388 Mr. A H. Cooke on the 



stood in the narrowest acceptation of the term), because they 

 are not sufficiently constant and because the groups in which 

 they occur are subject to considerable variations in form {sono 

 i pill polimorfi). 



" We, and other followers of the English school, while 

 looking upon species as in theory very useful and, indeed, 

 necessary for the study of the science, regard them as being 

 to a large extent conventional and arbitrary, and believe that 

 species vary according to the physical conditions in which 

 they find themselves placed. Thus we should regard it as an 

 abnormality, an exception to every rule, if there occurred in 

 the Eed Sea a Mediterranean species which had not undergone 

 some modification. 



" Besides the varieties indicated above there exist in the 

 Eed Sea certain shells differing indeed from their Mediter- 

 ranean congeners, but sufficiently akin to them to stand as 

 their representatives, so to speak, on the further side of the 

 isthmus. 



" Instances of this parallelism are found in Nassa gibhosula 

 and circumcincta^ Cerithium vidgatum and Riippellii^ Cerith. 

 conicum and CaiUaudi^ Chiton sicuhis and affim's, Diplodonta 

 rotwndata and Savignyi, Cardium edule and isthmicum. 



" Ought we to comprehend under the heading of geogra- 

 pihical variety shells from the Eed Sea, so near to the Mediter- 

 ranean ? 



" Under this denomination are usually distinguished forms 

 derived from a recognized type, which, by removal from the 

 central point of their creation, have become gradually modified, 

 and differ from it the more the greater is tlie distance which 

 separates them from the point of departure. This is not the 

 case with the shells now under consideration, because at 

 certain points of the isthmus scarcely 100 kilometres separate 

 the two faunas, and because such a short distance is not 

 enough to constitute the existence of a geographical variety. 



" These considerations have suggested the idea of distin- 

 guishing by a comprehensive term Red-Sea varieties and 

 species which correspond to Mediterranean species. We will 

 therefore give the name of equivalent varieties to those Medi- 

 terranean species which occur on the further side of the 

 isthmus only slightly modified ; and the name of equivalent 

 species to those representatives of Mediterranean species 

 which occur in the Eed Sea modified to a larger extent. 



" It is our belief that from any type, existent in a given 

 place, there can be derived equivalent species and equivalent 

 varieties if the locality has undergone more or less consider- 

 able changes ; that, again, there can be derived geographical 



