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Show 478 RECAPITULATION. CHAP. XIV. as the presence of peculiar species of bats, and the absence of all other mammals, on oceanic islands, are 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 parents formerly inhabited both areas ; and we almost invariably find that wherever many closely allied species inhabit two areas, some identical species common to both still exist. Wherever many closely allied yet distinct species occur, many doubtful forms and varieties of the same species 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 nearly all the plants and animals of the Galapagos archipelago, of Juan Fernandez, and of the other American islands being related in the most striking manner to the plants and animals of the neighbouring American mainland ; and those of the Cape de Verde archipelago and 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 constitute one grand natural system, with group subordinate to group, and with extinct groups often falling in between 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 species and genera within each class are so complex and circuitous. We see why certain characters are far more serviceable than others for classification ; -why adaptive characters, though of paramount importance to the being, are of hardly any CHAP. XIV. RECAPITULATION. 479 importance in classification . why h t d . f · ' c arac ers en ved ro.m rudimentary par. ts' though of n o serv. i·c e to the beinbg , are often of high classificatory val . d h 1 . 1 h ue , an w y em ryo ogica c aracters are the most valu bl f ll Th 1 ffi .. f a eoa. . e rea a mties o . all organic beings are due t o I· nh e-n. tance or community of descent. The natural t 1 . 1 sys em IS a genea ogiCa arrangement, in which we h . t d. h 1. ave o Iscover t e Ines of de.scent by the most permanent characters, however shght the~r vital importance may be. The framework of bones being the same in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse,-the same number of vertebroo forming the neck of the giraffe and of the elephant,-and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. The similarity of pattern in the wing and leg ?f a bat, though used for such different purpose,-in the Jaws and legs of a crab,-in the petals, stamens, and pistils of a flower, is likewise intelligible on the view of the gradual modification of parts or organs, which were alike in the early progenitor of each class. On the principle of successive variations not always supervening at an early age, and being inherited at a corresponding not early period of life, we can clearly see why the embryos of mammals, birds, reptiles, and fishes should be so closely alike, and should be so unlike the adult forms. We may cease marvelling at the embryo of an airbreathing mammal or bird having branchial slits and arteries running in loops, like those in a fish which has to breathe the air dissolved in water, by the aid of welldeveloped branchioo. Disuse, aided sometimes by natural selection, will often tend to reduce an organ, when it has become use~ess by changed habits or under changed conditions |