516 ORIGIN OF SPECIES 



lowland, on deserts and marshes, most of the inhabitants 

 within each great class are plainly related; for they are the 

 descendants of the same progenitors and early colonists. 

 On this same principle of former migration, combined in 

 most cases with modification, we can understand, by the aid 

 of the Glacial period, the identity of some few plants, and 

 the close alliance of many others, on the most distant moun- 

 tains, 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. Al- 

 though 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 differ- 

 ent proportions, from some other country or from each other, 

 the course of modification in the two areas will inevitablv 

 have been different. 



On this view of migration, with subsequent modification, 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 from any continent. Such cases as the 

 presence of peculiar species of bats on oceanic islands and 

 the absence of all other terrestrial mammals, are facts utterly 

 inexplicable on the theory of independent acts of creation. 



The existence of closely allied or representative species 

 in any two areas, implies, on the theory of descent with modi- 

 fication, that the same parent-forms formerly inhabited both 

 areas: and we almost invariably find that wherever many 

 closely allied species inhabit two areas, some identical 

 species are still common to both. Wherever many closely 

 allied yet distinct species occur, doubtful forms and vari- 



