It will be sufficiently apparent that this coincidence is too striking to be without a meaning. Zoologists are all agreed in their interpretation of this meaning: it is, that the history of the individual presents a summary of the history of the race, and goes through the stages of structure which its ancestors presented in their adult forms. The story of the gradual upward struggle of the animal kingdom, from its humble beginnings to its present wonderful complexity, is written in the growing tissues of every young creature. The principle that ancestral traits betray themselves is accepted as a truism in common life. Do we see young people rude and stupid? We say, perhaps, "No wonder; their grandfather was a drunken, worthless lout." Do we see a family of the poorest class clever, and industrious, and refined? We say, "They come of a good stock." When we speak in this way, we reason from the common experience of mankind, that children resemble their ancestors. Similarly, when zoologists find an embryo starting its existence from one cell, they say, "No wonder; its ancestors were unicellular." And when they find it assuming a two-layered form, they say, "Its ancestors were two-layered creatures." So certain are zoologists of the existence of an ancestral two-layered form, the parent at once of the existing Coelenterata and of the higher forms, that Professor Hæckel has given it a special name--Gastræa. The two-layered young stage of higher creatures, when it has a free-swimming existence, is called a Gastrula (Fig. 6). Both names, meaning stomach-animal, refer to the structure, which is, in a still simpler form, that of _Hydra_--a two-layered bag of cells, of which the inner layer, lining the cavity, performs the work of digestion. The lowest of the Vertebrata, the Lancelet (see p. 140), has a larva of this kind. The same reasoning which suggests the existence of an ancestral Gastræa-animal, suggests that of an ancestral Planula-animal; for the two-layered animals, on their part, present us with a monoblastic larva of the form already described (p. 34), called a Planula. Hence it is that zoologists look with such eagerness for forms, of which it can be said that they consist of one layer of cells only. The name Planula signifies "wandering animal," because the Planula larva swims about by means of cilia. [Illustration: FIG. 6.--Diagrammatic representation of a typical Gastrula, or two-layered larval form, highly magnified; optical section, longitudinal. _Ec_, Ectoderm or skin layer; _En_, Endoderm or stomach layer; _m_, mouth leading into the enteric cavity. The dots are the nuclei of the cells.] [Illustration: FIG. 7.--Diagrammatic representation of a typical Trochosphere, or ciliated larva, considerably magnified. _M_ is the mouth; the stomach and intestine are seen showing through the transparent body.]