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ESSAY ON CLASSIFICATION.

divisions, Fitzinger'employs those most generally in use.His subdivisions or grades of these two primary groupsare based upon a repetition of the same differences withintheir respective limits. The Invertebrata , in which thevegetative organs prevail, are contrasted with those inwhich the animal organs prevail; and the same distinc-tion is again drawn among the Vertebrata . Each of theseembraces two circles, founded upon the development ofone particular system of organs, etc. It cannot be ex-pected that the systems founded upon such principlesshould present a closer agreement with one another thanthose which are based upon anatomical differences; yet Iwould ask, what becomes of the principle itself, if its ad-vocates cannot even agree upon what anatomical systemsof organs their classes are founded ? According to Oken,the Mollusks (Acephala, Gasteropoda , and Cephalopoda )represent the system of circulation; at least, in the lastedition of his system, he views them in that light, whilstFitzinger considers them as representing the system ofsensibility. Oken identifies the Articulata (Worms , Crus­ tacea , and Insects) with the system of respiration; Fitz-inger with that of motion, with the exception of theWorms , including Radiata, which he parallelizes with thesystem of reproduction, etc. Such discrepancies mustshake all confidence in these systems, though they shouldnot prevent us from noticing the happy comparisons andsuggestions to which the various attempts to classify theanimal kingdom in this way have led their authors. Itis almost superfluous to add, that, great as the disagree-ment is between the systems of different physiophiloso-phers, we find quite as striking discrepancies between thedifferent editions of the system of the same author.