The emotion of love between the sexes has as yet received no thorough scientific treatment. No writer so far as I can find has treated it from a genetic standpoint. The literature upon the subject is therefore meager. In his recent treatise upon "The Psychology of the Emotions," Ribot remarks: "The sex-instinct, the last in chronological order with man and the higher animals, gives rise to the emotion of love with its numerous individual varieties. Most psychologists have been very sparing of details where it is concerned, and one might mention certain voluminous treatises which contain no mention of it. Is this through exaggerated delicacy? Or is it because the authors think that their place has been usurped by the novelists who have so obstinately confined themselves to the study of this passion? But the novelist's mode of analysis is different from the psychological mode, and does not exclude it." This author then devotes one chapter of eleven pages to the treatment of the sexual instinct, which includes what he has to say upon sex-love. Brief as this treatment is, it is valuable, both for the facts it presents and for the problems it suggests. Havelock Ellis, who has perhaps done more than any other investigator in the field of the normal Psychology of Sex says in his most recent work: "It is a very remarkable fact that although for many years past serious attempts have been made to elucidate the psychology of sexual perversions, little or no endeavor has been made to study the psychologic development of the normal sexual emotions. Nearly every writer seems either to take for granted that he and his readers are so familiar with all the facts of normal sex psychology that any detailed statement is altogether uncalled for, or else he is content to write a few introductory phrases, mostly made up from anatomic, philosophic and historical work.
"Yet it is unreasonable to take normal phenomena for granted here as in any other region of medicine. A knowledge of such phenomena is as necessary here as physiology is to pathology or anatomy to surgery. So far from the facts of normal sex development, sex emotions and sex needs being uniform and constant, as is assumed by those who consider their discussion unnecessary, the range of variation within fairly normal limits is immense, and it is impossible to meet with two individuals whose records are nearly identical.
"There are two fundamental reasons why the endeavor should be made to obtain a broad basis of clear information on the subject. In the first place, the normal phenomena give the key to the abnormal, and the majority of sexual perversions, including even those that are most repulsive, are but exaggerations of instincts and emotions that are germinal in normal human beings. In the second place, what is normal cannot be determined until the sexual life of a large number of healthy individuals is known, and until the limits of normal sexuality are known the physician is not in a position to lay down any reasonable rules of sexual hygiene."
Although very short, the analysis of the sex passions in adults by Herbert Spencer in a part of one section in his "Principles of Psychology," is one of the best. Bain devotes one chapter to the Tender Emotion which he makes include Sex-love, the parental feelings, the benevolent affection, gratitude, sorrow, admiration and esteem. A very few pages are given to sex-love proper. Very suggestive paragraphs bearing either directly or indirectly upon the subject are to be found in the works of such writers as Moll, Sergi, Mantegazza, James, Janet, Delboeuf, Feré, Boveri, Kiernan, Hartmann, Dessoir, Fincke and others. There is a vast amount of literature upon the pathological phases of the subject which is to be considered in another chapter.