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GEOGRAPHICAL DISTRIBUTION 361 



I do not deny that there are many and serious difficulties in 

 understanding how many of the inhabitants of the more remote 

 islands, whether still retaining the same specific form or subse- 

 quently modified, have reached their present homes. But the 

 probability of other islands having once existed as halting-places, 

 of which not a wreck now remains, must not be overlooked. I will 

 specify one difficult case. Almost all oceanic islands, even the 

 most isolated and smallest, are inhabited by land-shells, generally 

 by endemic species, but sometimes by species found elsewhere, 

 striking instances of which have been given by Dr. A. A. Gould in 

 relation to the Pacific. Now it is notorious that land shells are 

 easily killed by sea- water; their eggs, at least such as I have 

 tried, sink in it and are killed. Yet there must be some unknown, 

 but occasionally efficient, means for their transportal. Would the 

 just-hatched young sometimes adhere to the feet of birds roost- 

 ing on the ground and thus get transported? It occurred to me 

 that land-shells, when hibernating and having a membraneous 

 diaphragm over the mouth of the shell, might be floated in 

 chinks of drifted timber across moderately wide arms of the sea. 

 And I find that several species in this state withstand uninjured 

 an immersion in sea-water during seven days. One shell, the Helix 

 pomatia, after having been thus treated, and again hibernating, 

 was put into sea-water for twenty days and perfectly recovered. 

 During this length of time the shell might have been carried by 

 a marine current of average swiftness to a distance of 660 geo- 

 graphical miles. As this Helix has a thick calcareous operculum I 

 removed it, and when it had formed a new membraneous one, I 

 again immersed it for fourteen days in sea-water, and again it 

 recovered and crawled away. Baron Aucapitaine has since tried 

 similar experiments. He placed 100 land-shells, belonging to ten 

 species, in a box pierced with holes, and im.mersed it for a fort- 

 night in the sea. Out of the hundred shells twenty-seven recovered. 

 The presence of an operculum seems to have been of importance, 

 as out of twelve specimens of Cyclostoma elegans, which is thus 

 furnished, eleven revived. It is remarkable, seeing how well the 

 Helix pomatia resisted with me the salt water, that not one of 

 fifty-four specimens belonging to four other species of Helix tried 

 by Aucapitaine recovered. It is, however, not at all probable that 

 land-shells have often been thus transported; the feet of birds 

 offer a more probable method. 



