PRINCIPLES OF CLASSIFICATION. 43 



54. The foregoing remarks are scarcely less applicable to the classifications of Professors 

 Schultze (xcvii) and Bronn (xi) than they are to that of M. D'Orbigny; since by them, 

 as by him, the jjiau of f/roxolh is regarded as the fundamental basis of S3'stematic arrangement, 

 so that their primary divisions are eminently artificial. They have been guided, however, in 

 their subdivision of Orders into Families by a higher appreciation of the characters furnished 

 by variations in the texture of fl/e hJicII, than was entertained by M. D'Orbigny ; and they have 

 thus been led to a more natural grouping of generic forms than his classification usually 

 presents. — As neither of their systems, however, has yet found its way to general acceptance, it 

 scarcely appears necessary here to enlarge upon their defects ; these being necessary results 

 of the imperfect method of study whicli tlieir authors have followed; and the object of the 

 present work being to sTibstitute a classification, whicli, however incomplete, shall, at any rate, 

 present an approximation to a natural system, as being based on the whole aggregate of the 

 ascertainable characters of the several types, instead of on a single feature which affords no 

 reliable indication of their real aflSnities. 



55. It is now universally admitted by Philosophical Zoologists, that the importance of the 

 characters furnished by the f^Icrleton, whether internal or external, of any animal, depends 

 entirely upon the relations which they bear to its general organization ; and that hence the 

 adoption of any such characters as a basis for classification can only be justified, when their 

 accordances and differences can be shown to be indicative of corresponding accordances and 

 differences in those parts of the organism whicli are of higher physiological import. Thus 

 the possession of a hivahe shell is universally admitted as differentiating the Mollusks which 

 bear it from those whose shell is univalve, the whole plan of structure of the animals in the two 

 cases being obviously different. But among " bivalve'' Mollusks there are two very distinct 

 types of structure (the Lamellibranchiate and the Palliobranchiate), whose essential dissimi- 

 larity, being only revealed by anatomical inquiry, would never have been recognised by the 

 mere Conchologist ; although, when he has once made himself acquainted with these types, he 

 finds no difficulty in distinguishing the shells they respectively include by the special 

 characters which they severally present. So among " univalve" Mollusks there is not less 

 diversity of type, shells composed of a single piece being found among Gastcropods, Ptero- 

 pods, and Cephalopods ; and the conformation of the shell is here so much less intimately 

 related to that of the animal by which it is constructed, that it is not always possible to refer 

 a shell with certainty to its proper place while the nature of its animal inhabitant is unknown. 

 Further, " univalve" shells are formed also by Annelida, and there are no well-defined characters 

 by which the tubes of a Serjjula (Annelid) and these of a Vermetm (Gasteropod Mollusk) can be 

 distinguished one from the other ; so that in a system of classification founded ujion the shell 

 alone they would be placed side by side, as would also the Crustacean Cirrhipedes and the 

 Gasteropod Chitons, because both these tribes of animals have their bodies protected by 

 multivalve shells. Among Zoophytes, again, whilst the " lamelliform" structure of the stony 

 Corals is so uniformly related to the Aetiniform type of organization, that the existence of 

 that structure in the oldest fossil affords a sure key to the nature of the animal which formed 

 it, the polyparies which constitute the skeletons of animals of the Ahyonian type are so 

 diverse in their composition and arrangement, that it only becomes safe to predicate the 



