CHAP. XV.] RECAPITULATION. 393 



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 modi- 

 fication, 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 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 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 modification, 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 varieties belonging to the same groups likewise occur. 

 It is a rule of high generality that the inhabitants of each area are 

 related to the inhabitants of the nearest source whence immigrants 

 might have been derived. We see this in the striking relation of 

 nearly all the plants and animals of the Galapagos archipelago, 

 of Juan Fernandez, and of the other American islands, to the 

 plants and animals of the neighbouring American mainland ; and 

 of those of the Cape de Verde archipelago, and of the other 

 African islands to the African mainland. It must be admitted 

 that these facts receive no explanation on the theory of creation. 



