The Problem of Orientation 75 



tion it may extend it receives a strong stimulus and 

 the larva writhes about helplessly for some time. 

 Sooner or later, however, it follows up the right 

 movement. Occasionally the larva may crawl for 

 some distance directly towards the light, but after 

 a time its movements carry it in the 'opposite direc- 

 tion. When once oriented, the direction of locomo- 

 tion of the larvae is comparatively straight." 



It was not denied that these different forms 

 showed a certain tendency to turn directly away from 

 the light, but it was contended that orientation was 

 mainly produced "indirectly by following up those 

 chance movements which bring respite from the stim- 

 ulus." The light reactions of the forms studied 

 "may be interpreted as a resultant of two motor re- 

 sponses; first, the activities of locomotion which 

 are set up by the stimulus of the light, and second, 

 the act of jerking back and bending the body from 

 side to side in response to a strong stimulus from 

 in front. Here are two instincts or reflexes, how- 

 ever we may be pleased to call them, which are in 

 a measure antagonistic in that the first is frequently 

 overcome by the second. The direction of the ex- 

 ternal stimulus determines which of these two in- 

 stinctive tendencies predominates." 



The more recent work of Harper on the earth- 

 worm Perichista bermudensis has shown that in 

 strong light the worm turns away from the stimu- 

 lus, but that in weaker light "orientation is the re- 

 sult of a trial and error method." And Mast on 



