RETINAL IMAGE TO ANIMAL REACTIONS. 115 



The natural response of the turtle to the sea is so obviously in- 

 stinctive and so uniform that it presents all the superficial traits of a 

 tropism, but when it is looked into, it appears to be a very precise 

 form of instinctive reaction to the details of the retinal image. When 

 loggerhead turtles were tested in a dark room provided with a single 

 light, they went neither toward the light nor away from it, but re- 

 mained for the most part quietly resting where they had been put. 

 Contrary to the view expressed by Hooker (1911), they are not 

 phototropic. They are active when their retinal fields are full of 

 detail and they move toward that part of the field in which the horizon 

 is most open. Under natural circumstances this usually brings them 

 to the sea, but it does not necessarily do so, and it is in no sense a 

 true tropic reaction. The young loggerhead turtle exhibits, then, an 

 activity that superficially resembles a tropism, but that in reality is 

 very diiTerent. In this respect the animal declares its higher nature. 



Most vertebrates respond in very precise ways to the details of 

 their retinal fields. Thus frogs and toads will seize and swallow 

 almost any small moving object, be it a pebble or a bit of wax 

 attached to a string, or a living insect. The motionless insect, like 

 the motionless pebble, escapes. It is something moving in a field 

 otherwise quiescent that excites the reaction. This reaction is de- 

 pendent, therefore, on a detailed retinal image associated with a 

 highly differentiated central nervous apparatus. 



By a strange coincidence a frog through a simple operation may 

 be reduced from an animal responding in the highly complex way 

 just described to one that reacts after the style of pure phototropism. 

 Frogs, like most other animals of their class, are sensitive to light 

 through the skin. If the anterior part of the head of a frog is cut 

 ofT transversely just behind the eyes, the operation deprives the ani- 

 mal at once of retinal images and of its higher nervous centers. 

 What is left of the animal still responds to light, but only through 

 the skin and by means of a much simpler central apparatus than it 

 had before the operation. Such a frog will maintain a natural sitting 

 posture, and, if near a window, it will turn till it faces the light, after 

 which it will commonly move forward from time to time toward the 

 window. It is in no way excited by small moving objects about it, 



PROC. AMER. PHIL. bOC, VOL. LXI, I, AUG. 3I, 1 922. 



