Chap. XV.] EECAPITULATION. 289 



namely, that on the same continent, under the most 

 diverse conditions, under heat and cold, on mountain 

 and lowland, on deserts and marshes, most of the inhabit- 

 ants within each great class are plainly related ; for they 

 are the descendants of the same progenitors and early 

 colonist^ On this same principle of former migration, 

 combined in most cases with modification, we can under- 

 stand, by the aid of the Glacial period, the identity of 

 some few plants, and the close alliance of many others, 

 on the most distant mountains, and in the northern and 

 southern temperate zones ; and likewise the close alliance 

 of some of the inhabitants of the sea in the northern and 

 southern temperate latitudes, though separated by the 

 whole intertropical ocean. Although two countries may 

 present physical conditions as closely similar as the 

 same species ever require, we need feel no surprise at 

 their inhabitants being widely different, if they have 

 been for a long period completely sundered from each 

 other ; for as the relation of organism to organism is the 

 most important of all relations, and as the two countries 

 will have received colonists at various periods and in 

 different proportions, from some other country or from 

 each other, the course of modification in the two areas 

 will inevitably have been different. 



On this view of migration, with subsequent modifica- 

 tion, we see why oceanic islands are inhabited by only 

 few species, but of these, why many are peculiar or 

 endemic forms. We clearly see why species belonging 

 to those groups of animals which cannot cross wide 

 spaces of the ocean, as frogs and terrestrial mammals, do 

 not inhabit oceanic islands ; and why, on the other hand, 

 new and peculiar species of bats, animals which can 

 traverse the ocean, are often found on islands far distant 



