428 THE ORIGIN OF SPECIES 



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 modification, that 

 the same parent forms formerly inhabited both areas: and we 

 almost invariably find that wherever many closely allied ^ecies 

 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 like- 

 wise 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 Gala- 

 pagos 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 inteUigible on the theory of 

 natural selection with its contingencies of extinction and diver- 

 gence of character. On these same principles we see how it is that 

 the mutual affinities of the forms within each class are so com- 

 plex and circuitous. We see why certain characters are far more 

 serviceable than others for classification ; why adaptive characters, 

 though of paramount importance to the beings, are of hardly any 

 importance in classification; why characters derived from rudi- 

 mentary parts, though of no service to the beings, are often of 

 high classificatory value; and why embryological characters are 

 often the most valuable of all. The real affinities of all organic 

 beings, in contra-distinction to their adaptive resemblances, are 

 due to inheritance or community of descent. The Natural System 

 is a genealogical arrangement with the acquired grades of differ- 

 ence, marked by the terms, varieties, species, genera, families, 



