The particular species of ant concerning which this discovery was made is _Formica rufescens_, or _Polyergus rufescens_—the reader may take his choice—and Darwin, who impresses the facts of ant-slavery upon the mind better than a dozen books specially devoted to ants or insects, says of it: “This ant is absolutely dependent on its slaves, and without their aid the species would certainly become extinct in a single year. The males and fertile females do no work of any kind, and the workers, or sterile females, though most energetic and courageous in capturing slaves, do no other work. They are incapable of making their own nests or of feeding their own larvæ. When the old nest is found inconvenient, and they have to migrate, it is the slaves which determine the migration, and actually carry their masters (one might just as well call _them their_ slaves) in their jaws. So utterly helpless are the masters, that when Huber shut up thirty of them without a slave, but with plenty of the food which they like best, and with their own larvæ and pupæ to stimulate them to work, they did nothing; they could not even feed themselves, and many perished of hunger. Huber then introduced a single slave, and she instantly set to work, fed and saved the survivors, made some cells, and tended the larvæ, and put all to rights. What can be more extraordinary than these well-ascertained facts?”[52] The slave-ant in this case is _Formica fusca_, and it is also held in bondage by another species of slave-maker, viz. _Formica sanguinea_—or the Blood-red Ant—as was likewise a discovery of Pierre Huber. This last species is found in the south of England, and its slave-making habits have been observed by Darwin, who opened fourteen nests and found a few slaves in all of them. “The slaves,” he tells us, “are black, and not above half the size of their red masters, so that the contrast in their appearance is great.”[52] The black ants were not often seen by Darwin to leave the nest, and others who have observed their habits in England have considered them as “strictly household slaves.”[52] Huber, however, whose observations were carried on in Switzerland, says that “their chief office is to search for aphides,”[52] and this would take them far afield. In Switzerland, however, slaves seem to be more numerous in the nests of the Blood-red Ant, and Darwin attributes the difference in their habits to this account. Huber also tells us that the Swiss slaves “habitually work with their masters in making the nest, and they alone open and close the doors in the morning and evening.”[52] This is done, I suppose, by placing pellets of earth in the mouth of the entrance-tunnel and removing them again; but there is one species of ant which would have only to place or remove itself, for this purpose, since its large head, by being wedged into the passage, stops it up, and thus fulfils the office of a front door. The ant that does this must be one