It is much easier to explain why forms too dissimilar cannot love each other. This impossibility is one of the most powerful means of preserving the living forms, extremely varied, in those conditions which are useful to their existence. When a living being has come out of the struggles of life, when it has yielded to external agents and enemies in a certain way, it transmits itself to future generations in that form and nature which are the fruit of a long and successful battle. Precisely for the same reason, an herbivorous animal, which is the offspring of another that has gained its flesh with herbs, cannot grow and multiply except by feeding on herbs. Imagine for a moment that organs and tissues feeding on meat should be grafted on to the organs and tissues of an herbivorous animal. What disorders would not arise! A fragment of carnivorous animal closed up in an organism which has teeth to chew herbs, gastric juice to digest herbs, intestinal tube to assimilate herbs, and olfactory nerves which find leaves and flowers delectable! The apparent stability of the species, which in fact resolves itself in a slow mutation, is nothing therefore but the unavoidable necessity for male and female to pour into the crucible of generation elements that can combine, metals that can fuse, forming a homogeneous and compact alloy. From the elementary physics of generation you may jump to the most ardent sympathies, to the juxtaposition of human characters in the nest of love, and you will see that the same law rules all and each of these facts. _Neither too similar nor too dissimilar._ Love is the sum of analogous but not identical forces; it is the complement of complements; it is the square of squares; it tolerates neither subtractions nor divisions. We shall see at every step of our studies the same laws which govern generation, or the so-called _physical love_, re-appear in the high spheres of love. For us, love is simply one function which, to be understood, must not be barbarously mutilated and disrupted so as to have one part of its limbs sent to the laboratory of physiology, and the other left in the library of the philosopher. Love is such energy that from the lowest grades of the most automatic instinct it ascends to the highest regions of the suprasensible, and perhaps no other psychical element reaches to more distant poles. Think of the shepherd of the high Apennines who loves a goat, and of Heine, who in the clutches of death wants to be brought to the Louvre to see the Venus of Milo once more, and you will have a pallid idea of the frontiers which this ardent, tenacious, violent, multiform passion called love seeks to conquer.