GEOGRAPHICAL DISTRIBUTION 195 



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— herbaceous forms having been developed into 

 trees, etc. — 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. 



1 do not deny that there are many and serious dif- 

 ficulties 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 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 



