ScHAKFF — Former Land-Bridge between N. Europe and N. America. 7 



disseminated. We do not even know that the experiments conducted in the 

 workroom are repeated under natural conditions. Mr. Kew, in liis well- 

 known work on the " Dispersal of Shells," has to acknowledge : — " Unfor- 

 tunately I do not know tliat any observation clearly indicating the 

 transportal of Molluscs or their eggs with drift-timber, &c., has ever been 

 made. The creatures have never been found, as far as I have ascertained, in 

 the crevices, or under the bark either of trees encountered upon the sea, or 

 of those stranded on foreign coasts.'" We jiossess, moreover, \xn-y numcrou.s 

 records of intentional introductions of species by man having cither entirely 

 failed to establish themselves, or having become extinct after several years 

 of apparent success. I have already had occasion to quote an example which 

 has come under my notice, and which is of particular interest, as it relates to 

 a species which seemed to Darwin to possess special facilities for dispersal, 

 viz., Ci/clostoma clegans.^ This snail is provided with a lid or operculum. 

 When, as Mr. Darwin tells us, a dozen of them were immersed for a fortnight 

 in sea-water, eleven specimens survived the treatment.' Being endowed with 

 an exceptional device for resisting the action of sea-water on its delicate 

 organism, and being abundant all over the western parts of Continental Europe, 

 we should expect Ci/clostoma clcgans to have been cou\'eyed by marine currents 

 to the Canary Islands, Madeira, or even to Ireland. None of these islands is 

 at too great a distance from the mainland to be beyond easy reach of a floating 

 object ; and yet it does not inhabit any of them. This is of particular interest 

 as regards Ireland ; because dead shells of this species have been picked up on 

 the Irish coast, indicating that marine currents do carry specimens, and ha\'e 

 probably transported living ones to that island for centuries. That C'l/clostoiua 

 clcgans has nevertheless failed to establish itself in Ireland seems to justify 

 the belief that other animals or even plants arriving in a similar manner 

 may find it equally difficult to do so. 



A more striking fact against the theory of " accidental or occasional " 

 introduction is furnislied by the land molluscs inhabiting the I'acific islands. 

 These are tenanted entirely by the most primitive groups of snails ; while 

 the more highly organized genera and even families which occur on the 

 neighbouring continents of Asia and America are unrepresented. " It is very 

 easy," remarks Dr. I'ilsbry, " to show that snails may ha\e been carried 

 from place to place by a hurricane, a fioating tree, or floating island, or their 

 eggs may find room in the pellet of earth clinging to a bird's feather ; Inil ii 



■ Kew, H. W., " Dispeisal of Sliolls." p. KiS. 

 ■•' Sulmrtt', U. F., " Eiuopuau Aiiinmls,"' |i. t. 

 ^ Umwin, C, " Oriyin of Speuies," p. 353. 



