At the not very distant time when animism reigned supreme, these facts were attributed to calculations of design. Nature, it was believed, occupied herself chiefly with perpetuating organised species; as for individuals, she disdained the care of them. We now know that Nature, as an anthropomorphic being, does not exist; that the great forces called natural are unconscious; that their blind action results, however, in the world of life, in a choice, a selection, a progressive evolution, or, to sum up, in the survival of the individuals best adapted to the conditions of their existence. Without any intention of Dame Nature, the preservation of the species was necessarily, before anything else, the object of selection; and during the course of geological periods primitive bi-partition gradually became transformed through progressive differentiation into bi-sexual procreation, requiring the concurrence of special and complicated apparatus in order to be effected. But, at the same time as procreation, other functions also became differentiated by the formation of special organs; the nervous system vegetated around the _chorda dorsalis_; and, finally, conscious life awoke in the nervous centres. Thenceforth the accomplishment of the great function of procreation assumed an entirely different aspect. In the lowest stages of the animal kingdom reproduction is effected mechanically and unconsciously. A paramœcium, observed by M. Balbiani, produced in forty-two days, by a series of simple bi-partitions, 1,384,116 individuals, who very certainly had not the least notion of the phenomena by which they transmitted existence. But with superior animals it is very different; in their case the act of procreation is a real efflorescence, not only physical, but psychical. For the study that I am now undertaking it will not be without use to recall the principal features of this amorous efflorescence, since it is, after all, the first cause of marriage and of the family. At the same time, not to lose our standpoint, it is important to bear in mind that at the bottom all this expenditure of physical and psychical force has for motive and for result, both in man and animal, the conjugation of two generative cellules. Haeckel has written a dithyramb on this subject in his _Anthropogenia_, which is in the main so true that I take pleasure in quoting it--“Great effects are everywhere produced, in animated nature, by minute causes.... Think of how many curious phenomena sexual selection gives rise to in animal life; think of the results of love in human life; now, all this has for its _raison d’être_ the union of two cellules.... There is no organic act which approaches this one in power and in the force of differentiation. The Semitic myth of Eve seducing Adam for the love of knowledge, the old Greek legend of Paris and Helen, and many other magnificent poems, do they not simply express the enormous influence that sexual love and sexual selection have exercised since