A somewhat higher level on the inclined plane is illustrated by what are
called "tropisms," obligatory movements which the animal makes,
adjusting its whole body so that physiological equilibrium results in
relation to gravity, pressure, currents, moisture, heat, light,
electricity, and surfaces of contact. A moth is flying past a candle;
the eye next the light is more illumined than the other; a physiological
inequilibrium results, affecting nerve-cells and muscle-cells; the
outcome is that the moth automatically adjusts its flight so that both
eyes become equally illumined; in doing this it often flies into the
candle.
It may seem bad business that the moth should fly into the candle, but
the flame is an utterly artificial item in its environment to which no
one can expect it to be adapted. These tropisms play an important rôle
in animal behaviour.
§ 2