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. Cl. 3. Gasteropoda. Cl. 4. Acephala. Cl. 5. Brachiopoda. Cl. 6. Cirrhopoda. Third Branch. ANIMALIA ARTICULATA. Cl. 1. Annelides. Cl. 2. Crustacea. Cl. 3. Arachnides. Cl. 4. Insects. Fourth Branch. ANIMALIA RADIATA. Cl. 1. Echinoderms. Cl. 2. Intestinal Worms. Cl. 3. Acalephæ. Cl. 4. Polypi. Cl. 5. Infusoria. But though Cuvier emancipated himself from the conception of a serial progression throughout the Animal Kingdom, sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differently-combined sets of attributes distinguishing the different sub-kingdoms, and swayed by the belief in a progressive development which was erroneously supposed to imply a linear arrangement of animals, they persisted in thrusting organic forms into a quite unnatural order. The following classification of Lamarck illustrates this. INVERTEBRATA. I. APATHETIC ANIMALS. } } Cl. 1. Infusoria. } Do not feel, and move only by their Cl. 2. Polypi. } excited irritability. No brain, no Cl. 3. Radiaria. } elongated medullary mass; no senses; Cl. 4. Tunicata. } forms varied; rarely articulations. Cl. 5. Vermes. }