MOLLUSKS, ARTHROPODS AND VERTEBRATES 219 



Torelle says (1903, p. 471): " Tests were made at midday 

 on a level tract of ground about two acres in extent which 

 contained neither trees nor any object that could cast a 

 shadow. Six frogs were tried. When freed, each moved 

 indifferently toward any point of the compass, but usually 

 kept on moving in the direction in which it began to move. 

 In several trials no movement resulted; the frog crouched 

 low between short bunches of grass, its head held close to 

 the ground. When dark black or dark brown screens were 

 placed in the middle of this area and the frogs placed 

 within five yards of them, the movement was toward and 

 into the shadow of the screen, where they usually remained 

 indefinitely." In various other experiments it was found 

 that frogs exposed in direct sunlight hopped toward shadows 

 in the neighborhood, no matter if this required movement 

 perpendicular to the rays of light. When first exposed the 

 frogs turned toward the light, but after being in this position 

 a few moments they turned and hopped toward the shadows. 

 After they are in the shade they usually turn so as to face 

 the light. These reactions seem to show that the frogs go 

 toward the shadow because they see and perceive it. The 

 following reactions of toads lead to the same conclusion. 



A. Bufo americanus 



a. Method. — Two horizontal Nernst glowers were so 

 arranged in a large dark room that the rays crossed at right 

 angles above a black table one meter square. The two 

 beams of light, 20 cm. wide at the place of intersection, 

 were parallel with the plane of the table, and the lower edge 

 of the beams was just high enough to clear the table, which 

 was therefore not illuminated. Both beams were absorbed 

 by the dead black walls of the room, which were several 

 meters from the table. The light intensity in the middle of 

 the field from one glower was 12.5 ca. m., and that from 

 the other was 25 ca. m. 



h. Orientation in light from two sources. — On July 7, 

 at 7 P.M., seven toads, two large ones and five small ones. 



