possible having been caused by the poison—if we may call it so—of the ant’s bite, in the same way as the sting of the gall-fly raises galls upon the oak.[80] Of course, from the moment that the ants appear the tuber is safe from any other insect, or small bird, or mammal that might otherwise do it harm. The ants in defending their nest would defend it, and it is on this principle of mutual advantage that such ant and plant alliances have been brought about. Thus the dry, arid plains, called savannahs, of tropical America support a species of acacia of which the thorns, characteristic of the family, grow in pairs and are shaped exactly like the horns of some oxen. Every pair of these horns becomes in time an ants’ nest, and if the tree be touched or shaken, the ants rush out full of fury in defence of their habitations. Thus every tree is tenanted by a large army of retainers, who almost more than the thorns themselves, which have been developed for the same purpose, protect it against browsing quadrupeds. Its thorns, however, would be no protection against the leaf-cutting ants in search of materials for their mushroom-beds, whereas these are kept at bay by a hostile species, smaller indeed, but armed with a powerful sting. “For these services,” says Belt, “the ants are not only securely housed by the plant, but are provided with a bountiful supply of food; and to secure their attendance at the right time and place, this food is so arranged and distributed as to effect that object with wonderful perfection. The leaves are bi-pinnate (double, that is to say), and at the base of each pair of leaflets, on the mid-rib, is a crater-formed gland, which, when the leaves are young, secretes a honey-like liquid. Of this the ants are very fond; and they are constantly running about from one gland to another to sip up the honey as it is secreted. But this is not all; there is a still more wonderful provision of solid food. At the end of each of the small divisions of the compound leaflet there is, when the leaf first unfolds, a little yellow, fruit-like body, united to it by a point at its base. Examined through a microscope, this little appendage looks like a golden pear. When the leaf first unfolds the little pears are not quite ripe, and the ants are continually employed going from one to another examining them. When an ant finds one sufficiently advanced it bites the small point of attachment; then, bending down the fruit-like body, it breaks it off and bears it away in triumph to the nest. All the fruit-like bodies do not ripen at once, but successively, so that the ants are kept about the young leaf for some time after it unfolds. Thus the young leaves are always guarded by the ants; and no caterpillar or larger animal could attempt to injure them without being attacked by the little warriors.” Thus, as Mr. Belt very aptly puts it, “the ants are really kept by the acacia as a standing