Experience teaches that a healthy and growing boy may spend six hours of force daily upon his studies, and leave sufficient margin for physical growth. A girl cannot spend more than four, or, in occasional instances, five hours of force daily upon her studies, and leave sufficient margin for the general physical growth that she must make in common with a boy, and also for constructing a reproductive apparatus. If she puts as much force into her brain education as a boy, the brain or the special apparatus will suffer. Appropriate education and appropriate co-education must adjust their methods and regimen to this law. Another detail is, that, during every fourth week, there should be a remission, and sometimes an intermission, of both study and exercise. Some individuals require, at that time, a complete intermission from mental and physical effort for a single day; others for two or three days; others require only a remission, and can do half work safely for two or three days, and their usual work after that. The diminished labor, which shall give Nature an opportunity to accomplish her special periodical task and growth, is a physiological necessity for all, however robust they may seem to be. The apportionment of study and exercise to individual needs cannot be decided by general rules, nor can the decision of it be safely left to the pupil's caprice or ambition. Each case must be decided upon its own merits. The organization of studies and instruction must be flexible enough to admit of the periodical and temporary absence of each pupil, without loss of rank, or necessity of making up work, from recitation, and exercise of all sorts. The periodical type of woman's way of work must be harmonized with the persistent type of man's way of work in any successful plan of co-education.