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 larvee 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 
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