40 THE DISPERSAL OF SHELLS. 



point are probably required, but I find it stated both by 

 Darwin and Wallace that salt water is immediately 

 or almost immediately fatal both to ova and adults.^ 

 It seems improbable, therefore, that the creatures are 

 transported for any considerable distance over the 

 open sea in the way indicated. Molluscs belonging to 

 fresh-water genera or species, but inhabiting water which 

 is salt or brackish, might almost certainly be thus 

 carried, and in a new home in the course of ages some 

 of their descendants might re-adapt themselves to fresh 

 water. But we cannot argue that, because certain 

 members of a genus live in salt or brackish water, other 

 members from fresh-water habitats are likely to with- 

 stand a voyage over the sea ; nor in the case of fresh- 

 water species having individuals living in the sea or in 

 brackish estuaries, in certain spots, can it be supposed 

 that examples from fresh waters could pass through 

 the sea unharmed. Some of the fresh-water species of 

 Neritina, as Dr. H. B. Guppy remarks in his " Solomon 

 Islands," have been widely dispersed, Neritina stibsul- 

 cata and N. cornea occurring both in the Solomon 

 Islands and in the Philippines, A^. inacgillivrayi and 

 N. petiti in the Fiji and Solomon Islands, and N,porcata 

 in Samoa and Fiji, as well as in the opposite extremi- 

 ties of the Solomon group ; yet it is doubtful whether 

 the animals have been drifted far over the sea, for 

 although possessing a stony and close-fitting operculum 

 they are probably unable to resist the action of salt 



^ "Origin," p. 344; Wallace, "Geographical Distribution," i. 

 p. 31. 



