CuAP. XII. ON OCEANIC ISLANDS. 301 



presence of aerial bats — the singular proportions of certain 

 orders of plants — herbaceous forms having been develoi)cd into 

 trees, etc., seem to me to accord better with the belief in the 

 cfiiciency of occasional means of transport, carried on during a 

 long course of time, than Avith the belief in the former connec- 

 tion 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 relations would not have been 

 much distiu-bed, and consequently tliey would have been modi- 

 fied citlier not at all or in a more equal manner. 



I do not deny that there arc many and serious difficulties 

 in understanding how many of the inhabitants of the more 

 remote ishxnds, whether still retaining the same specific form 

 or subsequently modified, have reached their present homes. 

 Hut the probability of islands having existed as halting-places, 

 of which not a wreck now remains, must not he overlooked. 

 T will specify one such dillicult case. Almost all oceanic 

 islands, even the most isolated and smallest, are inhabited l)y 

 land-shells, generally by endemic species, but sometimes l)y 

 species found elsewhere — striking instances of which have 

 been given by Dr. A. A. Gould in relation to the Pacific. 

 Kow, 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 l)e, according to our view, some 

 luiknown, but occasionally eflicient, means for their trauspor- 

 tal. ^Vould the just-hatched young sometimes adhere to the 

 fe(>t of l)irds roosting on the ground, and thus get transported? 

 It occurred to me that land-shells, when hj'bcrnating, and hav- 

 ing a nuMnljranous dia]ihragm over the mouth of the shell, 

 might be floated in chinks of drifted tunl)er across moderately 

 wide arms of the sea. And I found that several species in this 

 state withstood 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 da\s, and ])erfectly recovered. During tliis length of 

 time tlie shell might have been carried, by a marine current of 

 average sv.iftness, to a distance of GGO geogra])liical miles. 

 As this Helix has a thick calcareous operculum, I removed it, 

 and, when it had formed a new membranous one, I again im- 

 mersed it for fourteen days in se:irwater, and again it recov- 

 ered and crawled away. Baron Aucapitaine has recently tried 

 similar experiments: he placed 100 land-shells, belonging to 

 IG 



