204 LIGHT AND THE BEHAVIOR OF ORGANISMS 



sity. If the change of intensity is greater when the oral 

 end is turned toward the right than when it is turned 

 toward the left, it is stimulated and bends farther toward 

 the left. The direction of bending is generally independent 

 of the direction of the rays, but the extent of bending 

 usually is not. 



Holmes and Harper both pointed out that swinging 

 movements toward the source of light are checked because 

 the animal becomes more and more sensitive as the anterior 

 end extends toward it. This end is however not only 

 checked under such conditions, it is also stimulated and 

 swings farther in the opposite direction. This is an impor- 

 tant factor in the process of orientation, that can hardly be 

 said to be included in the explanation of orientation by 

 selection of random movements, as described by Holmes. 



It may now be asked: Is the orienting stimulus in this 

 form due to a change of light intensity or to the effect of 

 light acting constantly as a directive stimulus ? 



There is no doubt that a change of intensity causes defi- 

 nite reactions in the earthworm, and that reactions thus 

 produced may result in orientation either by the selection 

 of random or trial movements, or by inducing more defi- 

 nitely prescribed movements, as in the blowfly larvae. But 

 this does not indicate that constant light cannot also pro- 

 duce orienting stimulations. There is however no evidence 

 showing that it does. Even in case a worm lies perfectly 

 quiet and very gradually starts to turn from the light w^hen 

 laterally illuminated, as can be readily demonstrated, it is 

 impossible to say whether the stimulus causing the reaction 

 is due to the effect of constant intensity or to change of 

 intensity. And when the worm is in motion, ever extending 

 and contracting the anterior end and changing its position 

 so that the effective intensity is continually changing, it is 

 of course impossible to predict what would take place if 

 the sensitive elements could be exposed to light having a 

 constant intensity. If light is thrown upon a specimen 

 which is perfectly quiet it begins to move, but in this case 



