During middle age, life's active period, there is an equilibrium between the body's waste and repair: one equals the other. The machine, when properly managed, then holds its own. A French physiologist fixes the close of this period for the ideal man of the future at eighty, when, he says, old age begins. Few have such inherited power, and live with such physiological wisdom, as to keep their machine in good repair,--in good working-order,--to that late period. From the age of twenty-five or thirty, however, to that of sixty or sixty-five, this equilibrium occurs. Repair then equals waste; reconstruction equals destruction. The female organization, like the male, is now developed: its tissues are consolidated; its functions are established. With decent care, it can perform an immense amount of physical and mental labor. It is now capable of its best work. But, in order to do its best, it must obey the law of periodicity; just as the male organization, to do its best, must obey the law of sustained effort. When old age begins, whether, normally, at seventy or eighty, or, prematurely, at fifty or thirty, repair does not equal waste, and degeneration of tissue results. More cells are destroyed by wear and tear than are made up from nutriment. The friction of the machine rubs the stuff of life away faster than it can be replaced. The muscles stiffen, the hair turns white, the joints crack, the arteries ossify, the nerve-centres harden or soften: all sorts of degeneration creep on till death appears,--_Mors janua vitae._ There the curves unite, and men and women are alike again. Sleep, whose inventor received the benediction of Sancho Panza, and whose power Dryden apostrophized,-- "Of all the powers the best: Oh! peace of mind, repairer of decay, Whose balm renews the limbs to labor of the day,"--