170 Mr. J. G. Jeffreys on the Marine Testacea 



the Mediterranean. How can this, or any other littoral species, 

 many of which are common to the British and Mediterranean 

 seas, find its way from one to the other ? Voluntary locomotion, 

 it is tolerably clear to all who know the proverbial slowness of 

 pace at which a snail, whether land or marine, can travel, would 

 require an immense time to complete the journey, even if the 

 animal knew or could find its way. Bivalves, being destitute of 

 a head or eyes, would of course labour under a greater disadvan- 

 tage; and besides, their motion is never progressive, but is 

 effected by eccentric and irregular leaps. The only other modes, 

 therefore, in which this great change of position could be 

 accounted for, are, either that the shell-fish may be torn from 

 their submarine abodes, and carried perforce by the current, or 

 that they may be in their embryonic state wafted to the place of 

 destination. The former mode would require it to be taken for 

 granted that there exist no rocks or other obstacles in the course 

 of their passage, that the current reaches the sea-bottom (which 

 is more than doubtful), and that the shell-fish in question live 

 within the range of the current. The other supposition can 

 only apply to the Bivalves, Brachiopods and Chitons, whose 

 embryo or fry are free and tolerably active swimmers, under- 

 going during that period of their existence a singular metamor- 

 phosis, as I have myself witnessed in the case of the common 

 oyster. But as the fry are developed and attain their normal 

 state within a few days at the furthest after being excluded from 

 their parent, and then become fixtures for life, or nearly so in the 

 case of the Chitons, it is hardly possible that the time allotted 

 to the first stage of their existence would enable them to traverse 

 such a vast distance. If we reject Forbes's proposition that the 

 species I have before mentioned are Boreal, Celtic, British, or 

 Glacial, and consider them as Mediterranean, the same difficulty 

 arises ; and we shall not find the mode of transit from the 

 Mediterranean to the British seas more easy or probable when 

 we reflect that the only ingress into or egress out of the Medi- 

 terranean is through the Straits of Gibraltar, and especially 

 if the only current which flows through that passage is an 

 indraught, and sets fronij instead of to, the Atlantic. The 

 popular idea of a counter- or under-current from the Mediter- 

 ranean outwards is (to say the least) not proved ; and I do not 

 think Lieutenant Maury, in his recent and excellent treatise on 

 the physical geography of the sea, has made out a strong case 

 in its support, in opposition to the opinions of Admiral Smyth 

 and Sir Charles Lyell. If there is such an outer- or under-current 

 from the Mediterranean into the Atlantic, shell-fish might, it is 

 true, be transported from the former to the latter ; but they 

 would in that case be, metaphorically as well as literally, " at 



