PHOTOTAXIS 57 



eyes are capable of forming images. If animals are stimu- 

 lated by two sources of light of the same degree of intensity 

 but of different area, the forms without eyes or in which the 

 eyes produce no image turn practically as often to the one 

 source of light as the other while animals with eyes producing 

 a distinct image will, if positively phototactic, generally 

 turn toward the light of larger area. With the perfection 

 of the organs of vision the primitive phototactic tenden- 

 cies of animals may become modified so as to afford the 

 basis for the reactions to special objects of the visual field 

 which we find in the more highly developed instincts. 



In many animals there is a strong reflex tendency to keep, 

 so to speak, in statu quo with the visual field. This tendency 

 accounts for many cases of so-called rheotropism as is shown 

 by the experiments of Hadley on the lobster and Lyon on 

 fishes. The same tendency is, as we have seen, manifested 

 also in compensatory motions. Radl has shown that in 

 Daphnia and some other forms in which the eyes are mov- 

 able there is an effort on the part of the eyes to become 

 oriented to the rays of light. In the vertebrates in which 

 the eyes are freely movable the same tendency is more or 

 less pronounced. This trait may be connected with the 

 involuntary tendency of animals to follow the movements 

 of objects which cross their field of vision. Vision in the 

 lower animals is concerned much more with the movements 

 than with the form of objects. It may be possible to trace 

 more accurately than has been done the relation between 

 phototaxis, compensatory motions of the head and body, 

 and the involuntary tendency to follow moving objects 

 with the eyes. This field of investigation has been but 

 little explored, but it is one which promises to be fruitful 

 of significant results. 



