The familial instinct, more or less developed, exists in the greater number of vertebrates, and in many invertebrates. From an early period it must have been an object of selection, since it adds considerably to the chances of the duration of the species. With some species (ants, bees, termites) this instinct has expanded into a wide social love, resulting in the production of large societies of complex structure, in which the family, as we understand it, is unknown. I lay stress on this fact, for it is of great importance in theoretical sociology; it proves, in fact, that large and complicated societies, with division of social labour, can be maintained without the institution of the family. We are not, therefore, warranted in pretending, as is usually done, that the family is absolutely indispensable, and that it is the “cellule” of the social organism. Let us observe, by the way, that the expression “social organism” is simply metaphorical, and we must beware of taking it literally, as Herbert Spencer, with a strange naïveté, seems to have done. Societies are agglomerations of individuals in which a certain order is necessarily established; but it is almost puerile to seek for, and to pretend to find in them, an actual organisation, comparable, for example, to the anatomic and physiologic plan of a mammal. Terminating this short digression, I revert to my subject by summing up the results of our examination of sexual associations among the animals. In regard to marriage, as well as to the family, nature has no preference; all means are welcome to her, provided the species profits by them, or, at least, does not suffer too much from them. We find amongst animals temporary unions, at the close of which the male ceases absolutely to care for the female; but we also find, especially among birds, numbers of lasting unions, for which the word marriage is not too exalted. It does not appear that polyandry--that is, a durable society between one female and many males--has been practised by animals. The female, nearly always weaker than the male, could not reduce a number of them to sexual servitude, and the latter have never been tempted to share one female systematically. On the contrary, they are often polygamous. But it is especially amongst mammals that polygamy is common, and it must often have had its _raison d’être_ either in the sexual proportion of births, or in a greater mortality of males. These are reasons I shall have to refer to later, in speaking of human polygamy. But if polygamy is frequent with mammals, it is far from being the conjugal _régime_ universally adopted; monogamy is common, and is sometimes accompanied by so much devotion, that it would serve as an example to human monogamists.