The first impeding cause is one already alluded to. On looking at the analogous instance of muscular increase from muscular action, it will be granted at once that, in some constitutions, there is a much greater susceptibility of change than in others. In the nervous system, the same principle of the influence of the original type undoubtedly holds good; and while some are easily susceptible of mental impressions and cerebral improvement, others are the reverse. Here, then, is one ground of difference of result. Another fact in regard to muscular development is, that while it is favoured by due exercise, it is prevented alike by insufficient and by excessive action, and that _what constitutes due exercise to one, may be insufficient for another, and excessive for a third_. From this follows the acknowledged axiom—That exercise ought to be adapted in kind and degree to the individual constitution, otherwise it will fail to increase either the muscles or the general strength. I have elsewhere[1] shown that the same law applies to the brain and nervous system, and that, if we act regardless of its existence, we inevitably fail in successfully attaining our object. From ignorance of physiology, however, on the part of teachers and parents, and ignorance of the connection subsisting between the brain and the mind, this law has been utterly neglected in practice. In our larger schools, accordingly, we have from one hundred to one hundred and fifty boys in each class, or from five hundred to six hundred in all, subjected to precisely the same amount of work, and to the same general management, in so far as the period of confinement and mental activity are concerned; and the individual powers and wants of each constitution are as little consulted, as if the whole were cast of the same material, and the same mould—and the result is what we behold and lament. In some, the degree of mental exercise is adapted to their capability, and they improve; in others, it falls much short, and their powers languish from inaction; while in a third portion it goes as far beyond the limit, and their minds and organs are worn out and impaired.