4 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



stimulus passes to the central nervous system, and 

 from there to the muscles of the wings, and the moth 

 is caused to fly into the flame. This reflex process 

 agrees in every point with the heliotropic effects of 

 light on plant organs. Since plants possess no nerves, 

 this identity of animal with plant heliotropism can 

 offer but one inference these heliotropic effects must 

 depend upon conditions which are common to both 

 animals and plants. At the end of my book on helio- 

 tropism I expressed this view in the following words : 

 " We have seen that, in the case of animals which 

 possess nerves, the movements of orientation toward 

 light are governed by exactly the same external con- 

 ditions, and depend in the same way upon the external 

 form of the body, as in the case of plants which possess 

 no nerves. These heliotropic phenomena, conse- 

 quently, cannot depend upon specific qualities of the 

 central nervous system (i)." On the other hand, 

 the objection has been raised that destruction of the 

 ganglion-cells interrupts the reflex process. This 

 argument, however, is not sound, for the nervous 

 reflex arc in higher animals forms the only protoplas- 

 mic bridge between the sensory organs of the surface 

 of the body and the muscles. If we destroy the gan- 

 glion-cells or the central nervous system, we interrupt 

 the continuity of the protoplasmic conduction between 

 the surface of the body and the muscles, and a reflex 

 is no longer possible. Since the axis-cylinders of the 

 nerves and the ganglion-cells are nothing more than 

 protoplasmic formations, we are justified in seeking 



