tion of organism to organism.

I do not deny that there are many and grave difficulties 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 overlooked. 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 transportal. 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.
{398}

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
almost every product of the land and water bears the unmistakeable stamp of
the American continent. There are twenty-six land-birds, and twenty-five of
these are ranked by Mr. Gould as distinct species, supposed to have been
created here; y