It is not uninstructive to mark how slowly and gradually the naturalists have been groping their way to a right classification in the ichthyic department of their science, and how it has been that identical perception of resemblance, having _cartilage_ for its general idea, to which the author of the “Vestiges” attaches so much importance, that has served mainly to retard their progress. Not a few of the more distinguished among their number deemed it too important a distinction to be regarded as merely secondary; and so long as it was retained as a primary characteristic, the fishes failed to range themselves in the natural order;—dissimilar tribes were brought into close neighborhood, while tribes nearly allied were widely separated. It failed, as has been shown, to influence Linnæus; and though he no doubt pressed his peculiar views too far when he degraded the glutinous hag into a worm, and elevated the Sharks and Rays into reptiles, it is certainly worthy of remark, that, in the scheme of classification which is now regarded as the _most natural_,—that of Professor Muller, modified by Professor Owen,—the ichthyic worms of the Swede are placed in the first and lowest order of fishes,—the _Dermopteri_,—and the greater part of his ichthyic reptiles, in the eleventh and highest,—the _Plagiostomi_. Cuvier yielded, as has been shown, to the idea of resemblance founded on the _material_ of the ichthyic framework, and so ranged his fishes into two parallel lines. Professor Oken, after first enunciating as law that “the characteristic _organ_ of fishes is the osseous system,” confessed the “great difficulty” which attaches to the question of skeletal “texture or substance,” and finally gave up the distinction founded on it as obstinately irreducible to the purposes of a natural classification. “The cartilaginous fishes,” he says, “appear to belong to each other, and are also usually arranged together; yet amongst them we find those species, such as the Lampreys, which obviously occupy the lowest grade of all fishes, while the Sharks and Rays remind us of the Reptilia.” And so, sinking the consideration of texture altogether, he placed the family of the Lamprey, including the glutinous hag, at the bottom of the scale, and the Sharks and Rays at the top. Agassiz’s system, peculiarly his own, has had the rare merit, as I have shown, of furnishing a key to the history of the fish in its several dynasties, which we may in vain seek in any other. His divisions,—if, retaining his strongly-marked Placoids and Ganoids, as orders stamped in the mint of nature, we throw his perhaps less obviously divisible Ctenoids and Cycloids into one order,—the corneous or horn-covered,—are scarcely less representative of periods than those great classes of the vertebrata, mammals, birds, reptiles, and fishes, which we find not less regularly ranged in their order of succession in the geologic record than in the “Animal Kingdom”