at a habit is a
tendency toward a certain type of action in a certain situation. The
basis of this tendency is in the nervous system. In order to understand
it we must consider what the nervous system is like. Nerves terminate at
one end in a sense organ and at the other end ultimately in a muscle.

In Figure II, A is a sense organ, B a nerve going from the sense organ
to the brain C. D, E, F, G, and H are motor nerves going from the brain
to the muscles. Now, let us show from the diagram what organization
means and what tendency means. At first when the child sees the
expression "nine times nine," he does not say "eighty-one." The stimulus
brings about no definite action. It is as likely to go out through E or
F as through D. But suppose we can get the child to say "nine times nine
equals eighty-one." We can write the expression on the blackboard and
have the child look at it and say "nine times nine equals eighty-one."
Suppose the act of saying "eighty-one" is brought about by the
nerve-current going out through nerve-chain D. By repetition, we
establish a bond. A stimulus of a particular kind comes through A, goes
over B to C, and out over D, making muscles at M bring about a very
definite action in saying "eighty-one."

[Illustration: FIGURE II.--THE ORGANIZATION OF TENDENCIES]

From the point of view of physiology, the process of habit-formation
consists in securing a particular nerve coupling, establishing a
particular nerve path, so that a definite form of stimulation will bring
about a definite form of response. A nerve tendency is simply the
likelihood that a stimulus will take a certain course rather than any
other. This likelihood is brought about by getting the stimulus to take
the desired route through the nervous system to a group of muscles and
to continue following this route. The more times it passes the same way,
the greater is the probability that at any given time the stimulus will
take the accustomed route and bring about the usual response. At first
any sort of action is possible. A nerve stimulus can take any one of the
many routes to the different muscles. By chance or by conscious
direction, the stimulus takes a certain path, and by repetition we fix
and make permanent this particular route. This constitutes a nerve
tendency or habit.

=Plasticity.= Our discussion should have made it clear that habit is
acquired nature, while instinct is inherited nature. Habit is acquired
tendency while ins