When once it is established that man is a mammal like any other, and only distinguished from the animals of his class by a greater cerebral development, all study of human sociology must logically be preceded by a corresponding study of animal sociology. Moreover, as sociology finally depends on biology, it will be necessary to seek in physiological conditions themselves the origin of great sociological manifestations. The first necessity of societies is that they should endure, and they can only do so on the condition of providing satisfaction for primordial needs, which are the condition of life itself, and which imperatively dominate and regulate great social institutions. Lastly, if man is a sociable animal, he is not the only one; many other species have grouped themselves in societies, where, however rudimentary they may be, we find in embryonic sketch the principal traits of human agglomerations. There are even species--as, for example, bees, ants, and termites--that have created true republics, of complicated structure, in which the social problem has been solved in an entirely original manner. We may take from them more than one good example, and more than one valuable hint. My present task is to write the history of marriage and of the family. The institution of marriage has had no other object than the regulation of sexual unions. These have for their aim the satisfaction of one of the most imperious biological needs--the sexual appetite; but this appetite is only a conscious impulse, a “snare,” as Montaigne calls it, which impels both man and animal to provide, as far as concerns them, for the preservation of their species--to “pay the ancestral debt,” according to the Brahmanical formula. Before studying the sexual relations, and their more or less regulated form in human societies, it will not be out of place to say a few words on reproduction in general, to sketch briefly its physiology in so far as this is fundamental, and to show how tyrannical are the instincts whose formation has been determined by physiological causes, and which render the fiercest animals mild and tractable. This is what I shall attempt to do in the following chapter. II. _Reproduction._