60. Equally, throughout the whole nature, may be traced the law
that exercised faculties are ever tending to resume their original
state. Not only after continued rest, do they regain their full
power not only do brief cessations partially reinvigorate them; but
even while they are in action, the resulting exhaustion is ever
being neutralized. The two processes of waste and repair go on
together. Hence with faculties habitually exercised--as the senses
of all persons, or the muscles of any one who is strong--it happens
that, during moderate activity, the repair is so nearly equal to
the waste, that the diminution of power is scarcely appreciable;
and it is only when the activity has been long continued, or has
been very violent, that the repair becomes so far in arrear of
the waste as to produce a perceptible prostration. In all cases,
however, when, by the action of a faculty, waste has been incurred,
_some_ lapse of time must take place before full efficiency can be
reacquired; and this time must be long in proportion as the waste
has been great.
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