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 exceeds that which Nature has provided for; the expenditure for other purposes falls below what it should have been; and evils of one kind or other are inevitably entailed.  Let us briefly consider these evils.

Supposing the over-activity of brain to exceed the normal activity only in a moderate degree, there will be nothing more than some slight reaction on the development of the body:  the stature falling a little below that which it would else have reached; or the bulk being less than it would have been; or the quality of tissue not being so good.  One or more of these effects must necessarily occur.  The extra quantity of blood supplied to the brain during mental exertion, and during the subsequent period in which the waste of cerebral substance is being made good, is blood that would else have been circulating through the limbs and viscera; and the growth or repair for which that blood would have supplied materials, is lost.  The physical reaction being certain, the question is, whether the gain resulting from the extra culture is equivalent to the loss?—­whether defect of bodily growth, or the want of that structural perfection which gives vigour and endurance, is compensated by the additional knowledge acquired?