The young Hydra, like its mother, is a simple sac with its wall composed of a double layer of cells, the cavity or stomach communicating directly with the stomach of the mother, so that the contractions of the body carry all the food taken by one into the stomach of the other, and inversely (Fig. 1). The parent and child live awhile in this way, but, whenever the latter has reached a certain size, it is detached and fixes itself on some near object, where it hunts on its own account. Soon the parent and offspring are indistinguishable, and during the summer they never cease to produce new Hydras. But, sometimes, in fertile waters rich in game, each Hydra retains its progeny, the little ones grow and produce new Hydras in their turn, and thus a new colony is founded. Trembly kept a long time a Hydra that carried twenty-two young ones of four different generations—a living genealogical tree.
That which is accidental in the common Hydra is quite normal with another fresh-water species, the Cordylophora lacustris, and in most marine Hydroids, in which the colonies often consist of innumerable individuals. But then new phenomena are seen. The social life becomes complicated, and a true division of labor occurs among the members of the same colony. At first all were alike, performing the same functions in the same manner. Specialization soon begins: some hunt, others digest, others reproduce; so that individuals that at first had no need of each other and lived united only in a careless way, become reciprocally necessary; the society thus acquires coherence and solidarity. In the Hydractinia we count not less than seven sorts of individuals: 1. Nourishers or gasterozoids; 2. Prehensers or dactylozoids, provided with bunches of stinging capsules; 3. Dactylozoids without stinging capsides; 4. Defenders; 5. Reproducers of individuals of both sexes; 6. Males; 7. Females. They are different in shape as well as in function; each taking the figure suited to its work, rising or falling in organization; so that division of labor brings with it, as in human society, inequality of conditions. The species thus become polymorphic.
Of these seven sorts of individuals that compose a colony of Hydractinia, the nourishers alone seem capable of living by themselves. The others have neither mouth nor tentacles, the sexual individuals are reduced to simple sacs, the defenders seem to be only sharp spines, between which the polyps can hide themselves (Fig. 5). It may seem an exaggeration to attribute individuality to these different parts. It may be said that they are simply organs; but organs of what? They are just as independent of each other, just as independent of the nourishers, as the latter can be of one another. They are, then, not organs of those Polyps. Can they be organs of the colony? It is already understood that the colony has the character of an individual, and the transformation we seek to demonstrate is admitted. But how can a colony acquire organs? Whence can they arise except from a transformation
