The whole teaching of this book, from social generalizations to the details of the wise management of girlhood, is based upon a single and simple principle, often referred to and always assumed in former writings from this pen, and in public speaking from many and various platforms. If this principle be invalid, the whole of the practice which is sought to be based upon it falls to the ground; but if it be valid, it is of supreme importance as the sole foundation upon which can be erected any structure of truth regarding woman and womanhood. Our first concern, therefore, must be to state this principle, and the evidence therefor. This will occupy not a small space: and the remainder will be amply filled with the details of its application to woman as girl and mother and grandmother, as wife and widow, as individual and citizen. Woman is Nature's supreme organ of the future, and it is as such that she will here be regarded. The purpose of adding yet another to the many books on various aspects of womanhood is to propound and, if possible, establish this conception of womanhood, and to find in it a never-failing guide to the right living of the individual life, an infallible criterion of right and wrong in all proposals for the future of womanhood, whether economic, political, educational, whether regarding marriage or divorce, or any other subject that concerns womanhood. A principle for which so much is claimed demands clear definition and inexpugnable foundation in the "solid ground of Nature." Cogent in some measure though the argument would be, we must appeal in the first place neither to the poets, nor to our own naturally implanted preferences in womanhood, nor to any teaching that claims extra-natural authority. Our first question must be--Do Nature and Life, the facts and laws of the continuance and maintenance of living creatures, lend countenance to this idea; can it be translated from general terms, essentially poetic and therefore suspect by many, into precise, hard, scientific language; is it a fact, like the atomic weight of oxygen or the laws of motion, that woman is Nature's supreme instrument of the future? If the answer to these questions be affirmative, the evidence of the poets, of our own preferences, of religions ancient and modern, is of merely secondary concern as corroborative, and as serving curiosity to observe how far the teachings of passionless science have been divined or denied by past ages and by other modes of perception and inquiry. Therefore this is to be in its basis none other than a biological treatise; for the laws of reproduction, the newly gained knowledge regarding the nature of sex, and the facts of physiology, afford the evidence of the essentially biological truth which has been so often expressed by the present writer in the quasi-poetic terms already set forth. Let us, then, first remind ourselves how the individual, whether male or female, is to be looked upon in the light of