of the Piedmontese Coast. 171 



sea/^ because, according to Maury's charts and account of the 

 Gulf-stream, the course northward of that great current lies far 

 beyond the range of the Mediterranean outlet into the Atlantic. 

 If there is no such outer current, shell-fish leaving the British 

 shores would, after crossing the English or Irish Channel, have 

 to traverse, by a circuitous route, the western coasts of Europe 

 by means of the great Arctic current, which is supposed to pass 

 under the Gulf-stream, before they could reach and enter the 

 Strait of Gibraltar. Either of these suppositions therefore, except 

 perhaps with respect to pelagic or floating shell-fish (such as the 

 Pteropods and lanthina communis), does not appear to me well 

 founded, and still less probable in the case of shell-fish which 

 permanently adhere to rocks, or their fry, for the reasons I have 

 before given. I therefore cannot help thinking that the migra- 

 tion or diffusion, beyond a limited range, of marine shell-fish by 

 means of oceanic currents now in operation is physically impro- 

 bable, and that it is unnecessary thus to account for the present 

 distribution of these animals. 



For the same reasons, I am not disposed to admit the theory 

 which has been propounded and maintained by so many natu- 

 ralists, that certain areas now exist, containing species peculiar to 

 each, and having each its own separate nucleus or centre from 

 which these species have radiated. 



In the last edition of Lyel?s ' Principles of Geology' is a map 

 showing the extent of land in Europe which can be proved to 

 have been covered by the sea during the earlier part of the ter- 

 tiary, or the eocene, period ; and a wide opening from the Bay 

 of Biscay to the Gulf of Lyons, in the upper part of the Me- 

 diterranean, appears to have formerly connected that sea with 

 the North Atlantic. It has been also proved by Brocchi, Phi- 

 lippi, and Searles Wood, that a large proportion of shells now 

 living in the Mediterranean are identical with fossil species from 

 the tertiary strata, both in Italy and Great Britain, and vice 

 versd; and it is not too much to assume, that in former seras 

 marine currents existed by which animals might have been trans- 

 ported from one to the other of those districts, or rather that 

 they were then diffused throughout a larger area than at pre- 

 sent. Whether the original birthplace or nucleus of these shell- 

 fish was in that part of the ocean which is now called the North 

 Atlantic, or in the Mediterranean, is immaterial; — all I contend 

 is, that the areas of geographical distribution, as proposed by 

 the late Professor Eorbes and others, are too much restricted to 

 existing circumstances, and that they ought rather to be referred 

 to a prior state of things. As yet, we want infinitely more in- 

 formation and data as regards the distribution of recent and 

 fossil shells, as well as a more accurate discrimination of species 



