r the
withdrawing of a hand from flame, such is the physiology of the
process. But where an immediate adjustment cannot be
made by an instinctive response, where satisfaction is not
secured by the passage of a sensory stimulus to an immediate
motor response, the nervous impulse is, as it were, deflected
to the brain area, auditory, visual, or whatever it may be,
which is associated with that particular type of sensation.
The path to the brain area is far from simple; the nervous impulse,
which might be compared to an electric current, must
pass through many nerve junctions known as "synapses," at
which points there is some not completely understood chemical
resistance offered to the passage of the nerve current. On
passing through the network of nerves in the brain area, the
current passes back again through a complicated maze of connections
to a motor nerve which insures a muscular response.
The first time a stimulus passes through this network the
resistance offered at the nerve junction or synapse is very high;
at succeeding repetitions of the stimulus the resistance is
reduced, the nerve current passes more rapidly and fluently
over the paths it has already traveled, and the action resulting
becomes as direct and automatic as if it were an original
reflex action.[1]

[Footnote 1: See McDougall: _Physiological Psychology_.]

THE ACQUISITION OF NEW MODES OF RESPONSE. Expressed in
less technical language this means simply that human beings
can learn by experience, and that they tend to repeat actions
they have once learned. Where an animal is perfectly adjusted
to its environment, all stimuli issue in immediate and
nicely adjusted responses. This happens only where the
environment is very simple and stable, and where in
consequence no complexity of structure or action is necessary. In
the clam and the oyster, and in some of the lower vertebrates,
perhaps, instinctive activity is almost exclusively
present. But in the case of man, so complicated are the
situations to which he is exposed that random instinctive
responses will not solve his problems. He must, as with his
highly modifiable nervous system he can, acquire new modes
of response which will, in the complexity of new situations
serve as effectively as his original tendencies to act would
serve him in a simpler and stabler environment. A human
being in a modern city cannot live by instinct alone; he must
acquire an enormous number of habits to meet the var