III 
ON INSTINCT AND WILL IN ANIMALS! 
1. In the biological literature one still finds authors who 
treat the “instinct” or the “will” of animals as a circum- 
stances which determines motions, so that the scientist who 
enters the region of animated nature encounters an entirely 
new category of causes, such as are said continually to pro- 
duce before our eyes great effects, without it being possible 
for an engineer ever to make use of these causes in the physi- 
cal world. “Instinct” and “will” in animals, as causes 
which determine movements, stand upon the same plane as 
the supernatural powers of theologians, which are also said 
to determine motions, but upon which an engineer could not 
well rely. 
My investigations on the heliotropism of animals led me 
to analyze in a few cases the conditions which determine the 
apparently accidental direction of animal movements which, 
according to traditional notions, are called voluntary or 
instinctive. Wherever I have thus far investigated the 
cause of such ‘‘voluntary” or “instinctive” movements in 
animals, I have without exception discovered such circum- 
stances at work as are known in inanimate nature as deter- 
mining movements. By the help of these causes it is pos- 
sible to control the “‘voluntary” movements of a living ani- 
mal just as securely and unequivocally as the engineer has 
been able to control the movements in inanimate nature. 
What has been taken for the effect of “will” or “instinct” 
is in reality the effect of light, of gravity, of friction, of 
chemical forces, etc. The following may be added by way 
of fuller explanation : 
1 Pfliigers Archiv, Vol. XLVITI (1899), p. 407. 
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