THE NERVOUS SYSTEM 



47 



have this specialized structure, adaptive response is 

 a function of that structure only. Illumination on 

 this point is found in the experiments made by Loeb 

 upon the heliotropic reactions of plants and animals 

 and on the sustained reflexes of animals whose central 

 nervous system has been removed. Loeb showed that 

 the phenomenon of heliotropism, that is, of orientation 

 with respect to light, which in animals was formerly 

 attributed to a "psychic" faculty, is essentially identi- 



From Loeb's " The Mechanistic Conception of Life." 

 FIG. 3. POSITIVE HELIOTROPISM OF A MARINE WORM (SPIROGRAPHIS). 



"The light fell into the aquarium from one side only and the worms all 

 bent their heads toward the source of light, as the stems of positively helio- 

 tropic plants would do under the same conditions." 



cal in both plants and animals and that it occurs in 

 both in accordance with the laws of photochemical 

 action. (Fig. 3.) The fact that some animals ex- 

 hibiting this reaction have a true central nervous 

 system while plants have none is aside from the prob- 

 lem. Other animals having a diffuse type of nervous 

 system, but no central organ, also show typical 

 phototropism (Crozier). 



If it be true, as these experiments seemed to show, 



