temperatures a similar proportion of females appeared. It is probable that this error was due to sexually intermediate stages, arrhenoplasmic females at the high temperature, thelyplasmic males at the low temperature. Where the problem is more complicated, as in the case of cattle, to say nothing of human beings, the process of investigation will yield still less harmonious results, and the correction of the interpretation which will have to be made by allowing for the disturbance due to the existence of sexually intermediate forms will be much more difficult. The study of comparative pathology of the sexual types is as necessary as their morphology, physiology and development. In this region of inquiry as elsewhere, statistics would yield certain results. Diseases manifestly much more abundant in one sex might be described as peculiar to or idiopathic of thelyplasm or arrhenoplasm. Myxœdema, for instance, is idiopathic of the female, hydrocele of the male. But no statistics, however numerous and accurate, can be regarded as avoiding a source of theoretical error until it has been shown from the nature of any particular affection dealt with that it is in indissoluble, functional relation with maleness or femaleness. The theory of such associated diseases must supply a reason why they occur almost exclusively in the one sex, that is to say, in the phrase of this treatise, why they are thelyplasmic or arrhenoplasmic. CHAPTER III THE LAWS OF SEXUAL ATTRACTION CARMEN: “L’amour est un oiseau rebelle, Que nul ne peut apprivoiser: Et c’est bien en vain qu’on l’appelle S’il lui convient de refuser. Rien n’y fait; menace ou prière: L’un parle, l’autre se tait; Et c’est l’autre que je préfère; Il n’a rien dit, mais il me plaît. * * * * * L’amour est enfant de Bohême Il n’a jamais connu de loi.” It has been recognised from time immemorial that, in all forms of sexually differentiated life, there exists an attraction between males and females, between the male and the female, the object of which is procreation. But as the male and the female are merely abstract conceptions which never appear in the real world, we cannot speak of sexual attraction as a simple attempt of the masculine and the feminine to come together. The theory which I am developing must take into account all the facts of sexual relations if it is to be complete; indeed, if it is to be accepted instead of the older views, it must give a better interpretation of all these sexual phenomena. My recognition of the fact that M and F (maleness and femaleness) are distributed in the living world in every possible proportion has led me to the discovery of an unknown natural law, of a law not yet suspected by any philosopher, a law of sexual attraction. As observations on human beings first led me to my results, I shall begin with this side of the subject.