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Rh vague as it is, it is really the last word of our wisdom in the matter.

It must be noticed that the growth of structural modification in living matter may be more rapid than in any lifeless mass because the incessant nutritive renovation of which the living matter is the seat tends often to corroborate and fix the impressed modification, rather than to counteract it by renewing the original constitution of the tissue that has been impressed. Thus, we notice after exercising our muscles or our brain in a new way, that we can do so no longer at that time;