GEOGRAPHICAL DISTRIBUTION OF ANIMALS
birds, which cross great stretches of the ocean with ease. Though the land birds belong to peculiar species, and most of them to peculiar genera, they are nevertheless of unmistakably South American types and belong to South American families. Large lizards, one a land species and the other marine, as well as the huge land tortoises that have given their name to the islands, are abundant. When the islands were first discovered there were fifteen species of these monstrous tortoises, each island and islet having its own species, but many of these have been extirpated. The species of land birds, like those of the tortoises, are peculiar to a single island each; the genera are mostly common to the group, as the islands are so far apart that communication between them is difficult.
On the theory of evolution these remarkable facts are easily explained. The ancestors of the existing birds and reptiles came, rarely and at long intervals, from the mainland of South America, and after settling in the islands became slowly modified, so that they were placed in genera nearly allied to those of the continent, yet different from them; and, in the isolation of the individual islands the species were free to develop into new forms peculiar to each.
In the Galapagos we witness the results of what may be called a great evolutionary experiment, under conditions unaffected by human interference, and such conditions are rare; but the Cape Verde islands, in the Atlantic, display very similar relations in their animals and plants. The species there are peculiar, but their affinity is as unmistakably African as that of the Galapagos species is South American.
Bermuda and Madeira are almost as far from the North American and African coasts, respectively, as the Galapagos Islands are from Ecuador, but they have hardly any peculiar
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