11. Commencing at the age of thirteen, strength of grip in boys shows a marked accentuation in its rate of increase, and this increase continues as far as our observations extend, namely, to the age of twenty. In girls no such great acceleration in muscular strength at puberty occurs, and after sixteen there is little increase in strength of grip. The well-known muscular differentiation of the sexes practically begins at thirteen. 12. As with strength of grip, so with endurance as measured by the ergograph; boys surpass girls at all ages, and this differentiation becomes very marked after the age of fourteen, after which age girls increase in strength and endurance but very slightly, while after fourteen boys acquire almost exactly half of the total power in these two features which they acquire in the first twenty years of life. 13. The development of vital capacity bears a striking resemblance to that of endurance, the curves representing the two being almost identical. Physiological age, according to studies made in New York City, should be considered in grading, not only for physical culture classes but for all high school or continuation classes. Dr. C. Ward Crampton, assistant physical director, while examining boys in the first grade of the High School of Commerce, noticed a greater variation in physical advancement than in years. He kept careful watch of the educational progress and discovered three clear divisions: (1) boys arrived at puberty,--postpubescent; (2) boys approaching maturity,--pubescent; (3) boys not yet approaching maturity,--prepubescent. The work in lower grades they had all passed satisfactorily, but in high school only the most advanced class did well. Practically none of the not-yet-maturing boys survived and few of the almost mature. In other words, the high school course was fitted to only one of the three classes of boys turned out of the grammar schools. The others succumbed like hothouse azaleas at Christmas time, forced beyond their season. Physiological age, not calendar years or grammar school months, should determine the studies and the companions of children after the tenth year. Physiological strength and vitality, not ability to spell or to remember dates, should be the basis of grading for play and study and companionship among younger children. Vitality, power to endure physically, should be the test of work and recreation for adults. Physicians may be so trained to follow directions issued by experts that physical examinations will disclose the chief enemies of vitality and the approximate limits of endurance. Teachers may train themselves to recognize signs of fatigue in school children and to adapt each day's, each hour's work to the endurance of each pupil. One woman principal has written: