Darwin, Wallace and the other 19th century naturalists who traveled widely were fascinated by the distribution of animals and plants in their habitats around the world. Why do the Galapagos Islands of South America and the Cape Verde Islands off Africa have strikingly different fauna and flora, despite having similar environments? Why does the Arctic have polar bears and Antarctica penguins?
These patterns impressed Darwin deeply. To him, they argued that species arose in single centers by descent with modification from existing species, and that their geographic range was limited by their ability to migrate to other suitable environments.
The distribution of flora and fauna of the oceanic islands provided Darwin with some of his strongest arguments. The islands contain a small number of species because immigration from the mainland was difficult, he said. Some categories of life are absent altogether, such as batrachians -- frogs, toads, and newts -- even though they would seem to be adapted for such habitats. The reason? They are killed by saltwater, so could not reach the islands by migration. Terrestrial mammals aren't found on oceanic islands more than 300 miles from the mainland. But bats, with their long-distance flying ability, are plentiful.
Another point: Most of the species on islands, while distinct from other species, are most closely related to species on the nearest mainland. Therefore, Darwin said, the island inhabitants must have migrated from the original, mainland area where the species originated. That explains why the species on the Galapagos Islands most closely resemble those on the nearby South American mainland, and those in the Cape Verdes resemble those of west Africa.
Aside from the islands, Darwin was intrigued by unusual distributions of animals and plants across the continents. He concluded that changes in locations of climatic zones over time -- the advance and retreat of glaciers, for example -- could explain some of the patterns in animals' habitats.
Just as intriguing to Darwin, and even more apparent now, is the fact that fossils of possible ancestors of living species are often found in the same parts of the globe where their descendants live today. Darwin observed this in the South American fossils he collected, relatives of today's capybaras and armadillos. Apes today live only in Africa and Asia, and that is where the fossils most resembling modern apes are also found. There are no apes, fossil or living, known from anywhere in the Americas.
These same patterns are just as impressive today. And since Darwin's day, advances in scientific understanding have shown how accurate his conclusions were. For example, plate tectonics, undreamed of when Darwin was forming his ideas, fits elegantly into Darwin's theory as another major influence on dispersal, helping to produce the patterns in the distribution of both fossils and living organisms seen around the world in modern times.