144 HELIX DESERTORUM. [CH. Ill 



islands, discusses the problems offered entirely from the 

 point of view of former land connection. 



Such land Mollusca as are operculate can of course 

 close their shells and so endure with impunity events that 

 would be fatal to others ; even the temporary operculum 

 formed by Helix is effective in this way. It is important 

 too to notice that fresh-water Mollusca can in some cases 

 endure brackish water, but it is stated by both Darwin 

 and Wallace that adults and ova are immediately destroyed 

 by salt water. This is of course fatal to the theory that 

 floating timber and brushwood can convey fresh-water 

 Mollusca in safety from shore to shore. Ice on the other 

 hand may well be carrier; Anodonta and Succinea and 

 Paludina have been frozen and have survived after libe- 

 ration. With regard to land Molluscs their tenacity of 

 life has become almost proverbial through the famous case 

 of the Helix desertorum in the British Museum. More 

 important is the record that out of 100 land shells im- 

 mersed in the sea in a box pierced with holes for a 

 fortnight 27 recovered; but of these 11 (out of a total of 

 12) were the operculate Cyclostoma. Helix however may 

 sometimes survive a similar treatment. 



In this way it is possible, or, according to Prof. 

 Semper 1 , likely that ocean currents propelling tree trunks 

 or even " floating islands " such as are known in tropical 

 seas may have effected the dispersal of land shells. But 

 in such cases it must be often necessary that the Molluscs 

 in question had secreted themselves in the crevices of the 

 bark of trees, a habit which has been apparently assigned 



1 Animal Life. Int. Sci. Series. 



