Cuvier, to whom we owe this classification of the animal kingdom into four great divisions, would have been the first to recognize the chaotic condition in which he left this last division, and would have acquiesced in the separation of the PROTOZOA, which has since been made. This fifth division includes many of the microscopic animals known as _Infusoria_; and receives its name from the idea that these simplest of all animals represent, as it were, the beginnings of life.[19] But Cuvier’s arrangement is open to a more serious objection. The state of science in his day excused the imperfection of classing the Infusoria and parasites under the Radiata; but it was owing, I conceive, to an unphilosophical view of morphology, that he placed the molluscs next to the Vertebrata, instead of placing the Articulata in that position. He was secretly determined by the desire to show that there are four very distinct types, or plans of structure, which cannot by any transitions be brought under one law of development. Lamarck and Geoffroy St. Hilaire maintained the idea of unity of composition throughout the animal kingdom;—in other words, that all the varieties of animal forms were produced by successive modifications: and several of the German naturalists maintained that the vertebrata in their embryonic stages passed through forms which were permanent in the lower animals. This idea Cuvier always opposed. He held that the four types were altogether distinct; and by his arrangement of them, their distinctness certainly appears much greater than would be the case on another arrangement. But without discussing this question here, it is enough to point out the fact of the enormous superiority in intelligence, in sociality, and in complexity of animal functions, which insects and spiders exhibit, when compared with the highest of the molluscs, to justify the removal of the mollusca, and the elevation of the articulata to the second place in the animal hierarchy. Nor is this all. If we divide animals into four groups, these four naturally dispose themselves into two larger groups: the first of these, comprising Vertebrata and Articulata, is characterized by a _nervous axis_ and a _skeleton_; the second, comprising Mollusca and Radiata, is characterized by the absence of both nervous axis and skeleton. It is obvious that a bee much more closely resembles a bird, than any mollusc resembles any vertebrate. If there are many and important differences between the vertebrate and articulate types, there are also many and important resemblances; if the nervous axis is _above_ the viscera, and forms the dorsal line of the vertebrate, whereas it is _underneath_ the viscera, and forms the ventral line in the articulate, it is, nevertheless, in both, the axis of the body, and in both it sends off nerves to supply symmetrical limbs; in both it has similar functions. And while the articulata thus approach in structure the vertebrate type,