The wide bearing and importance of the truths derived from physiology will become more and more apparent, as we examine another branch of the subject, and ascertain from an observation of facts around us, how far the present relations of men and women in civilized countries, are based upon sound principles of physiology. It is necessary to know how far these principles are understood and carried out from infancy onward, whether efforts for the improvement of the race are moulded by physiological methods of human growth, and what are the inevitable consequences which result from departure from these principles. According to a rational and physiological view of life, the family should be cherished as the precious centre of national welfare; every custom, therefore, which tends to support the dignity of the family and which prepares our youth for this life, is of vital importance to a nation. Thus the slow development of the sexual faculties by hygienic regime, by the absence of all unnatural stimulus to these propensities, by the constant association of boys and girls together, under adult influence, in habitual and unconscious companionship, the cultivation in the child’s mind of a true idea of manliness and the perception that self-command is the distinctive peculiarity of the human being, are the ordinary and natural conditions which rational physiology requires. On the contrary, every custom which insults the family and unfits for its establishment, which degrades the natural nobility of human sex, which sneers at it and treats this great principle with flippancy, which tends to kill its Divine essence, all such influences and such customs are a great crime against society, and directly opposed to the teaching of rational physiology. An extended view of social facts, not only in different classes of our own society, but also in those countries with which we are nearly related, is of the utmost value to the parent. Physiological knowledge would be valueless to the mass of mankind, if its direct bearing upon the character and happiness of a nation could not be shown. So in considering the sexual education of youth according to the light of sound physiology, the social influences which affect the natural growth of the human being are an important part of applied physiology.