e individual, much less for different functions, at different
stages of advancement, in different individuals. Much more experimental
work is needed before definite general results can be stated.

This experimental work, however, is suggestive along several lines, (1)
It seems possible that habits of skill, involving direct sensori-motor
bonds, are more permanent than memories involving connections between
association bonds. In other words, that physical habits are more lasting
than memories of intellectual facts. (2) Overlearning seems a necessary
correlate of permanence of connection. That is, what seems to be
overlearning at beginning stages is really only raising the material to
the necessary level above the threshold for retention. How far
overlearning is necessary and when it becomes wasteful are yet to be
determined. (3) Deterioration is hastened by competing connections. If
during the time a particular function is lying idle other bonds of
connection are being formed into some parts or elements of it, the rate
of forgetting of the function in question is hastened and the
possibility of recall made more problematic. The less the interference,
the greater will be the permanence of the particular bonds.

A belief maintained by some psychologists is in direct opposition to
this general law that disuse causes deterioration. It is usually stated
something like this, that periods of incubation are necessary in
acquiring skill, or that letting a function lie fallow results in
greater skill at the end of that period, or briefly one learns to skate
in summer and swim in winter. To some extent this is true, but as stated
it is misleading. The general law of the effect of disuse on a memory is
true, but under some circumstances its effect is mitigated by the
presence of other factors whose presence has been unnoted. Sometimes
this improvement without practice is explained by the fact that at the
last practice period the actual improvement was masked by fatigue or
boredom, so that disuse involving rest and the disappearance of fatigue
and boredom produces apparent gain, when in reality it but allows the
real improvement to become evident. Sometimes a particular practice
period was accompanied by certain undesirable elements such as worry,
excitement, misunderstandings, and so on, and therefore the improvement
hindered or masked, whereas at the next period under different
conditions there would be less interference and theref