We complain that Nature, in place of making our bodies of cast iron, so to speak, makes them, on the contrary, vulnerable at every point. The reason is, surely, that the less we are constituted like cast iron--the more vital and complex, intelligent and responsive, our tissues are, accordingly--the more conducive to change and advance (because the more sensitively re-active to subtler and psychical stimuli) they are likewise. We cannot be, at the same time, hardy and obtuse, yet exquisitely sensitive. Living tissue-cells are characterised, beyond all other developments, by a range of contrasting abilities. An arm serves as softest cushion for a child's head, or, by stiffening of its muscles, becomes rigid as steel. An eye that sees for miles will focus to a pin-point. But being, as we are, still in the making, our tissues necessarily have limitations--and the defects, accordingly, of both their sets of qualities. High sensitiveness of function is necessarily attended by corresponding complexity and delicacy of structure. Such structural delicacy obliges us to adapt environment to its complexities. It is thus an incentive to progress. It obliges us, as well, to moderate our activities, and, by thus restricting the output of our cruder powers, our resources are husbanded and directed into higher channels. The purpose of the complex differentiations which handicap the adolescent girl is obvious. The curving bones, the expanding pelvis, the rounded contours, the inhibited muscles, the languors and recurring disabilities, are designed to restrict activity, physical and mental. Physicists tell us that the Conservation of Motion and the Conservation of Energy are one and the same thing. This must be true, as well, of _Vital_ Energy. The conservation of Vital Activity subtends the Conservation of Vital resources. The new developments are by no means incidental merely to the new processes; they are an integral part of The Plan. In half-closing the doors on avenues of active output, Nature conserves the Woman-powers for more intrinsic use. Every brain and body-cell is raised thereby to higher levels both of constitution and of function. As stored _mechanical_ energy becomes transformed into the higher form of _electrical_ energy, so the power stored in Woman's anabolic cells is raised to higher evolutionary forms. Thus she becomes fitted to be mother of the Child--the blossom of the Race. Her part in the child will contain the inherence of these new higher evolutionary values, as the father's part in it will contain the inherence of the concrete powers he has developed. And while her body spontaneously raises all its issues in order to fit her to be a Mother, so it develops powers and functions adapting her to serve as soft environment, physical and attributal, for the rearing of her child.