"[I]n lower organisms, there is at once great reproductive power and a complete absence of sexual attraction, from the very fact that the sexes are not differentiated. With more advanced organisms, the sexual attraction increases as the reproductive force diminishes, until, at the summit, with man, the strongest sexual love becomes visible, even in the case of a total absence of reproduction. And thus, if at the two extremes of animal life we find, on the one hand, reproduction without sexual love, and, on the other, sexual love without reproduction."