Age after age brings forward varying phases of thought, when some particular facts of life are thrown into unusual prominence, such special development of thought serving to mould the society of that generation, giving it a special stamp, and thus advancing the progress of humanity one step forward. Of all the ideas gradually worked out and gained as the permanent possession of human society, the slowest in growth is the idea of the true relations of the sexes. The instinct of sex always exists as the indispensable condition of life and the foundation of society. It is the strongest force in human nature. Whatever else disappears, this continues. Undeveloped, no subject of thought, but nevertheless as the central fire of life, Nature guards this inevitable instinct from all possibility of destruction. But as an idea, thought out in all its wide relations, shaped in human practice in all its ennobling influences, it is the latest growth of civilization. In whatever concerns the subject of sex, customs are blindly considered sacred, and evils deemed inevitable. The mass of mankind seems moved with anger, fear, or shame, by any effort made to consider seriously this fundamental idea. It must necessarily come forward, however, in the progress of events, as the subject of primary importance. As society advances, as principles of justice and humanity become firmly established, as science and industry prepare the way for the more perfect command of the material world, it will be found that the time has come for the serious consideration of this first and last question in human welfare, for the subject of sex will then present itself as the great aid or obstacle to further progress. The gradually growing conviction will be felt that, as it is the fundamental principle of all society, so it is its crowning glory. In the relations of men and women will be found the chief cause of past national decline, or the promise of indefinite future progress. The family, being the first simple element of society--the first natural product of the principle of sex--the whole structure of society must depend upon the character of that element, and the powers that can be unfolded from it. Morality in sex will be found to be the essence of all morality, securing principles of justice, honour, and uprightness in the most influential of all human relations, and as it is all-important in life, so it is all-important in the education which prepares for life. A great social question lies, therefore, at the foundation of the moral education of youth, and influences more or less directly each step of education. It becomes indispensable to consider the relation of this subject to the various stages of education, and the methods by means of which education may guide and strengthen youth in their entrance into wider social life.