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What intrigued Darwin most about the two rhea species was that, similar as they were, they overlapped very little in geographic distribution. The greater rhea inhabited the Pampas and northern Patagonia, as far south as Argentina's Río Negro, which drained to the coast at about 41° south latitude; the lesser rhea replaced it beyond the Río Negro and occupied southern Patagonia. Together with the evidence of extinct South American mammals, the implications of rhea diversity and distribution would prove almost as suggestive to Darwin as the patterns he would later find among the finches and mockingbirds of the Galápagos.

How do species originate, and how do they come to be where they are? The orthodox story, still firmly embraced by European science at the time of the Beagle voyage, was that God had created species independently, in sequential batches (to compensate for extinctions), and had chosen to place them, almost arbitrarily, in their particular locales—kangaroos in Australia, giraffes and zebras in Africa, rheas and sloths and armadillos in South America, extinct and living forms clustered closely in space and time. But to Darwin, both the extinct mammals (along with their living counterparts among sloths and armadillos) and the two rheas (occupying adjacent regions of habitat) suggested something more rational: the ideas of relatedness and succession among closely allied species. The living tree sloths and armadillos seemed to have succeeded earlier such forms in time, inhabiting roughly the same terrain during different epochs of Earth's history. (Those earlier forms of sloth were true sloths; the earlier armored creatures are now known as glyptodonts, a family distinct from but closely related to living armadillos.) The two rheas, similar but not identical, likewise seemed to succeed each other—but in space, across the horizontal dimension of landscape. The clustering in time and in space thus hinted that each group had descended, with modification, from common ancestors: rheas from rheas, sloths from earlier sloths, armadillos from an armadilloish or glyptodontish precursor, possibly far larger than armadillos living today. That's the explanation to which Darwin felt drawn, because it seemed more economical, more inductive, and more persuasive than the creationist scenario.

How important were the South American data in shaking his faith in the orthodox view—persuading him that evolution was a reality for which he should seek a material explanation? Darwin himself would give several answers to that question over the length of his lifetime. His answers ranged, in essence, from very important, but less so than the Galápagos birds, to crucially important, period.

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