THE METHOD OF TRIAL AND ERROR. 243 



In no other group of organisms does the method of trial and error 

 so completely dominate behavior, perhaps, as in the infusoria. In this 

 group the entire organization seems based on this method. But reac- 

 tions on this plan are found abundantly elsewhere. In Amoeba the 

 present author has shown that many of the reactions are of this charac- 

 ter. (See the preceding paper on the movements and reactions of 

 Amoeba.) When stimulated mechanically or by a chemical, the 

 Amoeba does not move directly away from the source of stimulus, but 

 merely in some other direction than that toward the side stimulated. 

 If this leads to a new stimulus, the animal tries another direction. By 

 continued stimuli Amoeba may be driven in a definite direction. The 

 conditions necessary for this are that movement in any other direction 

 shall lead to stimulation. 



Reaction on the plan of trial and error is, perhaps, best seen in 

 Amoeba in the method by which 

 a specimen suspended in the water 

 finds and attaches itself to a solid 

 object. The suspended Amoeba 

 sends out pseudopodia in all di- 

 rections. If the tip of one of these 

 pseudopodia comes in contact with 

 a solid object it becomes attached ; 

 the protoplasm begins to flow in 

 that direction, and all the other 

 pseudopodia are withdrawn. The 

 Amoeba then passes to the solid 

 and creeps over its surface. Thus FIG. 81.* 



the Amoeba has tried sending out pseudopodia in all directions ; that 

 which has been successful in finding a solid determines the direction of 

 movement. 



In bacteria the reactions to light, to chemicals, and to mechanical 

 stimuli are essentially like those of the infusoria. The details as to the 

 direction of turning, etc., are not known, owing to the minuteness of 

 these organisms. But the essential point is that when the bacteria are 

 stimulated effectively they change the direction of movement. Such 

 change is repeated until the organisms are brought into a position 

 where there is no effective stimulation. The behavior is clearly that 

 of trial and error. 



* FIG. 81. Different positions occupied in the usual reaction to stimuli in 

 Oxytricha. The animal swings its anterior end in a circle, occupying succes- 

 sively positions i, 2, 3, 4, 5, 6, and receiving a sample of water from each direc- 

 tion in which the anterior end is pointed. When the stimulus ceases the animal 

 may swim forward in any of these directions. (The backward swimming which 

 precedes or accompanies the turning toward the right side is not represented.) 



