of the Piedmontese Coast. 169 



by oceanic currents. But it must be borne in mind that the 

 great Gulf-stream, which is the only current that could be 

 effectual for such a purpose, sets from south to north, and 

 that the indraught current from the Atlantic into the Mediterra- 

 nean, through the Straits of Gibraltar, sets from west to east. 

 Major Rennell was indeed of opinion (according to Admiral 

 Smyth, whose excellent and elaborate work on the Mediterra- 

 nean I have had frequent occasion to consult), that there is a 

 general tendency of the Atlantic waters between 30° and 45° of 

 north latitude, and from 100 to 130 leagues off the land, to 

 move towards the Strait of Gibraltar, at a rate of not less than 

 from fourteen to seventeen miles in twenty-four hours ; although 

 this opinion, the Admiral says, ought to be received cum grano 

 saliSy especially if depth be admitted as a condition of these 

 400,000 square miles. However, granting that this may to 

 some (I am not prepared to say to what, or any) extent account 

 for the migration or diffusion of species from the Lusitanian 

 coast to the British or Mediterranean seas, it cannot be con- 

 sidered to operate in the opposite direction ; and I therefore do 

 not see how any species which we may for the present call, with 

 Prof. Forbes, " peculiarly Northern ^^ or " Boreal ^' (such as Chi- 

 ton Hanleyi)y or " Celtic^^ (as Lucina flexuosa), or " peculiarly Bri- 

 tish^^ {siS Jeffreysiadiaphana),ov "Glacial^-* (as Cardium Suecicum) , 

 can find their way into the Mediterranean, while the Gulf-stream 

 continues its present course. We will even assume that there is 

 a counter-current (although our present hydrographical know- 

 ledge does not warrant the assumption) from the north to the 

 south, — I would ask, how is it possible that species, which, like 

 Chiton Hanleyij inhabit the coralline zone, can be transported 

 across the Atlantic to such a distance ? The stationary habits 

 of the adult animal, which passes its life adhering to stones 

 and shells, forbid the idea of its voluntary migration. In its 

 embryo state this Chiton is doubtless (as Mr. Clark has 

 shown in the 'Annals' for December 1855 with respect to its 

 congener, Chiton cinereus), like many other Mollusca, free, and 

 capable of swimming about with considerable activity ; but this 

 stage of growth only lasts three or four days, when the meta- 

 morphosis or final development takes place, and the creature, 

 having " sown its wild oats,'' settles down for the rest of its 

 life, and only crawls about for a short distance in search of food. 

 The same remark occurs to me w^ith respect to the littoral 

 species, such as Skenea planorbis (a well-known inhabitant of 

 Great Britain, and now for the first time noticed by me as 

 Mediterranean), which would appear to be physically incapable 

 of crossing the stream of the English Channel in order to diffuse 

 itself along the western coasts of Europe and gain ingress into 



