l8o MAN, THE ANIMAL 



and effector in the worm occur several kinds of 

 nerve cells limited entirely to the ganglia which 

 receive stimuli indirectly and indirectly pass on 

 stimuli to the muscles. No one understands what 

 takes place in these associational nerve cells as 

 they are termed; so that the actual mechanism of 

 reflex action is not understood. It really explains 

 nothing to say that it is a reflex act. The nervous 

 reactions which man is not aware of while they 

 are taking place are for convenience of description 

 termed reflex but we do not know that they are in 

 any way different from those reactions of which 

 he is conscious. Hence it does not seem that 

 much is to be gained by an elaborate presentation 

 of reflex action as it is related to learning. 



In 19 1 2 Yerkes gave an account of his experi- 

 ments in trying to modify the behavior of earth- 

 worms. He devised a simple apparatus (Fig. 55), 

 by means of which he was able to test the ability 

 of earthworms to "learn" to follow a simple path 

 and to avoid an injurious chemical (or electrical) 

 stimulus by reacting negatively to a peculiar tact- 

 ual stimulus which regularly preceded the chemical 

 stimulus. He had in mind two questions : ( i ) 

 Can the worm profit by experience: and (2) can 

 it "associate" the tactual stimulus with the chemi- 

 cal and acquire the habit of regularly responding 

 to the sandpaper as though it anticipated the un- 

 pleasant stimulus from which it always did move 



