I refer, of course, to the theory that Plato in his Symposium puts into the mouth of Aristophanes and which deals not only with the origin of the sexual instinct but also with its most important variations in relation to the object. ‘Human nature was once quite other than now. Originally there were three sexes, three and not as to-day two: besides the male and the female there existed a third sex which had an equal share in the two first.... In these beings everything was double: thus, they had four hands and four feet, two faces, two genital parts, and so on. Then Zeus allowed himself to be persuaded to cut these beings in two, as one divides pears to stew them.... When all nature was divided in this way, to each human being came the longing for his own other half, and the two halves embraced and entwined their bodies _and desired to grow together again_.’[35] Are we to follow the clue of the poet-philosopher and make the daring assumption that living substance was at the time of its animation rent into small particles, which since that time strive for reunion by means of the sexual instincts? That these instincts—in which the chemical affinity of inanimate matter is continued—passing through the realm of the protozoa gradually overcome all hindrances set to their striving by an environment charged with stimuli dangerous to life, and are impelled by it to form a protecting covering layer? And that these dispersed fragments of living substance thus achieve a multicellular organisation, and finally transfer to the germ cells in a highly concentrated form the instinct for reunion? I think this is the point at which to break off.