CHAP. XIII. 



on Oceanic Islands. 



353 



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, 

 hut occasionally efficient means for their transportal. Would the 

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

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

 land-shells, when hybernating and having a membranous 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 hybernating 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 geographical miles. As 

 this Helix has a thick calcareous operculum, I removed it, and 

 when it had formed a new membranous 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 immersed it for a fortnight in the sea. Out of the 

 hundred shells, twenty-seven recovered. The presence of an oper- 

 culum 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. 



On the Belations of the Inhabitants of Islands to those of the 

 nearest Mainland. 

 The most striking and important fact for us is the affinity of the 

 species which inhabit islands to those of the nearest mainland, 

 without being actually the same. Numerous instances could be 

 given. The Galapagos Archipelago, situated under the equator, 

 lies at the distance of between 500 and 600 miles from the shores 

 of South America. Here almost every product of the land and of 

 the water bears the unmistakeable stamp of the American continent. 



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