I have conceived love to be the most powerful and at the same time the least studied of human affections. Surrounded by a triple forest of prejudice, mystery and hypocrisy, civilized men know it too often only through stealth and shame. Poets, artists, philosophers, legislators, snatch a morsel now and then from the flesh of the great god, and hurry away to conceal it as a precious booty of forbidden fruit. To study love as a phenomenon of life, as a gigantic power which moulds itself in a thousand ways among various races and in various epochs, and as an element of health for the individual and for the generations, has appealed to me as a great and worthy undertaking. THE AUTHOR. THE BOOK OF LOVE "... _this precious jewel Upon the which is every virtue founded._" --DANTE. INTRODUCTION GENERAL PHYSIOLOGY OF LOVE Many years ago I wrote that to live means _nutrition and generation_, and the deeper I cast the sounding-line into the dark abysses of life, the more I am convinced that this definition faithfully depicts the most striking characteristics of all creatures which, from bacteria to man, come to life, grow and die on the face of our planet. If, however, I wished still further to simplify my idea, reducing life to its simplest and most essential form, I would say without fear of betraying the truth, that _to live means to generate_. Every living body is perishable, but before dying it has the power of reproducing the form that has made it capable of living; and that whirlwind which absorbs and rejects, which assimilates new atoms and repels old ones, and which so clearly represents the eternal picture of life in all its manifestations, is also the most faithful representation of every form of generation. Nutrition is a real genesis, and in the great chemical laboratory of living beings we have at all times before our eyes the reproduction of histological elements of organs and individuals. We lose hair, epithelia, white corpuscles every day; and yet every day we generate hair, epithelia and leucocytes: this is an every-day generation in the body of man. A nail falls off, a new one takes its place: this is the reproduction of an organ. We generate children similar to ourselves: this is the reproduction of an entire organism, the true _generation_. But in one of our offspring we see re-repeated a mole which is on our nose: this is the reproduction of an organ within an organism. On the other hand, one race generates another race, one species another species; and here we see a broader genesis by which from the reproduction of a cell through another cell we gradually pass to the generating of an organ, of an individual, of a race, of a species.