186 ABSENCE OF TERRESTRIAL [Cuap. XIII. 



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, — herbaceous 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 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 

 difficulties in understanding how many of the in- 

 habitants of the more remote islands, whether still 

 retaining the same specific form or subsequently modi- 

 fied, have reachecj 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 else- 

 where, — 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. 



