It is natural, at present, we will suppose, for every pair of birds in a colony of terns or guillemots to feel affection for, and to tend, their own young. Were this affection, and the active expression of it, to extend to the young of other members of the community, then, as every pair of birds would probably be able to supply the wants of more than its own young, a lesser number than the whole community would be sufficient for nursery work, leaving the others free for--what we cannot say, but nature might evolve her product out of the material thus placed at her disposal. Some new activity might well arise, which, if fostered, would be of advantage to the general commonwealth. But let us consider the old ones. Terns, as we have seen, are vigorous in the defence of their eggs and young. They mob and attack any one--be it bird, beast, or man--who trespasses upon their breeding-grounds. If, therefore, only about half the colony were needed for the nurture of the young, and thus gradually came to be the equivalent of the workers amongst ants and bees, in the other half there would exist the elements of a soldier caste. Of these it would become at first the more special, and in time the exclusive business, to drive all enemies away from the ternery; and since efficiency in so important an office might well outweigh the otherwise ill effects of a loss of fertility in certain members of the commonwealth, the soldiers, both male and female, might, in the continuous prosecution of their task, come gradually to lose the sexual instinct, which, again, would allow the others to lay, with advantage, a greater number of eggs. I have mentioned the case of a dog making regular daily expeditions to a ternery, in order to feast upon the eggs; and if one dog could commit havoc like this, what might not some wild egg-eating species do, if not efficiently kept away? It is obvious that the eggs thus destroyed might amount to more in number than those by the loss of which they would be saved to the community; and, on the other hand, a caste whose sole task it was to guard the eggs and young might be competent to guard a greater number than the whole community would be, if "a divided duty" claimed their attention.