- 

 ment directed with reference to the stimulus or as simply a change 

 of movement in a structurally determined direction, the aboral, 

 evoked by the stimulus. As a matter of fact, the movement is 

 determined functionally as much as structurally, since, as Jen- 

 nings shows, Paramcecium is asymmetrical in the distribution of 

 sensory areas. The oral groove is the sensory region par excel- 

 lence, and so aboral turning may be regarded as a directed move- 

 ment, in accord with the views of Yerkes ('06). 



Jennings' view is borne out by the fact that Paramcecium does 

 show an apparent disregard to the direction of the stimulus, 

 always turning aborally no matter whether stimulated locally in 

 any region or all over the body and uniformly. It does not follow 

 that when a stimulus is applied uniformly all over the body, it 

 affects all parts alike, for there are marked differences m sensitive- 

 ness. The oral groove is the most sensitive region and receives 

 stimuli of many kinds first, through currents drawn from in front, 

 as Jennings shows. Or at any rate it may be most affected by 

 stimuli applied anywhere and reaching it by conduction. Mast 

 has discussed this point of view with respect to several forms of 

 Protozoa ('06). In view of this, aboral turning would not be 

 interpreted as a mere change of movement, but as a movement 

 directed away from the stimulus. This would bring Paramoe- 



