CHAP. XIII.] MAMMALS ON OCEANIC ISLANDS. 331 



of the sea separating two mammalian faunas, and the degree of 

 their affinity, a relation which is quite inexplicable on the theory 

 of independent acts of creation. 



The foregoing statements in regard to the inhabitants of oceanic 

 islands, namely, the fewness of the species, with a large propor- 

 tion consisting of endemic forms the members of certain groups, 

 but not those of other groups in the same class, having been 

 modified the absence of certain whole orders, as of batrachians 

 and of terrestrial mammals, notwithstanding the presence of aerial 

 bats, the singular proportions of certain orders of plants, her- 

 baceous forms having been developed into trees, &c., seem to me 

 to accord better with the belief in the efficiency of occasional 

 means of transport, carried on during a long course of time, than 

 with the belief in the former connection of all oceanic islands 

 with the nearest continent ; for on this latter view it is probable 

 that the various classes would have immigrated more uniformly, 

 and from the species having entered in a body their mutual rela- 

 tions would not have been much disturbed, and consequently they 

 would either have not been modified, or all the species in a more 

 equable manner. 



I do not deny that there are many and serious difficulties in 

 understanding how many of the inhabitants of the more remote 

 islands, whether still retaining the same specific form or subse- 

 quently modified, have reached their present homes. But the 

 probability of other islands having once existed as halting-places, 

 of which not a wreck now remains, must not be overlooked. I 

 will 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, but 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 

 tarried by a marine current of average swiftness, to a distance of 

 660 geographical miles. As this Helix has a thick calcareous 



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