V.] INTRODUCTION. Ixxxiii 



has been also left in an unsettled and unsatisfactory 

 state. Woodward lays it down as a rule, that, "in order 

 to constitute a distinct province, it is considered neces- 

 sary that at least one-half the species should be peculiar, 

 a rule which applies equally to plants and animals." 

 On the other hand, M' Andrew, after admitting that a 

 considerable portion of the species of Mollusca inhabiting 

 any one zoological province may be found in other pro- 

 vinces, says, "It is not by a simple comparison of the 

 list of species that we can determine the similarity or 

 divergence of the fauna of separate localities, as the differ- 

 ence between them may consist in a few characteristic 

 forms, which may be especially developed in each." This 

 wide divergence of opinion as to the rule or " law " of 

 distribution, between two such able and experienced 

 naturalists, renders further inquiry into the facts of the 

 case indispensable, especially when it is considered that 

 so many of the " species " referred to by Woodward and 

 of the "forms" (by which it is presumed genera are 

 meant) indicated by M 'Andrew are questionable or still 

 sub judice. That genera-makers may be found who will 

 separate such forms as Trivia from Cyprcea, Erato from 

 Margimlla, and Admete from Cancellaria, cannot be 

 helped, although most naturalists deprecate and disavow 

 such trifling distinctions. But until a complete concord- 

 ance has been established and recognized between all the 

 forms, whether generic or specific, of the Mollusca which 

 inhabit any one area, a solid and reliable foundation 

 cannot be obtained for erecting the superstructure of 

 distribution. No conchologist, whose mind is free from 

 prejudice, either as regards the authority of names or of 

 theories on the subject in question, can detect any greater 

 difference between specimens of Mitra Grcenlandica from 

 Spitzbergen and Mitra ebenus from Naples, than he can 



