276 Animal Instincts and Tropisms 



light, the other half in the shade. If at the be- 

 ginning of the experiment the positively heliotropic 

 animals are in the direct sunlight at a, they promptly 

 move toward the window, gathering at the window 

 end c of the tube, although by so doing they go 

 from the sunshine into the shade. 1 This experi- 

 ment is in harmony with our idea that the effect 

 of light consists in turning the head of the animal 

 and subsequently its whole body toward the source of 

 light. By going from the strong light into the shade 

 the reaction velocity in both eyes is diminished equally 

 and hence there is no reason for the animal to change 

 its orientation, though its progressive motion may be 

 stopped for an instant by the change. But at the 

 boundary between sunlight and daylight a sudden 

 change from strong to weak light occurs. If the 

 energy gradient determined the direction of the posi- 

 tively heliotropic animal, the motion should stop at 

 the boundary from strong to weak light, which may 

 happen for an instant but which will not interfere with 

 the progressive motion of the animal. 



8. Graber had found that when animals are put into 

 a trough covered half with blue and half with red glass, 

 those that are "fond" of light go under the blue, those 

 that are "fond" of darkness go under the red glass. 

 The writer pointed out that this result should be 

 expected on the basis of his theory of heliotropism, if 



3 Loeb, ]., Dynamics of Living Matter, p. 126. 



