66 Studies in Animal Behavior 



larger area which is the more potent in causing the 

 turning of the body. With the development of im- 

 age-forming eyes it becomes possible for animals 

 to respond to objects and not to mere differences 

 in the amount of light and shade. The image-form- 

 ing type of eye is stimulated by a decrease as well 

 as by an increase of illumination on particular parts 

 of its surface. This stimulation is generally as- 

 sociated with an involuntary turning toward the 

 source of stimulation. Hence, the automatic turn- 

 ing of the head toward a new object in the field of 

 vision, and the tendency to follow the movements 

 of bodies with movements of the eyes. The eyes 

 of animals are notoriously quick to respond to 

 movements. The moving thing is the stimulating 

 thing. With the evolution of the image-forming 

 type of eye and the development of acute sensitive- 

 ness to change of illumination of particular parts 

 of the retinal surface, the general tendency to go 

 toward or away from light may pass into an in- 

 voluntary tendency to become oriented toward par- 

 ticular moving objects in the field of vision. When 

 an animal reacts in a definite way to objects im- 

 pressed on its retina we commonly say that it sees. 

 These reactions to objects come to be very com- 

 plex and specialized. They come to depend upon 

 the size, form and color of the moving object. But 

 it is not improbable that they have their primary 

 roots in the positive and negative phototaxis of 

 simpler organisms. Josiah Royce in his Outlines of 



