XXVI CLASSIFICATION OF HELICES. 



systematic malacology, the role played by degeneration, and in form- 

 ally adopting and suitably characterizing the main European genera 

 as originally outlined by Schmidt. In the preliminary classification- 

 proposed by the writer (Proc. Acad. Nat. Sci. Phila. 1892, p. 392) 

 these European groups were placed as subgenera of Helix, but a 

 fuller study of the subject has resulted in the adoption of the genera 

 defined by von Ihering. 



III. NEW CLASSIFICATION OF HELICES. 



It will be seen by reference to the preceding pages that the classi- 

 fication of Helices has been based hitherto mainly upon the modifi- 

 tions of a single organ, such as the shell or the jaw ; and that even 

 the best of these classifications have yet given no clue to the rela- 

 tions the various groups of different life-areas bear toward one an- 

 other, nor have they even remotely suggested any phylogenetic lines. 

 In the present volume the attempt has been made to found a system 

 of grouping based upon several organs, and one expressive of the 

 facts of phylogeny and zoogeography. 



Single-organ classifications are even more than usually dangerous 

 in Pulmonates for we find that they have, like their ancestors the 

 Tectibranchs, an extremely plastic shell which shows many cases 

 of parallel or " converging" development, and frequently becomes- 

 reduced to a functionless remnant, in members of widely different 

 families, and their mouth parts are subject to great changes in 

 nearly allied groups. The Prosobranchs show no such wide range 

 of mutability in either shell or radula. 



It is generally held by biologists that a classification which takes 

 cognizence of several totally diverse, uncorrellated organs, is more 

 reliable than one based upon a single organ ; for the reason that 

 while some one organ or system of directly correllated organs, may 

 independently assume similar forms in members of different stocks 

 or phyla, when they are subjected to similar conditions of life, the 

 probabilities are remote that several organs not directly correllated 

 will be simultaneously so modified. Again, the ancestral form of a 

 certain organ may be retained in several groups widely diverse in 

 other respects; and moreover, the taxonomic value of a given struc- 

 ture varies widely in different families or genera. 



Another consideration of weight in selecting characters for a phy- 

 logenetic classification, is the fact that peripheral organs, or those- 

 directly acted upon by external forces, are most readily remoulded. 



