In this ascending evolution from the instinctive to the idealistic form there is a decisive moment—viz., the appearance of the individual choice. This is the special criterion which differentiates instinct from emotion. Sexual instinct contents itself with a specific satisfaction; sexual love does not. And as choice manifests itself among the superior representatives of the animal kingdom, not only by sanguinary combats between the males, or by the more pacific tournaments which precede sexual selection, but, in the absence of all rivalry and competition, by the exclusive preference of one male for one female, chosen among many others whom he might possess, we may admit _a fortiori_ that primitive humanity must very soon have left behind the stage of _Venus Volgivaga_. We know that Schopenhauer, and after him Hartmann, have tried to determine the reasons of choice; but such attempts must always be partial failures, because we can never be sure of discovering all the unconscious factors. For the rest, the psychology of love contains many other mysteries. Whence comes the blind violence which astonishes and sometimes terrifies the calm spectator? Bain thinks it can be explained by the concentration of the attention on an individual, and by the fact that intensity and unity of object are associated in love. But this applies also to other passions, such as ambition and hatred. Herbert Spencer, in the analysis already quoted, attributes it to the complexity of the passion, love being an aggregate of heterogeneous tendencies all converging to one end and carrying the individual in the same direction. At bottom, the irresistible element is in the sexual instinct, and only exists in virtue of it; instinctive activity alone has such power. This is what Schopenhauer calls, in metaphysical terms, the Genius of the species, which makes of the individual an instrument for the furtherance of its ends. We might also call it, in biological terms, according to the hypothesis of Weismann now in vogue: the continuity of the germ-plasm which energetically manifests and affirms itself, safeguarding the rights of the species against individual fancies. But all these metaphors explain nothing, add nothing to the simple verification of the fact. Sexual instinct remains the centre round which everything revolves; nothing exists but through it. Character, imagination, vanity, imitation, fashion, time, place, and many other individual circumstances or social influences give to love—as emotion or passion—an unlimited plasticity. It is the task of the novelists to describe all its various shapes, and one which they have not failed to perform. II.