The system never does two things well at the same time. The muscles and the brain cannot _functionate_ in their best way at the same moment. One cannot meditate a poem and drive a saw simultaneously, without dividing his force. He may poetize fairly, and saw poorly; or he may saw fairly, and poetize poorly; or he may both saw and poetize indifferently. Brain-work and stomach-work interfere with each other if attempted together. The digestion of a dinner calls force to the stomach, and temporarily slows the brain. The experiment of trying to digest a hearty supper, and to sleep during the process, has sometimes cost the careless experimenter his life. The physiological principle of doing only one thing at a time, if you would do it well, holds as truly of the growth of the organization as it does of the performance of any of its special functions. If excessive labor, either mental or physical, is imposed upon children, male or female, their development will be in some way checked. If the schoolmaster overworks the brains of his pupils, he diverts force to the brain that is needed elsewhere. He spends in the study of geography and arithmetic, of Latin, Greek and chemistry, in the brain-work of the school room, force that should have been spent in the manufacture of blood, muscle, and nerve, that is, in growth. The results are monstrous brains and puny bodies; abnormally active cerebration, and abnormally weak digestion; flowing thought and constipated bowels; lofty aspirations and neuralgic sensations; "A youth of study an old age of _nerves_."