390 FRANK W. BANCROFT 



When by chance some reflex left the animals turned away from the 

 light the motor reflexes stopped, and the forward swimming was 

 resumed. This was called orientation by the method of trial and 

 error. 



But when Jennings, in the latter half of the same paper came to 

 investigate the behavior of Euglena an essentially different reac- 

 tion was observed. Euglena executed motor reflexes similar to 

 those described for Stentor. But the organism also became ori- 

 ented in a uniformly illuminated field by swimming in a sweeping 

 curve which soon brought it into orientation without the selec- 

 tion of any random movements. However, in spite of the regular 

 nature of the orienting process, Jennings puts this gradual orien- 

 tation of Euglena also into the category of ' trial and error.' As it 

 moves in its spiral course the swervings to or from the light are 

 interpreted as trials, erroneous if away from, successful if towards 

 the light. 



Now it is to be noted that this is an entirely different kind of 

 'trial and error' from that previously discussed, and it has been 

 criticised by a number of investigators. Thus Holmes ('05, p. 

 110) says: 



But to view the matter in this way is to go far towards obliterating 

 the distinction between orientation through trial and error and orien- 

 tation by the direct method. In the mode of phototactic response here 

 considered Euglena does not react by a number of indiscriminate move- 

 ments until the right one is accidentally hit upon, but by a direct reflex 

 whose effect is to bring the organism more nearly parallel to the direc- 

 tion of the rays. 



Torrey also ('07, p. 317) has objected to this point of view. He 

 says : 



Jennings's figure indicates that Euglena is both unterschiedsempfind- 

 lich and heliotropic. At a (fig. 1) the reversal in the direction of the light 

 which has been coming from the direction in which the creature has been 

 swimming produces a sudden change in the intensity of stimulation, a 

 shock which results in the swerving from the previous course as indicated 

 between a and c. The organism recovers rapidly only to be subjected 

 to the constant stimulus of a steady light from one direction to the end 

 of the experiment. The result of the action of the constant stimulus is a 

 path from c to 5 . . . . perfectly in harmon}^ with the tropic schema. 



