ZOOLOGY

Now, if scientific cla ification were really based on the idea of likene alone, and the grouping together of individuals into kinds in forming a natural system were regulated by a consideration of their resemblances only, then the modification of the Aristo tel i an system involved in Cuvier's discovery would be merely an extension of the group Enaima at the expense of the group Anaima; and the Anelli des, or red-blooded worms, must have been united with birds, fishes, and other Sanguineous animals. But the zoologist, in the formation of a natural sv-stem, has to be governed at every step by the idea of difference as well as by that of likene . Coincident with the discovery above mentioned was the perception that if the Anelli des resembled beasts in the colour of their blood, they differed from them in most other e ential points; and thus the circulating fluid was rejected as a ground for the primary groups of the animal kingdom, and other characters were eagerly sought for. Lamarck conceived that he had discovered the best substitute for the Aristo tel i an primary character in the vertebral column; this structure being present in all the Enaima of Aristotle, and absent in all the Anaima. He proposed, therefore, the name of Vertebrata for the one cla , and Invertebrata for the other. The defect of this primary division of the animal kingdom was soon perceived to arise from the neglect of a third fundamental idea in a cla ificatory science, viz. that of proportion, or relative value, in the primary groups; and in the attempt to remedy this defect the important discovery was made, that the vertebral column was a modification of structure subordinately related to a particular condition of an organic system of much higher importance in the animal body than the skeleton, viz. the nervous system. A knowledge of the anatomy of this system hence became e ential to the zoologist; and the result of a long series of minute and elaborate di ections was the detection of at least three modifications of the nervous system of equal importance with that in regard to which a skull and vertebral column are dependent and subordinate. Hence arose the proposition by Cuvier to divide the animal kingdom primarily into four provinces or subkingdoms: viz. Vertebrata, Mollusca, Articulata, and Radiata; or, as they have been termed, in accordance with the modifications of the nervous system respectively characterizing them, Myelencephala, Heterogangliata, Homogangliata, and Acrita. All previous primary divisions of the animal kingdom*, having been proposed in ignorance of the true characters by which such groups exist in nature, are now abandoned by the common consent of naturalists, and nothing would be gained by quoting them in this place. Herein the Linnsean method is inferior to the Aristo tel i an system: the cla Vermes of the System a Natures, as it included the Ostracoderm a and Malakia of Aristotle, afterwards the Mollusca of Cuvier, with the true Fermes, was a retrograde step in this branch of zoological science. The subkingdom Vertebrata, or Myelencephala, is subdivided into cla es, according to the modifications of the respiratory and circulating organs. These modifications are e entially four in.number, which may be thus expre ed: — 1. Lungs suspended freely in a thoracic cavity; subdivided into minute air-cells. Heart divided into four cavities; pulmonic and systemic circulations distinct. 2. Lungs adherent to the walls of a thoracic cavity, communicating with large air-cells in other cavities of the body. Heart with four cavities; pulmonic and systemic circulations distinct. 3. Lungs suspended freely in, or attached to the parietes of a thoracic-abdominal cavity; not divided into minute air-cells. Heart with four or three cavities; pulmonic and systemic blood mixed in the general circulation. 4. Gills for respiration, with or without lungs. Heart with two cavities, transmitting all the blood to the breathing organs. The first condition of the respiratory and circulating organs coexists with a viviparous generation, and lacteal organs for the nourishment of the new-born young; whence the cla was termed Zootoca t by Aristotle, and Mammal i a by Linnaeus; and the latter term is retained, because it expre es a character which is truly peculiar to the cla . The total or partial clothing of hair, and the other organic coexistences which proclaim the naturalne of the group in question, are mentioned under the term Mammal i a. The second condition of the respiratory and circulating organs characterizes the cla of Birds. It is a ociated in this cla with an oviparous generation, a covering of feathers, and those other modifications of the vertebrate type of structure which are detailed under the * If we except some of the schemes of cla ification of the animal kingdom by John Hunter, found among his manuscripts after his death. (See Preface to Palmer's edition of his Animal Ecojiomy.) t The terra Zootoca has been degraded by a modem naturalist to designate a subgenus of lizards, distinguiiihed by their viviparous generation. [s. 1354]