While the female spider often kills and eats the male, monkeys, and parrots give proof of such a great mutual attachment that when one of the conjoints dies the other sinks into complete despair, ceases to eat, and perishes in its turn. In this domain we find singular adaptations to special conditions of existence. Among the bees and ants, a third class of individuals, or neuters, formed by differentiation of females, do not copulate, and lay at the most a few eggs which are not fecundated and which occasionally develop by parthenogenesis. Among the termites, another species of social ants, a similar state of things exists, but the neuters, or workers, are derived from the male sex as well as the female and their sexual organs are quite rudimentary. The third sex, or worker, not only has a cerebral development superior to the sexual individuals, but also inherits the social sympathetic irradiations of the sexual appetite, which results in his devotion to a brood which is not his own. Among the social insects the males are little more than flying sexual organs, which after copulation are incapable of leading an independent existence and die of hunger and exhaustion in the case of ants or termites, or are massacred by the workers in the case of bees. The fecundated females, on their part, become breeding machines whose activity is incessant. Among the ants, however, the females are at first capable of nourishing a few larvæ by the aid of a portion of their eggs and their secretions, till the workers are hatched, who henceforth undertake all the work including the maternal care of the brood. Whoever has observed the fidelity of a pair of swallows and the way in which the male and female nourish and rear their young, must be struck by the analogy to the conjugal and family love of the faithful type of human beings. This is especially remarkable when the same couple return every year to the old nest. This family life of the swallows does not prevent a certain social life, which manifests itself in organized attacks on birds of prey, and in combined emigration in the autumn and spring. On the other hand, we are instinctively indignant at the want of fidelity in other animals, between conjoints, parents and offspring (dogs and rabbits, for instance), because we involuntarily expect to find in them our own moral sense, which is not at all just.