But while there is this structural gradation among orders, establishing a relative rank between them, are classes and branches also linked together as a connected chain? That such a chain exists throughout the Animal Kingdom has long been a favorite idea, not only among naturalists, but also in the popular mind. Lamarck was one of the greatest teachers of this doctrine. He held not only that branches and classes were connected in a direct gradation, but that within each class there was a regular series of orders, families, genera, and species, forming a continuous chain from the lowest animals to the highest, and that the whole had been a gradual development of higher out of lower forms. I have already alluded to his division of the Animal Kingdom into the Apathetic, Sensitive, and Intelligent animals. The Apathetic were those devoid of all sensitiveness except when aroused by the influence of some external agent. Under this head he placed five classes, including the Infusoria, Polyps, Star-Fishes, Sea-Urchins, Tunicata, and Worms,--thus bringing together indiscriminately Radiates, Mollusks, and Articulates. Under the head of Sensitive he had also a heterogeneous assemblage, including Winged Insects, Spiders, Crustacea, Annelids, and Barnacles, all of which are Articulates, and with these he placed in two classes the Mollusks, Conchifera, Gasteropoda, and Cephalopoda. Under the head of Intelligent he brought together a natural division, for he here united all the Vertebrates. He succeeded in this way in making out a series which seemed plausible enough, but when we examine it, we find at once that it is perfectly arbitrary; for he has brought together animals built on entirely different structural plans, when he could find characters among them that seemed to justify his favorite idea of a gradation of qualities. Blainville attempted to establish the same idea in another way. He founded his series on gradations of form, placing together, in one division, all animals that he considered vague and indefinite in form, and in another all those that he considered symmetrical. Under a third head he brought together the Radiates; but his symmetrical division united Articulates, Mollusks, and Vertebrates in the most indiscriminate manner. He sustained his theory by assuming intermediate groups,--as, for instance, the Barnacles between the Mollusks and Articulates, whereas they are as truly Articulates as Insects or Crabs. Thus, by misplacing certain animals, he arrived at a series which, like that of Lamarck, made a strong impression on the scientific world, till a more careful investigation of facts exposed its fallacy.