udy in the present state of our science.

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 trul