80 STUDIES IN GENERAL PHYSIOLOGY 



appearing in the left at the same distance from the median 

 plane. In short, we can say that the symmetrical plane of 

 an animal from a morphological standpoint is also the 

 symmetrical plane from a physiological standpoint. 



This distribution of irritability on the surface of an 

 animal determines the orientation of dorsiventral animals 

 toward a source of light. If the median plane lies in the 

 direction of the rays of light, the symmetrical points of the 

 surface of the animal are struck by the rays at an equal 

 angle. The effects of the stimuli on the right and left halves 

 of the body annihilate each other, since they are equal in 

 intensity and opposite in direction. The light can therefore 

 produce no tendency to turn to the right or the left. When, 

 however, the median plane is oriented obliquely toward the 

 source of light, unequal forces act upon symmetrical ele- 

 ments, and a tendency to turn must arise which continues 

 until the median plane coincides with the direction of the 

 rays of light. 



This dependence of irritability on the form of the body 

 causes Musca larvae to move away from the source of light 

 precisely in the direction of the rays, and plant lice to move 

 just as precisely in the direction of the rays toward the 

 source of light. The heliotropic movements of an animal 

 are therefore dependent on the symmetrical relations of its 

 body, in the same manner as was shown by Sachs to be the 

 case in plants. 



To show how far these conceptions of heliotropic phenom- 

 ena in animals differ from the prevailing notions on the 

 subject, especially those of the Darwinians, I shall give the 

 views of Romanes on this subject. Romanes mentions the 

 well-known facts that insects of all kinds fly into the flame, 

 that many birds are attracted by the light of lighthouses, 

 and fishes by the lanterns. He explains the phenomenon as 

 follows: " The habit must be attributed to mere curiosity, or 



