Students of the physical sciences discovered in the nineteenth century a universal law of Nature, always believed by the wisest since the time of Thales, but never before proven, which is now commonly known as the law of the conservation of energy. When we say to a child, "You cannot eat your cake and have it," we are expressing the law of the conservation of matter, which is really a more or less accurate part-expression of the law of the conservation of energy. The law that from nothing nothing is made--and further, though here this concerns us less, that nothing is ever destroyed--is the only firm foundation for any work or any theory whether in science or philosophy. The chemist who otherwise bases his account of a reaction is wrong; the sociologist who denies it Nature will deny. It was the sure foundation upon which Herbert Spencer erected the philosophy of evolution; and every page of this book depends upon the certainty that this law applies to woman and to womanhood as it does to the rest of the universe. Further, it may be shown that certain less universal but most important generalizations made by two or three biologists are indeed special cases of the universal law. There is, first, the law of Herbert Spencer, which states that for every individual there is an inevitable issue between the demands of parenthood and the demands of self; and there is, secondly, the law of Professors Geddes and Thomson, which asserts that this issue specially concerns the female as compared with the male sex, the distinguishing character of femaleness being that in it a higher proportion of the vital energy is expended upon or conserved for the future and therefore, necessarily, a smaller proportion for the purposes of the individual. It is of service to one's thinking, perhaps, to regard Geddes and Thomson's law as a special case of Spencer's, and Spencer's as a special case of the law of the conservation of energy. First, then, somewhat of detail regarding the law of balance between expenditure on the self and expenditure upon the race; and then to the all-important application of this to the case of womanhood--for upon this application the whole of the subsequent argument depends.