st on premature or undue growth of any one part, she
will, with more or less protest, concede the point; but that she may do
your extra work, she must leave some of her more important work undone.
Let it never be forgotten that the amount of vital energy which the body
at any moment possesses, is limited; and that, being limited, it is
impossible to get from it more than a fixed quantity of results. In a
child or youth the demands upon this vital energy are various and
urgent. As before pointed out, the waste consequent on the day's bodily
exercise has to be met; the wear of brain entailed by the day's study
has to be made good; a certain additional growth of body has to be
provided for; and also a certain additional growth of brain: to which
must be added the amount of energy absorbed in digesting the large
quantity of food required for meeting these many demands. Now, that to
divert an excess of energy into any one of these channels is to abstract
it from the others, is both manifest _a priori_, and proved _a
posteriori_, by the experience of every one. Every one knows, for
instance, that the digestion of a heavy meal makes such a demand on the
system as to produce lassitude of mind and body, frequently ending in
sleep. Every one knows, too, that excess of bodily exercise diminishes
the power of thought--that the temporary prostration following any
sudden exertion, or the fatigue produced by a thirty miles' walk, is
accompanied by a disinclination to mental effort; that, after a month's
pedestrian tour, the mental inertia is such that some days are required
to overcome it; and that in peasants who spend their lives in muscular
labour the activity of mind is very small. Again, it is a familiar truth
that during those fits of rapid growth which sometimes occur in
childhood, the great abstraction of energy is shown in an attendant
prostration, bodily and mental. Once more, the facts that violent
muscular exertion after eating, will stop digestion; and that children
who are early put to hard labour become stunted; similarly exhibit the
antagonism--similarly imply that excess of activity in one direction
involves deficiency of it in other directions. Now, the law which is
thus manifest in extreme cases, holds in all cases. These injurious
abstractions of energy as certainly take place when the undue demands
are slight and constant, as when they are great and sudden. Hence, if
during youth the expenditure in mental labour exc