The Problem of Orientation 83 



stances. Now organisms of any kind in going to 

 or from the light are subjected to frequent changes 

 of light intensity as they deviate to one side or the 

 other of their course. Do the fluctuations of light 

 intensity so caused produce stimulations that affect 

 the orientation of the animal? The possibility that 

 stimulations produced by such fluctuations in the in- 

 tensity of light might play a role in orientation was 

 suggested in my paper on the phototaxis of Rana- 

 tra. As this insect, when one eye is blackened over, 

 nevertheless, in some cases goes nearly straight to 

 the light, it was pointed out that "were the insect 

 so constituted as to respond to an increase of light 

 entering the left eye by a turn to the left and to a 

 decrease of light by a turn to the right, we can 

 understand how, when once pointed to the light, a 

 straight course might be preserved. If the insect 

 turned towards the right there would be an in- 

 crease of light entering the left eye which we might 

 suppose stimulates the insect to turn in the opposite 

 direction. Deviations to the left would cause a 

 diminution of light entering the left eye, which we 

 might suppose acts as a stimulus to turn to the right 

 side. The right eye may be supposed to act, mu- 

 tatis mutandis, in a similar manner." 



The conclusion reached, however, was that re- 

 sponses to fluctuations of light intensity alone did 

 not give an adequate explanation of the orientation 

 of this form, although they might afford a co- 

 operative factor. Such responses probably do play 



