It is obvious that where the generative power is restricted to a certain season, it cannot be the sexual instinct that keeps male and female together for months or years. Nor is there any other egoistic motive that could probably account for this habit. Considering that the union lasts till after the birth of the offspring, and considering the care taken of this by the father, we may assume that the prolonged union of the sexes is, in some way or other connected with parental duties. I am, indeed, strongly of opinion that the tie which joins male and female is an instinct developed through the powerful influence of natural selection. It is evident that, when the father helps to protect the offspring, the species is better able to subsist in the struggle for existence than it would be if this obligation entirely devolved on the mother. Paternal affection and the instinct which causes male and female to form somewhat durable alliances, are thus useful mental dispositions which, in all probability, have been acquired through the survival of the fittest. But how, then, can it be that among most animals the father never concerns himself about his progeny? The answer is not difficult to find. Marriage is only one of many means by which a species is enabled to subsist. Where parental care is lacking, we may be sure to find compensation for it in some other way. Among the Invertebrata, Fishes, and Reptiles, both parents are generally quite indifferent as to their progeny. An immense proportion of the progeny therefore succumb before reaching maturity; but the number of eggs laid is proportionate to the number of those lost, and the species is preserved nevertheless. If every grain of roe, spawned by the female fishes, were fecundated and hatched, the sea would not be large enough to hold all the creatures resulting from them. The eggs of Reptiles need no maternal care, the embryo being developed by the heat of the sun; and their young are from the outset able to help themselves, leading the same life as the adults. Among Birds, on the other hand, parental care is an absolute necessity. Equal and continual warmth is the first requirement for the development of the embryo and the preservation of the young ones. For this the mother almost always wants the assistance of the father, who provides her with necessaries, and sometimes relieves her of the brooding. Among Mammals, the young can never do without the mother at the tenderest age, but the father’s aid is generally by no means indispensable. In some species, as the walrus,[96] the elephant,[97] the _Bos americanus_,[98] and the bat,[99] there seems to be a rather curious substitute for paternal protection, the females, together with their young ones, collecting in large herds or flocks apart from the males. Again, as to the marriage of the Primates, it is, I think, very probably due to the small number of young, the female bringing forth