the separation of the sexes? The influence of all the other passions which agitate the human heart cannot weigh in the balance with love, which inflames the senses and fascinates the reason. On the one hand, we celebrate in love the source of the most sublime works of art, and of the noblest creations of poetry and of music; we venerate it as the most powerful factor in civilisation, as the prime cause of family life, and consequently of social life. On the other hand, we fear love as a destructive flame; it is love that drives so many to ruin; it is love that has caused more misery, vice, and crime than all other calamities together. Love is so prodigious, its influence is so enormous on psychic life and the most diverse functions of the nervous system, that in regard to it we are tempted to question the supernatural effect of our natural explanation. Nevertheless, comparative biology and the history of development conduct us surely and incontestably to the simplest, most remote source of love; that is to say, the elective affinity of two different cellules--the spermatic cell and the ovulary cell.”[1] III. _Rut and Love._ In a former work on the evolution of morality I have described the manner in which the hereditary tendencies and instincts arise from habit, induced in the nervous cellules by a sufficient repetition of the same acts. The instinct of procreation has, and can have, no other origin. The animal species, during the long phases of their evolution, have reproduced themselves unconsciously, and by very simple processes, which we may still observe in certain zoophytes. By degrees these mere sketches of animals have become perfected and differentiated, and have acquired special organs over which the biological work has been distributed; thenceforth the play of life has echoed in the nervous centres, and has awakened in them impressions and desires, the energy of which strictly corresponds to the importance of the functions. Now, there is no more primordial function than procreation, since on it depends the duration even of the species; and for this reason the need of reproduction, or the rut, breaks out in many animals like a kind of madness. The psychic faculties of the animal, whether great or small, are then over-excited, and rise above their ordinary level; but they all tend to one supreme aim--the desire for reproduction. At this period the wildest and most unsociable species can no longer endure solitude; both males and females seek each other; sometimes, even, they are seen to form themselves into groups, or small provisional societies, which will dissolve again after the coupling time is over.