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all-important as to gain for him the name of " the social animal," there is no differentiation of individuals adapted only for certain kinds of work. Among these highest examples of social animals, the presence of an advanced mental endowment, the specialization of the mental power, the power of reason, have taken the place of and made unnecessary the structural differentiation of individuals. The honey-bee workers do different kinds of work: some gather food, some care for the young, and some make wax and build cells, but the individuals are interchangeable; each one knows enough to do these various things. There is a structural differentiation in the matter of only one special work or function, that of reproduction.
With the ants there is, in some cases, a considerable structural divergence among individuals, as in the genus Atta of South America with six kinds of individuals— namely, winged males, winged females, wingless soldiers, and wingless workers of three distinct sizes. In the case of other kinds with quite as highly organized a communal life there are but three kinds of individuals, the winged males and females and the wingless workers. The workers gather food, build the nest, guard the "cattle" (aphids), make war, and care for the young. Each one knows enough to do all these various distinct things. Its body is not so modified that it can do but one kind of thing, which thing it must always do.
The increase of intelligence, the development of the power of reas6ning, is the most potent factor in the development of a highly specialized social life. Man is the example of the highest development of this sort in the animal kingdom, but the highest form of social development is not by any means the most perfectly communal.
268. Advantages of communal life.—The advantages of communal or social life, of co-operation and mutual aid, are real. The animals that have adopted such a life are among the most successful of all animals in the struggle for existence. The termite individual is one of the most defenseless, and, for those animals that prey on insects, one of the most toothsome luxuries to be found in the insect world. But the termite is one of the most abundant and widespread and successfully living insect kinds in all the tropics. Where ants are not, few insects are. The honeybee is a popular type of a successful life. The artificial protection afforded the honey-bee by man may aid in its struggle for existence, but it gains this protection because of certain features of its communal life, and in Nature the honey-bee takes care of itself well. The Little Bee People of Kipling's Jungle Book, who live in great communities in the rocks of Indian hills, can put to rout the largest and fiercest of the jungle animals. Co-operation and mutual aid are among the most important factors which help in the struggle for existence. Its great advantages are, however, in some degree balanced by the fact that mutual help brings mutual dependence. The community or society can accomplish greater things than the solitary individuals, but co-operation limits freedom, and often sacrifices the individual to the whole.
COMMENSALISM AND PARASITISM
269. Association between animals of different species.—
The living together and mutual help discussed in the last chapter concerned in each instance a single species of animal. All the various members of a pack of wolves or of a community of ants are individuals of the same species. But there are many instances of an association of individuals of different kinds of animals. In many cases of an association of individuals of different species one kind derives great benefit and the other suffers more or less injury from the association. One kind lives at the expense of the other. This association is called parasitism. In some cases, however, neither kind of animal suffers from the presence of the other. The two live together in harmony and presumably to their mutual advantage. In some cases this mutual advantage is obvious. This kind of association is called commensalism or symbiosis.
270. Commensalism.—A curious example of commensalism is afforded by the different species of Remoras (Echeneididm) which attach themselves to sharks, barracudas, and other large fishes by means of a sucking disk on the top of the head (Fig. 201). This disk is made by a modification of the dorsal fin. The Remora thus attached to a shark may be carried about for weeks, leaving its host only to secure food. This is done by a sudden dash through the water. The Remora injures the shark in no way save, perhaps, by the slight check its presence gives to the shark's speed in swimming.
Whales, similarly, often carry barnacles about with them. These barnacles are permanently attached to the skin of the whale just as they would be to a stone or
Fig. 201.—Remora, with dorsal fin modified to be a sucking plate by which the fish attaches itself to a shark.
wooden pile. Many small crustaceans, annelids, mollusks, and other invertebrates burrow into the substance of living sponges for shelter. On the other hand, the little boring sponge (Cliona) burrows in the shells of oysters and other bivalves for protection. Some species of sponge "are never found growing except on the backs' or legs of certain crabs." In these cases the sponge, with its many plant-like branches, protects the crab by concealing it from its enemies, while the sponge is benefited by being carried about by the crab to new food supplies.
Small fish of the genus Nomeus may often be found accompanying the beautiful Portuguese man-of-war (Physalia) as it sails slowly about on the ocean's surface (Fig. 202). These little fish lurk underneath the float and among the various hanging thread-like parts of the Physalia, which are provided with stinging cells.
In the nests of the various species of ants and termites many different kinds of other insects have been found. Some of these are harmful to their hosts, in that they feed on the food stores gathered by the industrious and provident ant, but others appear to feed only on refuse or useless substances in the nest. Some may even be of help to their hosts. Over one thousand species have been recorded by collectors as living habitually in the nests of ants and termites.
271. Symbiosis.—Of a more intimate character, and of more obvious and certain mutual advantage, is the wellknown association called symbiosis. The hermit-crab always takes for his habitation the shell of another animal, often that of the common whelk. All of the hind part of the crab lies inside the shell, while its head with its great claws project from the opening of the shell. On the surface of the shell near the opening there is often to be found a sea-anemone, or sea-rose (Fig. 203). This sea-anemone is fastened securely to the shell, and has its mouth opening and tentacles near the head of the crab. The sea-anemone is carried from place to place by the hermit-crab, and in this way is much aided in obtaining food. On the other hand, the crab is protected from its enemies by the well-armed and dangerous tentacles of the sea-anemone. If the seaanemone be torn away from the shell inhabited by one of these crabs, the crab will wander about, carefully seeking for another anemone. When he finds it he struggles to loosen it from its rock or from whatever it may be growing on, and does not rest until he has torn it loose and placed it on his shell.
There are numerous small crabs called pea-crabs (Pin