common to North America and Europe. 469 



better to leave those families out of the question, which pre-| 

 sent great discrepancies, and base my conclusions upon the 

 genera nearly equally abundant in species upon both sides, 

 which will be found to be the case in the Physadse. Thus we 

 have about 50 species of Physa, Limnea, and Planorbis, of 

 which three, or 6 per cent, are European : but with Ancylus, 

 the number of Physadae is raised to 60, reducing the percent- 

 age to Jive. If we reject Ampullaria as a southern form not 

 admissible into the fauna of Europe, the remaining portions 

 of Lamarck's Peristomata will not cause the result to vary, so 

 that we may safely assume the last named number as indicat- 

 ing the proper proportion. 



In preparing lists like the foregoing one, we are met by 

 several difficulties, the most important turning upon the iden- 

 tity or non-indentity of certain shells, apparently the same, but 

 found upon different continents ; with no apparent existing 

 means of traversing the intervening ocean. The question at 

 issue resolves itself into two propositions: 1. Animals occurring 

 in separate regions, which they could not have attained by 

 crossing the intermediate space, however much they may re- 

 semble each other, are distinct species. 



2. vSimilar animals, under whatever circumstances they may 

 be found, constitute but one species. 



At the very onset we are met by the question. What is a 

 species ? and sides will be taken according to the answer each 

 one is ready to adopt.* The definition of a species does not 



American form. Some conchologists consider the two genera identical, an opinion 

 which the characters and geographical distribution will not justify, notwithstanding 

 the near alliance between some of the species. 31elanopsis is not an American 

 genus, the species referred to it by M. Deshayes, (Melania nupera Say,) constituting 

 with several others a distinct form, of which Lithasia geniculata Hald. is the type. 



* " We have agreed that a species shall be that distinct form originally so created, 

 and producing, by certain laws of generation, others like itself. There is this incon- 

 venience attending the use of it by naturalists, that it assumes as a fact, that which^ 

 in the present state of science, is in many cases a fit subject of inquiry ; namely, 

 that species, according to our definition, do exist throughout nature. It is too conve- 

 nient a term to be dispensed with, even as an assumption ; only care should he taken 

 that we do not accept the abstract term for the fact." Bichcno. Lin. Trans, xv, 4S2. 

 " There is no law whatever hitherto established, by which the limits of variation to a 

 given species can be satisfactorily assigned, and until some such law be discovered, 



