Chap. XII. OCEANIC ISLANDS. 397 



looked. I will here give a single instance of one of 

 the cases of difficulty. 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. Dr. Aug. A. Gould has 

 given several interesting cases in regard to the land- 

 shells of the islands of the Pacific. Now it is notorious 

 that land-shells are very easily killed by salt; their 

 eggs, at least such as I have tried, sink in sea-water 

 and are killed by it. Yet there must be, on my view, 

 some unknown, but highly efficient means for their trans- 

 portal. Would the just-hatched young occasionally 

 crawl on and 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 found that 

 several species did in this state withstand uninjured an 

 immersion in sea-water during seven days : one of these 

 shells was the Helix pomatia, and after it had again 

 hybernated I put it in sea-water for twenty days, 

 and it perfectly recovered. As this species has a 

 thick calcareous operculum, I removed it, and when it 

 had formed a new membranous one, I immersed it for 

 fourteen days in sea-water, and it recovered and 

 crawled away : but more experiments are wanted on 

 this head. 



The most striking and important fact for us in regard 

 to the inhabitants of islands, is their affinity to those of 

 the nearest mainland, without being actually the same 

 species. Numerous instances could be given of this 

 fact. I will give only one, that of the Galapagos 

 Archipelago, situated under the equator, between 500 

 and 600 miles from the shores of South America. Here 



