Love conjoins in a quite unique way the =two= principal classes of vital manifestations--the lower vegetative and the higher animal life; and it thus constitutes the highest and the most intense expression of the =unity= of life (Schopenhauer’s “focus of the will;” Weismann’s “continuity of the germ-plasma”). Whoever wishes to understand the developmental tendencies of love as they manifest themselves at the present day in the course of human history, whoever desires to grasp how remarkably love has been developed, enriched, and ennobled in the course of civilization, must at the outset gain a clear understanding of this apparently dualistic, but in reality thoroughly monistic, nature of the passion. The matter may be expressed also in this way--that he who has scientifically investigated love, who has based his conception of it philosophically, and has personally experienced it, will become a convinced monist in relation to life, at least, and to the organic world, and will be compelled to regard every dualistic division into a physical and a spiritual sphere as something quite artificial. In love above all is manifested this mystery of the life force, as for centuries the poets, the artists, and the metaphysicians have declared, and more especially as the great natural philosophers of the eighteenth and nineteenth centuries have proved--above all Charles Darwin and Ernst Haeckel. There is, indeed, no more happily chosen metaphor, none that better describes the fundamentally monistic nature of love, than the saying of the old æsthetic J. G. Sulzer--that love is a =tree=, that it has its =roots= in the physical sphere, but that its =branches= extend high above the physical world, expanding more and more, branching more and more abundantly into the sphere of the spiritual.[1] It is certainly impossible to find a more appropriate comparison. Thereby we show clearly the intimate =organic= connexion between the physical and spiritual phenomena of love; it is rooted for ever in Mother Earth, but it grows always upwards into the subtle ether. Just as the arborescence of the tree has a richer, more manifold, more extensive development than the root, so also it is in the =spiritual= form that love is first capable of extending upwards and in all directions, compared with which its physical capacity for development is minimal and strictly limited. =But just as the arborescence of the tree grows from, and is supplied with nutriment by, the root, so also the higher love is inevitably founded upon a sensory basis. Even while= love becomes spiritually richer, it remains as irrevocably as ever dependent upon the physical.[2] To put the matter briefly, the future =developmental possibilities= of human love rest purely in the spiritual sphere, but they are inseparably connected with the far less variable physical phenomena of sexuality.