The Articulates are the respiratory animals in this classification: they represent respiration. The Worms, breathing, as he asserts, through the whole surface of the skin, without special breathing organs, are the lowest; the Crustacea, with gills, or aquatic breathing organs, come next; and he places the Insects highest, with their branching tracheæ, admitting air to all parts of the body. The Vertebrates, or Flesh Animals, with their four classes, represent the Bones, the Muscles, the Nerves, and the Organs of Sense. This theory, according to which there are as many great divisions as there are structural systems or combinations of systems in the Animal Kingdom, seemed natural and significant, and there was something attractive in the idea that man represents, as it were, the synthetic combination of all these different systems. Oken also, in his exposition of his mode of classification, showed an insight into the structure and relations of animals that commended it to the interest of all students of Nature, and entitles him to their everlasting gratitude. Nevertheless, his theory fails, when it is compared with facts. For instance, there are many Worms that have no respiration through the skin, while his appreciation of the whole class is founded on that feature; and in his type representing circulation, the Mollusks, there are those that have no heart at all. It would carry me too far into scientific details, were I to explain all the points at which this celebrated classification fails. Suffice it to say that there is no better proof of the discrepancy between the system and the facts than the constant changes in the different editions of Oken's own works and in the publications of his followers founded upon his views, showing that they were themselves conscious of the shifting and unstable character of their scientific ground. VI.