But however violent the love of birds for their progeny may be, it lasts only a short time, and is suddenly extinguished when the young can manage for themselves. It is then quite surprising to see the parents drive away by strokes of the beak the little ones they had been nursing with such devoted tenderness a few days before. The birds of several species, however, teach their young to fly before separating from them. The white-headed sea-eagle carries them on its back to give them lessons in flying; grebes, swans, and eiders teach their young to swim, etc. But the family is only of short duration among birds and animals generally, unless, as is the case with some gallinaceæ, the male keeps a few of his daughters to enrich his harem. As a matter of fact, both with birds and other animals, the paternal or maternal sentiment hardly lasts longer than the rearing time. When once the young are full grown, the parents no longer distinguish them from strangers of their species, and it is thus even with monogamic species when the conjugal tie is lifelong; the marriage alone endures, but the family is intermittent and renewed with every brood. We may remark that it is almost the same with certain human races of low development. But, before speaking of man, it will be well to investigate conjugal union and the family amongst the animals nearest to man--the mammals. From the point of view of duration and strength of the affections, or that which we as men should call their morality, the mammals are far from occupying the first rank in the animal hierarchy; many birds are very superior to them. We find, however, great differences in the morals, according to the species. Many mammals have stopped at the most brutal promiscuity; males and females unite and separate at chance meetings, without any care for the family arising in the mind of the male. The females of mammals being always weaker than the males, no sexual association comparable to polyandry is possible in this class, since, even if she wished it, the female could not succeed in collecting a seraglio of males. But as to polygamy, it is quite different, and this is very common with mammals, especially with the sociable kinds, living in flocks. It is even a necessity of the struggle for existence. Sociability generally proceeds from weakness. The species that are badly armed for fierce combats, and that have besides some difficulty in finding food, are glad to live in association. Union is strength. The ruminants, for example, do this. Certain carnivorous animals, ill-furnished with teeth and claws, dogs also, and jackals, live in troops for the same reason--that of opposing a respectable front to the enemy. This life in common is certainly favourable to the development of social virtues; it cannot but soften primitive cruelty, and develop altruistic qualities; but it is little conducive to sexual restraint and monogamy. Thus the greater number of sociable mammals are polygamous.