of Worms with the Articulata is apparent to everyone, and we already see how these same Worms are related to Mollusca and Vertebrata. The theory, therefore, extends to the entire animal kingdom.
Now, what do we mean by association? When we say that animal organisms have been in great part produced by the transformation of animal societies into individuals, what do we mean by the term society? Are all societies in the way to become individuals? Many animals associate together, and their societies are sometimes admirably governed. The social manners of dogs, antelopes, beavers, and many birds are well known, while the complex and perfectly coördinated operations of societies of bees, ants, termites, are the admiration of the world. Do such societies ever become individuals? Certainly not. But there exist other animal societies in which the relations are closer—where the individuals are not only in immediate contact but in continuity of tissue with their neighbors. These societies are called colonies, but the individuals that compose them are not always indissolubly united together. They can separate from their companions, and live a long time and affirm their independence by forming new colonies. In the same zoological group of neighboring species, we find some individuals that always live solitary and others always associated, as for example the specially remarkable group of Polyps or Acalepha.
One species of this group, the brown Hydra (Hydra fusca), is common in stagnant waters and even in small garden basins. It has always excited the interest of naturalists and philosophers since Trembly made known its marvelous faculties. These Hydras ordinarily live solitary; but frequently the larger individuals are seen carrying smaller ones on the walls of their bodies. In a captured Hydra we can follow their development step by step. They are at first simple swellings,
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| Fig. 1.—a, diagrammatic section of Hydra; b, Hydra viridis, showing swellings in the body-wall; c, Hydra vulgaris, with an undetached bud enlarged; d, thread-cell of the Hydra, greatly magnified. |
in the center of which there is a prolongment of the cavity of the mother's body. These swellings enlarge and soon put out tentacles, and a mouth opens in the midst of the crown formed by them.
