This arrangement of classes is obviously based on apparent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, beginning with the most superior forms and ending with the most inferior forms. While the general and vague idea of perfection determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. Not only Linnæus but his opponents, who proposed other systems, were "under the impression that animals were to be arranged together into classes, orders, genera, and species, according to their more or less close external resemblance." This conception survived until the time of Cuvier. "Naturalists," says Agassiz, "were bent upon establishing one continual uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was: Natura non facit saltum. They called their system la chaine des êtres."
The classification of Cuvier, based on internal organization instead of external appearance, was a great advance. He asserted that there are four principal forms, or four general plans, on which animals are constructed; and, in pursuance of this assertion, he drew out the following scheme.
First Branch. ANIMALIA VERTEBRATA.
Cl. 1. Mammalia.
Cl. 2. Birds.
Cl. 3. Reptilia.
Cl. 4. Fishes.
Second Branch. ANIMALIA MOLLUSCA.
Cl. 1. Cephalapoda.
Cl. 2. Pteropoda.