The normality of the reaction to sex knowledge depends upon the physical and mental training of the child. Our thoughts concerning girls run in fixed grooves. We believe that, in babyhood, instinct leads them to prefer dolls to their brothers' guns and a little later renders them less active physically and more gentle and tractable mentally. Because of this supposed difference in instincts and because of a well-defined picture in our own minds of the final product we wish to evolve, we build a structure externally fair, but lacking the foundation to enable it to resist the stress of time and circumstance. Because of our traditionally different ways of dealing with girls and boys, we have produced girls who are not healthy little animals, but women in miniature with nervous systems too unstable to cope successfully with the strain of our modern complex life. The stability of the nervous system is dependent upon the proper development of the fundamental centers. Incomplete development of the lower parts means incomplete development in the higher. These fundamental centers are stimulated to growth and development especially by the activity of the large muscle masses. Not only is the development of the brain and nervous system dependent upon muscular activity, but the growth and activity of the vital organs as well,--the heart, lungs, and digestive system,--and the normality of sex life. All this we acknowledge in the case of the boy. Even with him, we fail to live up to our convictions, as is shown by the long hours of inactivity in school and the lack of suitable activities during recess periods. But on the whole we encourage the boy to run and climb and jump and take distinct pride in these accomplishments. The same accomplishments in our girls occasion alarm; we have an ideal of gentle womanhood. Even though unrestrained up to the time she attends school, the girl then enters upon the long career of physical repression which characterizes her training. Parents, teachers, neighbors, and schoolmates often seem to conspire to curb all the natural impulses upon which her health and rounded development depend. Aside from the reproductive organs, the physical mechanism of the girl is much like that of the boy. There is no peculiarity in the structure of the reproductive organs to prohibit vigorous activity. The development and health of these organs and their ligamentous supports are dependent primarily upon the quality and free circulation of the blood, both of which are preeminently the result of fresh air and exercise. If the muscular system in general is well developed, there is no reason why the muscular and ligamentous structure of the reproductive organs should not be equally well developed. To insure their proper development, exercise is essential.