366 WILLIAM L. DOLLEY, JR. 



its body. Should the eyes be the parts stimulated, any inter- 

 ference with one of these ought to result in a disturbance of the 

 direction of the butterfly's locomotion. Thus, if the cornea of 

 one eye were blackened, the insect in locomotion, being positively 

 phototropic, ought to move as though that eye were in shade; 

 namely, in a circle, with the unaffected eye toward the center." 



Mast holds that the precision with which some organisms with 

 but one functional eye perform circus movements appears to 

 add support to the 'continuous action theory,' but he also says 

 ('11, p. 222), as a result of his work on the toad, ''These results 

 show that, in this form and in all other forms which orient after 

 ■ one eye is destroyed, difference of effective intensity on opposite 

 sides does not regulate orientation." 



A glance at these various views shows that the movement of 

 animals in circles when one eye is blackened, or when one antenna 

 is removed, has been held by most of the investigators to support 

 the view that the orientation of animals is in accord with the 

 'continuous action theory' described above. This theory is op- 

 posed by one that may be called the 'change of intensity theory,' 

 the adherents of which hold that in some organisms, at least, light 

 does not produce orientation through its continuous action, but 

 by stimuH dependent upon the time rate of change of intensity. 

 According to this theory, an organism going toward a source 

 of hght, may turn to one side; but when this occurs, then, imme- 

 diately the photosensitive surfaces are exposed to a change of 

 intensity, and this causes a reaction which results in reorienta- 

 tion, after which the orienting stimulus ceases. 



The chief points at issue between the two theories concern the 

 following questions: (1) Does hght function in orientation through 

 its continuous action, or through a change of intensity? (2) 

 Does an animal, when oriented, continue to be affected by the 

 same stimulus that is effective in producing orientation? and 

 (3) Is bilateral symmetry essential in the process? 



In view of the bearing that circus movements have on the 

 theories as to the mechanism of normal orientation in animals, 

 and in view of the conflicting results recorded by previous workers 

 it seemed desirable to make a more thorough and a more extended 



