ing account of the symbiosis of the Aztec ants and the imbauba

tree:

"In the forests of South America grow the imbauba or so-called candelabra trees, species of the genus Cecropia, which well deserve their name, 'candelabra,' from the curious appearance given them by the outspringing bare branches, each bearing a tuft of leaves at the free end. These leaves are often attacked by the leaf-cutting ants of the genus Ecodoma, which roam by

tens of thousands over the various plants of the forest biting off the leaves, that they may fall to the ground, where they are again seized, bitten into pieces and the pieces carried into the nests of the ants. In the nests they serve as a medium on which grow certain molds or fungi, much liked by the ants. The candelabra tree protects itself from these leaf-robbing enemies by an association with another ant species, Azteca instabilis, which finds safe dwelling places in the hollow trunk of the tree and a special supply of food in a brownish fluid secreted by it. Along the tree trunk occur in regular order little pits through which the female Azteca can easily bore into the interior, where she lays her eggs and establishes colonies, so that soon the interior of the whole trunk swarms with ants which rush out whenever the tree is shaken. But this alone would not serve to protect the imbauba from the leaf cutters, for how could the Aztecs dwelling inside the tree know of the presence of the light-footed leaf-cutters without? But this is arranged for by the development on the outside of the tree, at the very points where the danger is greatest, namely, on the petioles of the younger leaves, of peculiar little hairy growths from which project small white grains which are very nutritious and not only eagerly eaten by ants, but garnered by them to carry into their nests, presumably as food for their larvæ. Thus right where protection is most needed the plant has developed a special organ attractive to the fierce Aztec ants, so that their constant presence at

FIG. 233.-Piece of a branch of the Imbauba tree, Cecropia, the leaves cut away, showing at the base of each petiole the small tuft on the food; at the right some of this ant food enlarged. (After Schimper.)

these points is an effective protection against the encroachments of the leaf-cutters, as courage and eagerness to fight other ants is already characteristic of the Aztecs. Not all candelabra trees live in symbiosis with ants or possess this special protection against the ravages of the leaf-cutter species. Schimper found in the forests of Brazil several species of Cecropia which never shelter ants in the chambers of the hollow trunk. Now these species do not develop the curious special food-producing organs at the bases of the leaf petioles. These species lack the means of attracting and retaining the ant guests. Only one species of candelabra tree, Cecropia peltata, has developed this arrangement, and it is plainly of no direct use for the tree except through the bringing to it of the protecting ants."

There are, of course, numerous other examples known of the symbiotic association of plants and animals; and if we were to follow the study of symbiosis into the plant kingdom we should find that in one of the large groups of plants, the familiar lichens which grow on rocks and tree trunks and old fences, every member lives symbiotically. A lichen is not a single plant, but is always composed of two plants, an alga (chlorophyll-bearing) and a fungus (without chlorophyll) living together in a most intimate, mutually advantageous association. But we must devote no more space to the consideration of this fascinating subject.

The simplest form of social life, or the living together of several to many individuals of the same species, is shown among those kinds of animals in which many individuals of one species keep together, forming a great band or herd. In this case there is not much division of labor, and the safety of the individual is not wholly bound up in the fate of the herd. Such animals are said to be gregarious in habit. The habit undoubtedly is advantageous in the mutual protection and aid afforded the individuals of the band. This mutual help in the case of many gregarious animals is of a very positive and obvious character. In other cases this gregariousness is reduced to a matter of slight or temporary convenience, possessing but little of the element of mutual aid. The great herds of reindeer in the north, and of the bison or buffalo which once ranged over the Western American plains, are examples of a gregariousness in which mutual protection from enemies, like wolves, seems to be the principal advantage gained. The bands of

wolves which hunted the buffalo show the advantage of mutual help in aggression as well as in protection. In this banding together of wolves there is active coöperation among individuals to obtain a common food supply. What one wolf cannot dothat is, tear down a buffalo from the edge of the herd-a dozen can do, and all are gainers by the operation.

On the other hand, the vast assembling of sea birds on certain ocean islands and rocks is a condition probably brought about rather by the special suitableness of a few places for safe breeding than from any special mutual aid afforded; still, these sea birds undoubtedly combine to drive off attacking eagles and hawks. Eagles are usually considered to be strictly solitary in habit (the unit of solitariness being a pair, not an individual); but the description, by a Russian naturalist, of the hunting habits of the great white-tailed eagle (Haliaëtos albicilla) on the Russian steppes shows that this kind of eagle at least has adopted a gregarious habit, in which mutual help is plainly obvious. This naturalist once saw an eagle high in the air, circling slowly and widely in perfect silence. Suddenly the eagle screamed loudly. "Its cry was soon answered by another eagle, which approached it, and was followed by a third, a fourth, and so on, till nine or ten eagles came together and soon disappeared." The naturalist, following them, soon discovered them gathered about the dead body of a horse. The food found by the first was being shared by all. The association of pelicans in fishing is a good example of the advantage of a gregarious and mutually helpful habit. The pelicans sometimes go fishing in great bands, and, after having chosen an appropriate place near the shore, they form a wide half-circle facing the shore, and narrow it by paddling toward the land, catching the fish which they inclose in the evernarrowing circle.

The wary Rocky Mountain sheep (Fig. 234) live together in small bands, posting sentinels whenever they are feeding or resting, who watch for and give warning of the approach of enemies. The beavers furnish a well-known and very interesting example of mutual help, and they exhibit a truly communal life, although a simple one. They live in "villages" or communities, all helping to build the dam across the stream, which is necessary to form the broad marsh or pool in which the nests or houses are built. Prairie dogs live in great villages

or communities which spread over many acres. They tell each other by shrill cries of the approach of enemies, and they seem to visit each other and to enjoy each other's society a great deal, although that they afford each other much actual active help is not apparent. Birds in migration are gregarious, although at other times they may live comparatively alone. In their long flights they keep together, often with definite leaders who seem to discover and decide on the course

of flight for the whole great flock. The wedge-shaped flocks of wild geese flying high and uttering their sharp, metallic call in their south

ward migrations are well known in many parts of the United States. Indeed, the more one studies the habits of animals the more examples of social life and mutual help will be found. Probably most animals are in some degree gregarious in habit, and in all cases of gregariousness there is probably some degree of mutual aid.

An interesting series of gradations from a strictly solitary through a gregarious to an elaborately specialized communal life is shown by the bees. Although the bumblebee and the honeybee are so much more familiar to us than other bee kinds that the communal life exemplified by them may have come to seem the usual kind of bee life, yet, as a matter of fact, there are many more solitary bees than social ones. The general character of the domestic economy of the solitary bees is well shown by the interesting little green carpenter bee, Ceratina dupla. Each female of this species bores out the pith from five or six inches of an elder branch or raspberry cane, and