All the foregoing remarks on the inhabitants of oceanic islands,—­namely, the scarcity of kinds—­the richness in endemic forms in particular classes or sections of classes,—­the absence of whole groups, as of batrachians, and of terrestrial mammals notwithstanding the presence of aerial bats,—­the singular proportions of certain orders of plants,—­herbaceous forms having been developed into trees, etc.,—­seem to me to accord better with the view of occasional means of transport having been largely efficient in the long course of time, than with the view of all our oceanic islands having been formerly connected by continuous land with the nearest continent; for on this latter view the migration would probably have been more complete; and if modification be admitted, all the forms of life would have been more equally modified, in accordance with the paramount importance of the relation 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.  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,