Our amusements and our customs are largely moulded by the same powerful attraction, viz., the mental and moral quality of the relations which are formed between the sexes. As civilization advances, and dense masses of human beings are crowded together in heterogeneous selfish strife, the destructive extremes of luxury and pauperism appear. From this state of society, where misery will do anything for money, and the satiety of luxury seeks fresh stimulus, speculation in this strongest part of our nature--sex--arises. Its creative use disappears, and it becomes a subject of merchandise. Every variety of effort is made to stimulate and debase the mental quality or sentiment of sex, and the strength of human passion furnishes an exhaustless field for corrupt speculation. It is therefore not the simple physical aspect of the reproductive powers which is remarkable in humanity. The physical instinct is shared with the rest of the animal creation. It is the unique and powerful mental and moral element, the principle that moulds and governs human sex, which produces such striking results in the life of our race. The mental or emotional element in these powers, both in relation to the action and reaction of mind and body, and the hereditary transmission of tendencies, will, therefore, largely engage the attention of the physiologist who truly studies our human nature. The distinctive moral character of human sex renders the exclusive study of physical phenomena in man as useless and unscientific a method of investigation as would be the study of music on dumb instruments. The distinctively mental character of human sex must therefore always be recognised as a guide in any physiological inquiry into the structure and functions of the physical organs especially appropriated to the use of sex. The clue to a true knowledge of sexual functions in man and woman is found in this striking peculiarity of the human race, viz., that these functions are largely dominated by mental action, and that sex in the human being does not mean simply the action of the physical organs, but also the conjoined mental principle directing those organs.