VI. From this similarity, or _homology_, of the gastrula in all classes of compound animals I drew the conclusion, in virtue of the biogenetic law (p. 81), that all the metazoa come originally from one simple ancestral form, the _gastraea_, and that this ancient (Laurentian), long-extinct form had the structure and composition of the actual gastrula, in which it is preserved by heredity. VII. This phylogenetic conclusion, based on the comparison of ontogenetic facts, is confirmed by the circumstance that there are several of these gastræades still in existence (_gastraemaria_, _cyemaria_, _physemaria_, etc.), and also some ancient forms of other animal groups whose organization is very little higher (the _olynthus_ of the sponges, the _hydra_, or common fresh-water polyp, of the cnidaria, the _convoluta_ and other cryptocæla, or worms of the simplest type, of the _platodes_). VIII. In the further development of the various tissue-forming animals from the gastrula we have to distinguish two principal groups. The earlier and _lower_ types (the _coelenteria_ or _acoelomia_) have no body cavity, no vent, and no blood; such is the case with the gastræades, sponges, cnidaria, and platodes. The later and _higher_ types (the _caelomaria_ or _bilateria_), on the other hand, have a true body cavity, and generally blood and a vent; to these we must refer the worms and the higher types of animals which were evolved from these later on, the echinodermata, mollusca, articulata, tunicata, and vertebrata. Those are the main points of my "gastræa theory"; I have since enlarged the first sketch of it (given in 1872), and have endeavored to substantiate it in a series of "Studies on the gastræa theory" (1873-84). Although it was almost universally rejected at first, and fiercely combated for ten years by many authorities, it is now (and has been for the last fifteen years) accepted by nearly all my colleagues. Let us now see what far-reaching consequences follow from it, and from the evolution of the germ, especially with regard to our great question, "the place of man in nature."