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 réle. 
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 zodlogist 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 zodlogist 
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