Affective Tendencies 363 
induced by real hunger, is—it is scarcely necessary to 
state—a natural consequence and of but secondary im- 
portance. It is only one of many forms in which we see 
the substitution of the part for the whole, and this phe- 
nomenon characteristic of all mnemonic physiological 
processes appears also in the tendency to physiological 
invariability, which is also essentially mnemonic as we 
shall see more clearly later on. These peculiar sensa- 
tions localized in the gastric mucous membrane and 
produced by its swelling or by some other more or less 
similar change caused by the empty condition of the 
stomach, usually take place before or simultaneously 
with the actual lack of histogenetic substance in the blood, 
and so finally became representative or vicarious signs of 
hunger. 
The same is true of thirst and of its localization in 
the upper part of the alimentary canal. 
We might pass on from hunger and thirst to the 
other more or less fundamental organic “appetites” or 
“needs.”’ All would show us in their different manifes- 
tations that they are directed simply and solely toward 
the restoration of the stationary physiological state, 
which has been lost or in some way disturbed. 
Thus there exists for every animal species an opti- 
mum of environment with reference to the degree of 
saturation of the solution in which the animal lives, to 
the temperature or to the intensity of light, etc., above 
and below which the organism cannot maintain its nor- 
mal physiological state and which the animal makes 
every effort to maintain. 
So for instance we see that the infusorian Paramac- 
cium at 28° C. reacts negatively toward a rising but not 
toward a falling temperature, whereas at 22° C. it 
