HELIOTEOPISM OF ANIMALS 



on the form of the body in so far as dorsiventral animals 

 move with their median planes in the direction of the rays 

 of light, in which position the rays fall upon symmetrically 

 situated points of the surface of their bodies at nearly equal 

 angles. In this way the fact that a moth flies into a flame 

 turns out to be the same mechanical process as that by which 

 the axis of the stem of a plant puts itself in the direction of 

 the rays of light. In both cases, however in the fatal 

 flight of the moth as well as in the orientation of plants 

 one point remains unexplained, namely: how can the light 

 so change the state of the protoplasm as to bring about the 

 mechanical effects just mentioned? At present we are not 

 able to form a clear idea of this. 



A second condition which has a determining influence 

 upon the mechanical effects of light on plants is the refran- 

 gibility of the rays. Sachs has shown that it is chiefly the 

 more refrangible rays which are able to bring about move- 

 ments in plant organisms. We shall see that quite gen- 

 erally the more refrangible rays are also more effective 

 mechanically in the animal kingdom. 



Thirdly, we shall prove that the orientation of animals as 

 well as of plants takes place when the intensity of the light 

 remains constant. Very often we observe, for example in our 

 eyes, that a change in the intensity of the light acts as a 

 stimulus. In addition to these essential considerations of 

 the effects of light in the animal kingdom, the following 

 factors play a r6le, namely: 



Fourthly, light causes the orientation of animals (as well 

 as of plants) only within certain limits of intensity. Fifthly, 

 temperature influences the movements of orientation in 

 animals and plants toward light which is true for all 

 phenomena of stimulation. 



To sum up: The conditions which control the movements 

 of animals toward light are identical, point for point, 



