Darwin's third line of evidence came from biogeography, the study of the geographic distribution of plants and animals. I have already mentioned what Darwin called his "Law of Succession": living organisms in an area most closely resemble fossils found in the same location. This implies that the former evolved from the latter. But Darwin found his strongest evidence on "oceanic islands"--those islands, such as Hawaii and the Galápagos, that were never connected to continents but arose, bereft of life, from beneath the sea.
What struck Darwin about oceanic islands--as opposed to continents or "continental islands" such as Great Britain, which were once connected to continents--was the bizarre nature of their flora and fauna. Oceanic islands are simply missing or impoverished in many types of animals. Hawaii has no native mammals, reptiles, or amphibians. These animals, as well as freshwater fish, are also missing on St. Helena, a remote oceanic island in the middle of the South Atlantic Ocean. It seems that the intelligent designer forgot to stock oceanic (but not continental!) islands with a sufficient variety of animals. One might respond that this was a strategy of the creator, as those organisms might not survive on islands. But this objection fails, because such animals often do spectacularly well when introduced by humans. Hawaii has been overrun by the introduced cane toad and mongoose, to the detriment of its native fauna.
Strikingly, the native groups that are present on these islands--mainly plants, insects, and birds--are present in profusion, consisting of clusters of numerous similar species. The Galápagos archipelago harbors twenty-three species of land birds, of which fourteen species are finches. Nowhere else in the world will you find an area in which two-thirds of the birds are finches. Hawaii has similar "radiations" of fruit flies and silversword plants, while St. Helena is overloaded with ferns and weevils. Compared with continents or continental islands, then, oceanic islands have unbalanced flora and fauna, lacking many familiar groups but having an over-representation of some species.
Moreover, the animals and the plants inhabiting oceanic islands bear the greatest similarity to species found on the nearest mainland. As Darwin noted when describing the species of the Galápagos, this similarity occurs despite a great difference in habitat, a fact militating against creationism:
Why should the species which are supposed to have been created in the Galápagos Archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which resembles closely the conditions of the South American coast: in fact there is a considerable dissimilarity in all these respects.
As the final peg in Darwin's biogeographic argument, he noted that the kinds of organisms commonly found on oceanic islands--birds, plants, and insects--are those that can easily get there. Insects and birds can fly to islands (or be blown there by winds), and the seeds of plants can be transported by winds or ocean currents, or in the stomachs of birds. Hawaii may have no native terrestrial mammals, but the islands do harbor one native aquatic mammal, the monk seal, and one native flying mammal, the hoary bat. In a direct challenge to creationists (and now also to advocates of ID), Darwin posed this rhetorical question:
Though terrestrial mammals do not occur on oceanic islands, aerial mammals do occur on almost every island. New Zealand possesses two bats found nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands?
The answer is that the creative force did not produce bats, or any other creatures, on oceanic islands. All of Darwin's observations about island biogeography point to one explanation: species on islands descend from individuals who successfully colonized from the mainland and subsequently evolved into new species. Only the theory of evolution explains the paucity of mammals, birds, reptiles, amphibians, and freshwater fish on oceanic islands (they cannot get there), the radiation of some groups into many species (the few species that make it to islands find empty niches and speciate profusely), and the resemblance of island species to those from the nearest mainland (an island colonist is most likely to have come from the closest source).