THE METHOD OF TRIAL AND ERROR. 



239 



This method of reaction seems very set and simple when considered 

 by itself. It is almost like that of a muscle which reacts by the same 

 contraction to all effective stimuli. The behavior of these animals 

 seems, then, of the very simplest character. To practically all strong 

 stimuli they react in a single definite way. 



But if we look closely at this simple method of reacting, we find it, 

 after all, marvelously effective. The organism, as we have seen, is 



revolving on its long axis. When, as a consequence of stimulation, it 

 swings its anterior end toward the side X, this movement is combined 



* FIG. 80 Diagrams of the movements in a reaction to a stimulus in an infu- 

 sorian, Paramecium (A), and in a rotifer, Anursea (Z?). The anterior end swings 

 about in a circle (turning continually toward the aboral or dorsal side). It thus 

 tries many different directions, at the same time receiving samples of water from 

 each of these directions, i, 2, 3, 4, 5, the successive positions taken, with the 

 currents of water at the anterior ends. If the stimulus ceases the organism 

 may stop in any of these positions, and swim forward in the direction so indi- 

 cated. (The backward swimming, which precedes or accompanies the turning, 

 is not represented.) 



