Oken, the great German naturalist, also attempted to establish a connected chain throughout the Animal Kingdom, but on an entirely different principle; and I cannot allude to this most original investigator, so condemned by some, so praised by others, so powerful in his influence on science in Germany, without attempting to give some analysis of his peculiar philosophy. For twenty years his classification was accepted by his countrymen without question; and though I believe it to be wrong, yet, by the ingenuity with which he maintained it, he has shed a flood of light upon science, and has stimulated other naturalists to most important and interesting investigations. This famous classification was founded upon the idea that the system of man, the most perfect created being, is the measure for the whole Animal Kingdom, and that in analyzing his organization we have the clue to all organized beings. The structure of man includes two systems of organs: those which maintain the body in its integrity, and which he shares in some sort with the lower animals,--the organs of digestion, circulation, respiration, and reproduction; and that higher system of organs, the brain, spinal marrow, and nerves, with the organs of sense, on which all the manifestations of the intelligent faculties depend, and by which his relations to the external world are established and controlled: the whole being surrounded by flesh, muscles, and skin. On account of this fleshy envelope of the hard parts in all the higher animals, Oken divided the Animal Kingdom into two groups, the Vertebrates and Invertebrates, or, as he called them, the "_Eingeweide und Fleisch Thiere_"--which we may translate as the _Intestinal Animals_, or those that represent the intestinal systems of organs, and the _Flesh Animals_, or those that combine all the systems of organs under one envelope of flesh. Let us examine a little more closely this singular theory, by which each branch of the Invertebrates becomes, as it were, the exponent of a special system of organs, while the Vertebrates, with man at their head, include all these systems. According to Oken, the Radiates, the lowest type of the Animal Kingdom, embody digestion. They all represent a stomach, whether it is the simple sac of the Polyps, or the cavity of the Acalephs, with its radiating tubes traversing the gelatinous mass of the body, or the cavity and tubes of the Echinoderms, inclosed within walls of their own. The Mollusks represent circulation; and his division of this type into classes, according to what he considers the higher or lower organization of the heart, agrees with the ordinary division into Acephala, Gasteropoda, and Cephalopoda.