RECAPITU LATION. 463 



tions, 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 spe- 

 cies, 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 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 varieties belong- 

 ing 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 rela- 

 tion 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 those of the 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. 



The fact, as we have seen, that all past and present 

 organic beings can be arranged within a few great classes, 

 in groups subordinate to groups, and with the extinct groups 

 often falling in between the recent groups, is intelligible 

 on the theory of natural selection with its contingencies of 

 extinction and divergence of character. On these same 

 principles we see how it is that the mutual affinities of 

 the forms within each class are so complex and circuitous. 

 We see why certain characters are far more serviceable than 

 others for classification, j why adaptive characters, though 



