al law is stated by James as follows: "When two
elementary brain-processes have been active together or in immediate
succession, one of them, on recurring, tends to propagate its excitement
into the other." This is but a technical statement of the simple fact
that nerve currents flow most easily over the neurone connections that
they have already used.

It is hard to teach an old dog new tricks, because the old tricks employ
familiar, much-used neural paths, while new tricks require the
connecting up of groups of neurones not in the habit of working
together; and the flow of nerve energy is more easily accomplished in
the neurones accustomed to working together. One who learns to speak a
foreign language late in life never attains the facility and ease that
might have been reached at an earlier age. This is because the neural
paths for speech are already set for his mother-tongue, and, with the
lessened plasticity of age, the new paths are hard to establish.

The connections between the various brain areas, or groups of neurones,
are, as we have seen in an earlier chapter, accomplished by means of
_association fibers_. This function requires millions of neurones, which
unite every part of the cortex with every other part, thus making it
possible for a neural activity going on in any particular center to
extend to any other center whatsoever. In the relatively unripe brain of
the child, the association fibers have not yet set up most of their
connections. The age at which memory begins is determined chiefly by
the development of a sufficient number of association fibers to bring
about recall. The more complex reasoning, which requires many different
associative connections, is impossible prior to the existence of
adequate neural development. It is this fact that makes it futile to
attempt to teach young children the more complicated processes of
arithmetic, grammar, or other subjects. They are not yet equipped with
the requisite brain machinery to grasp the necessary associations.

[Illustration: FIG. 18.--Diagrammatic scheme of association, in which V
stands for the visual, A for the auditory, G for the gustatory, M for
the motor, and T for the thought and feeling centers of the cortex.]

ASSOCIATION THE BASIS OF MEMORY.--Without the machinery and processes of
association we could have no memory. Let us see in a simple illustration
how association works in recall. Suppose you are passing an orchard and
see a tree loa