Chap. XII. 
OCEANIC ISLANDS. 
397 
equally modified, in accordance with the paramount im¬ 
portance of the relation of organism to organism. 
I do not deny that there are many and grave diffi¬ 
culties in understanding how several of the inhabitants 
of the more remote islands, whether still retaining the 
same specific form or modified since their arrival, could 
have reached their present homes. But the probability 
of many islands having existed as halting-places, of 
which not a wreck now remains, must not be over¬ 
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 four¬ 
teen days in sea-water, and it recovered and crawled 
away : but more experiments are wanted on this head. 
