the rate of deterioration depend upon the particular function tested. No
one function can serve as a sample for others. No one curve of
forgetting exists for different functions at the same stage of
advancement or for the same function at different stages of advancement
in the same 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