ON INSTINCT AND WILL IN ANIMALS 109 



ticularly that of the more refrangible ones, determines also 

 the orientation of the organs of a plant. By the help of 

 light the botanist controls the orientation of a plant at will. 

 Why should he maintain that the "will" or the "instinct" 

 of the plant co-operates with the rays of light when the 

 orientation is determined solely and unequivocally by the 

 latter ? The movements of an animal toward the light are, 

 however, as I have shown, identical point for point with 

 the movement of a plant toward the light. Wherever the 

 orientation of plants has been satisfactorily controlled experi- 

 mentally, light has, indeed, been considered the sole deter- 

 mining factor ; but in the case of animals, in which in similar 

 experiments light is without doubt also the sole determining 

 factor, "instinct" and "free will" have still been considered 

 to play a role. 



Just as the direction of the rays of light (particularly that 

 of the more refrangible ones) is the essential factor in the ex- 

 amples described above, and in many others given in my 

 papers on heliotropism, so in other cases it is gravity, in others 

 again contact with solid bodies, in still others chemical forces, 

 etc., which determine the movements of the animals. 



2. In order to state the cause which determines in each 

 instance the "voluntary" movements of an animal, I desig- 

 nated the movements by their external cause. I spoke 

 therefore, as has long been the custom in plant physiology, 

 of heliotropism when the direction of the rays of light 

 determines the direction of the movements of an animal or 

 its orientation ; of geotropism, when gravity, or of stereot- 

 ropism, when contact with solid bodies, determines the 

 orientation or the movements ; etc. 



A zoologist asked me reproachfully what had been gained 

 by designating as "stereotropism" what had been designated 

 as "instinct." I was discussing the fact that certain ani- 

 mals creep into the crevices of solid bodies, and the zoologist 



