396 GEOGEAPHICAL DISTRIBUTION. Chap. XII. 



a certain degree on the lapse of time, and as during 

 changes of level it is obvious that islands separated by 

 shallow channels are more likely to have been con- 

 tinuously united within a recent period to the main- 

 land than islands separated by deeper channels, we can 

 understand the frequent relation between the depth of 

 the sea and the degree of affinity of the mammalian 

 inhabitants of islands with those of a neighbouring con- 

 tinent, — an inexplicable relation on the view of inde- 

 pendent acts of creation. 



All the foregoing remarks on the inhabitants of 

 oceanic islands, — namely, the scarcity of kinds — the 

 richness in endemic forms in particular classes or sec- 

 tions of classes, — the absence of whole groups, 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 view of occasional means of transport having 

 been largely efficient in the long course of time, than 

 with the view of all our oceanic islands having been 

 formerly connected by continuous land with the nearest 

 continent ; for on this latter view the migration would 

 probably have been more complete ; and if modifi- 

 cation be admitted, all the forms of life would have 

 been more equally modified, in accordance with the 

 paramount importance of the relation of organism to 

 organism. 



I do not deny that there are many and grave diffi- 

 culties in understanding how several of the inhabitants 

 of the more remote islands, whether still retaining the 

 same specific form or modified since their arrival, could 

 have reached their present homes. But the probability 

 of many islands having existed as halting-places, of 

 which not a wreck now remains, must not be over- 



