THE ANNALS AND MAGAZINE OF NATURAL HISTORY. [THIRD SERIES.] No. 96. DECEMBER 1865. XLII. — On the Systematic Value of the Organs which have been employed as Fundamental Characters in the Classification of Mollusca. By Dr. 0. A. L. Morch. Whether classes, orders, and genera are founded in nature, or are only artificial divisions, is a question rarely disputed. It is, however, still a matter of discussion whether now-existing spe-cies are direct descendants of extinct forms from remote geo-logical periods, which have been gradually changed in the course of natural selection or in consequence of physical changes of the globe, or are entirely new creations of any one geological era. The habits of an animal often cause a considerable modification of the external form, size, or colour, which are often improperly considered of specific value ; but it must be admitted that such difierences are subject to limits which cannot be passed, and do not become hereditary. For instance, the corns of the human foot are, like the nails, a thickening of the epidermis ; but the former are produced by accidental pressure on the foot, while the latter originate in the fcetal structure of the animal. There is the same distinction between false and genuine species. Linne divided all creation into three kingdoms — animal, vegetable, and mineral. The last-named division is less logical, because minerals can only be considered as parts of the great celestial bodies, which may be regarded as inorganic beings with involuntary motion impressed upon them, corresponding with that of the heart or stomach of animals. Geofiroy St.-Hilaire has more correctly made the division into phanerobiotic and cryptobiotic kingdoms. A predestined scheme is shown in the chronology of our planet as well as in the development of organic beings. Thus the oceans with their lower types are regularly in course of time changed to islands and continents with their Ann. is Mag. N. Hist. Ser.3. Fo/.xvi. 26
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