174 The Animal Mind 



tion, it would be of little practical service. The other view, 

 that the important factor is the difference in the intensity of 

 stimulation of opposite points on the unoriented animal's 

 body, is that held by Verworn (417). Holmes points out that 

 no crucial test experiment of the two hypotheses has ever 

 been made. Such an experiment would require that a semi- 

 transparent animal should have two symmetrical points on its 

 body, a and b, stimulated with exactly equal intensity, each 

 by a ray of light coming from a different direction. Under 

 such circumstances, according to the theory of Verworn, the 

 animal ought to move straight forward (181). An attempt 

 to get evidence was made by Davenport and Cannon in a 

 study of Daphnia. They proposed the following question: 

 Do positively phototactic animals move more rapidly toward 

 their optimum intensity than toward an intensity below the 

 optimum? If orientation is determined, as the Verworn 

 theory supposes, by the relative intensity of light on different 

 points of the organism, then the absolute intensity of the 

 light ought not to affect it. If, on the other hand, the direc- 

 tion of the rays orients the animal, then precision of orienta- 

 tion should increase as the absolute intensity approaches the 

 optimum. Daphnia was found to move somewhat less 

 rapidly toward the light when the intensity of the latter was 

 reduced ; this fact was held to be due to diminished precision 

 of orientation and hence to tell for the theory of Loeb 1 (93). 



69. The Eyes in Phototaxis 



The directive theory of phototaxis is of little significance 

 in connection with the reactions to light of organisms whose 

 bodies are opaque and which have eyes. For the eyes seem 

 to be fundamentally concerned in orientation to light. That 



1 A discussion of the intensity, and direc^on theories will be found in Holt 

 and Lee's article on "The Theory of Phototactic Response" (187). 



