Heterosexuality arises from the reciprocal attraction and the coalescence of the reproductive cells of two individuals of distinct sexes; it forms the foundation and constitutes the most important element of the sexual relations of the higher animal world and of the human species; and it obtains through inheritance continually a more sharply defined expression. Since this fundamental phenomenon of the sexual impulse has been transmitted from the most ancient and simplest forms of the organic world and has been modified only in the direction of heterosexuality, it has come to pass, as Ewald Hering says at the end of his celebrated lecture on “Memory as a General Function of Organic Matter,” that organic matter has the strongest memory of the impulse of conjugation in its most ancient and most primitive form; thus this impulse at the present day continues to dominate mankind as an intensely powerful physical imperative, endowed with the strength of an elemental force, which, notwithstanding the gradually higher development of the brain, has remained during thousands of years undiminished in its potency, and indeed by the accumulative influence extending through thousands of generations has acquired a notable increase in intensity. We must assume that for untold generations always those animals and men have had the most numerous descendants in whom the sexual impulse was the most powerful; this powerful impulse being inherited, was transmitted once more to the next generation, and tended by natural selection continually to increase. This explanation of the indisputable gradual increase in the intensity of the sexual impulse, first given by the moral philosopher Paul Rée, is more illuminating than the theory propounded by Havelock Ellis of the increase of the sexual impulse by civilization, which was long ago maintained by Lucretius (“De Rerum Naturâ,” V. 1016). In support of this latter theory, it is asserted that among savage people the genital organs are less powerfully developed than among civilized races, but this can by no means be regarded as an established fact. Civilization has done no more than cause a fuller development of all sides of sexual love by a multiplication of physical and psychical =stimuli=; but it appears extremely doubtful if civilization itself is to be regarded as the immediate causal influence in the increase of the intensity of the sexual impulse. * * * * * Having studied the elementary phenomena of human love dependent upon the phylogenetic history of the human race, namely the union of the male and female reproductive cells, the question now arises as to the nature of the =psychical= processes, the character of the =sensations= that accompany this union of the sperm cells and the germ cells. What is the most primitive =psychical elementary phenomenon= of love?