The old notion that our bodies are changed every seven years, science has long since exploded. "The matter," said Mr. John Goodsir, "of the organized frame to its minutest parts is in a continual flux." Our bodies are never the same for any two successive days. The feet that Mary shall dance with next Christmas Eve will not be the same feet that bore her triumphantly through the previous Christmas holidays. The brain that she learns German with to-day does not contain a cell in its convolutions that was spent in studying French one year ago. Whether her present feet can dance better or worse than those of a year ago, and whether her present brain can _do_ more or less German and French than the one of the year before, depends upon how she has used her feet and brain during the intervening time, that is, upon the metamorphosis of her tissue. From birth to adult age, the cells of muscle, organ, and brain that are spent in the activities of life, such as digesting, growing, studying, playing, working, and the like, are replaced by others of better quality and larger number. At least, such is the case where metamorphosis is permitted to go on normally. The result is growth and development. This growing period or formative epoch extends from birth to the age of twenty or twenty-five years. Its duration is shorter for a girl than for a boy. She ripens quicker than he. In the four years from fourteen to eighteen, she accomplishes an amount of physiological cell change and growth which Nature does not require of a boy in less than twice that number of years. It is obvious, that to secure the best kind of growth during this period, and the best development at the end of it, the waste of tissue produced by study, work, and fashion must not be so great that repair will only equal it. It is equally obvious that a girl upon whom Nature, for a limited period and for a definite purpose, imposes so great a physiological task, will not have as much power left for the tasks of the school, as the boy of whom Nature requires less at the corresponding epoch. A margin must be allowed for growth. The repair must be greater and better than the waste.