The Reactions of Planarians to Light 139 



paper Cohn ('64) abandoned his first position and came to regard 

 intensity as the important element in light, a position also main- 

 tained by Famintzin ('67), Engelmann ('83), Oltmanns ('92), 

 Verworn ('01) and even by Loeb ('93b) in the case of Planaria 

 torva, which he found came to rest in accordance with the intensity, 

 and regardless of the direction, of the light. Davenport and Can- 

 non ('97) modified this point of view by attempting to show that 

 direction and intensity may each operate independently, producing, 

 respectively, " phototaxis" and "photopathy." Holt and Lee ('01) 

 followed with an excellent summary of the whole controversy, 

 emphatically maintaining, in opposition to Davenport and Can- 

 non, that intensity alone is the only possible operative factor in 

 light stimulation and that direction of the rays has no effect what- 

 soever except in determining a greater intensity of light with refer- 

 ence to one part of an organism as compared with other parts. 



Among more recent investigations Holmes ('03), experimenting 

 with the same organism that led Oltmanns to ascribe the greater 

 importance to intensity, namely, Volvox, declares himself in favor 

 of direction, while Zeleny ('05), on the other hand, gives an 

 instance of Serpulid larvae going both toward the source of the 

 light and away from it; that is, moving regardless of direction, in 

 order to arrive in regions of increased intensity. 



Carpenter ('05) found that the pomace fly, Ampelophila droso- 

 phila, will orient to the direction of light after it has first been 

 sufficiently aroused by the intensity of the light, while both Yerkes 

 ('99) and Towle ('00) maintain that direction and intensity are by 

 no means mutually exclusive, and that each may play a part simul- 

 taneously in determining the behavior of an organism. 



Lastly, it has been made clear by Parker ('03) that, besides 

 direction and intensity of light, the size of the source of illumina- 

 tion may determine the orientation. This theory explains why 

 butterflies alight upon a patch of reflected sunlight which produces 

 a large but faint retinal image instead of flying toward the sun 

 itself, which forms only a small but intense retinal image. In the 

 case of planarians, however, this phase of light stimulation is not 

 operative, since the eyes of these animals are incapable of forming 

 retinal images. 



