186 ABSENCE OP TERRESTRIAL [Chap. XIII. 



other groups in the same class, having been modified — 

 the absence of certain whole orders, as of batrachians 

 and of terrestrial mammals, notwithstanding the pres- 

 ence of aerial bats, — the singular proportions of certain 

 orders of plants, — herbaceous forms having been de- 

 veloped 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 hav- 

 ing entered in a body their mutual relations 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 difii- 

 culties in understanding how many of the inhabitants 

 of the more remote islands, whether still retaining the 

 same specific form or subsequently 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 difiicult case. Almost all oceanic 

 islands, even the most isolated and smallest, are in- 

 habited 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 effi- 

 cient means for their transportal. Would the just- 



