120 THE DISPERSAL OF SHELLS. 



in the course of many thousands of years. I have 

 collected a few notes on various suggested means of 

 distribution, but, as Darwin and A. de Candolle 

 agreed,' the putting together of merely possible means 

 is poor work ; nothing more can be done at present, 

 however, and we are not likely to obtain much in the 

 way of direct evidence until some traveller-naturalist 

 gives his whole attention to the subject, and carries on 

 a series of careful investigations. 



Ocean Currents. 



Professor Semper regarded the currents of the ocean 

 as beyond doubt the most important means of trans- 

 portal for land mollusca ; indeed, according to this 

 author, it is " more than probable that land-snails can 

 travel only in this way, and in no other, from one 

 island to another." ^ But it will be remembered that 

 Mr. Darwin concluded that the creatures are not likely 

 to be often thus transported, for, as stated in the 

 " Origin," they are known to be easily killed by sea- 

 water, and their eggs, at least such as were tried, sank 

 in it and were killed ; it was found by experiment, 

 however, that when hibernating and having a membran- 

 ous epiphragm over the mouth of the shell, several 

 species were able to withstand immersion for seven 

 days : 



"One shell, the Helix pomatia, after having been thus 



^ See '' Life and Letters," ii. (1888), p. 82. 



- K. Semper, "Animal Life," ed. 4, (1890), pp. 282, 296, 314. 



