r answer follows the sensory stimulus
almost instantaneously. The great advantage of establishing or
enregistering these reflex chains is that the answers are practically
ready-made or inborn, not requiring to be learned. It is not necessary
that the brain should be stimulated if there is a brain; nor does the
animal will to act, though in certain cases it may by means of higher
controlling nerve-centres keep the natural reflex response from being
given, as happens, for instance, when we control a cough or a sneeze on
some solemn occasion. The evolutionary method, if we may use the
expression, has been to enregister ready-made responses; and as we
ascend the animal kingdom, we find reflex actions becoming complicated
and often linked together, so that the occurrence of one pulls the
trigger of another, and so on in a chain. The behaviour of the
insectivorous plant called Venus's fly-trap when it shuts on an insect
is like a reflex action in an animal, but plants have no definite
nervous system.

What are Called Tropisms

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 role
in animal behaviour.

Sec. 2

Instinctive Behaviour

On a higher level is instinctive behaviour, which reaches such
remarkable perfection in ants, bees, and wasps. In its typical
expression instinctive behaviour depends on inborn capacities; it does
not require to be learned; it is independent of practice or experience,
though it may be improved by both; it is shared equally by all members
of the species of the same sex (for the female's instincts are often
different from the male's); it refers to particular conditions of life
that are of vital importance, though