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.E
.e living agoutis, the glyptodonts and the armadillos-why? "I think one of the possible explanations he was mulling over, even as early as 1832, was that one begat the other. Transmutation." But even Brinkman admits that there is only tenuous evidence, "no smoking gun," for his hypothesis about Darwin having converted to evolutionism long before ever striding ashore in the Galápagos.
He hinted at the subject in 1845, in the second edition of his Beagle narrative, revised by him to include coy hints about the theory he was still unprepared to publish. The relationships between fossil and living forms among the rodents, the sloths, the camels, and the armadillos were "most interesting facts," he noted. Further work by other investigators had meantime revealed the same kind of pattern in Brazil-fossil and living forms of anteater, of tapir, of monkey and peccary and possum. "This wonderful relationship in the same continent between the dead and the living," Darwin wrote, would "throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts." But what sort of light? What would that light reveal? Throwing light was one of his favorite metaphors, and it would return, but not for a decade and a half-not until he was ready to shine the blinding beam of his theory in public.
There's another intriguing question about the South American fossils and rheas: When did this evidence register on Darwin, tipping him toward the idea of evolution? The widely accepted view is that he returned from the Beagle voyage not yet an evolutionist, merely puzzled by what he had seen, and that he made the big leap to evolutionary thinking after his consultations in London, with John Gould and Richard Owen, about the bird and fossil specimens he had consigned to them. (Soon after that he began using a new term for the process: "transmutation.") But not everyone agrees.
"I think he was personally converted much earlier," a historian of paleontology named Paul D. Brinkman told me. We were sitting in his office at the North Carolina Museum of Natural Sciences in Raleigh, amid a portrait of young Darwin, a Jurassic Park poster, and photos of old ground sloth and glyptodont specimens. "Why would there be this resemblance between the fossil fauna and the extant fauna of this area? Why would they be so similar?" he asked, repostulating questions that Darwin must have framed. The ancient rodents and thne cryptic piece of testimony came from Darwin himself, near the end of his life, in the private autobiography he wrote for his family. "During the voyage of the Beagle," he reminisced, "I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos." He alluded also to the rheas and to the Galápagos species, differing island by island. "It was evident," Darwin wrote, "that such facts as these, as well as many others, could be explained on the supposition that species gradually become modified; and the subject haunted me." In years since, it has also haunted scholars.
The Beagle, having completed its South American survey work and then spent a year circumnavigating the world, reached England in October 1836. Darwin, then 27 and a seasoned naturalist, weary of travel, eager for home, was a changed man in other ways too. He no longer saw himself serving time in a country parsonage; he was committed to a life of science. And he had at least started to lose his belief in the immutability of species. It's not possible to know with certainty, but he seems by then to have identified the great question, though not yet the great answer, that would dominate the rest of his working life.
With his specimens outsourced for expert identification-the birds to Gould, the fossil mammals to Owen, the reptiles to a zoologist named Thomas Bell-he set about putting his thoughts in order and following out his suspicions. He brainstormed in his most private notebook about ostriches, guanacos, and whether "one species does change into another." If so, how might such transmutation occur?
About a year and a half later, after adding one crucial piece to his thinking (the idea of excess reproduction and struggle for existence, adopted from an essay on human population by Thomas Malthus), Darwin hit upon his theory: natural selection, whereby the best adapted individuals of each population survive to leave offspring and others don't. Then he nurtured, refined, developed, and concealed that theory for 20 years, until a younger man named Alfred Russel Wallace (see "The Man Who Wasn't Darwin" in National Geographic, December 2008) struck upon the same idea, forcing Darwin to rush to get his own ready for print
That was 1858. By then Darwin had begun writing a long, detailed, heavily footnoted treatise on natural selection, but it was only half finished. Panicked, feeling proprietary, yet also reawakened to the wondrous immediacy of the story he had to tell, he shoved the big book aside and quickly composed a more streamlined account. This shorter, slapdash version would be merely an "abstract" of the theory and its supporting data, he claimed. He called it "my abominable volume" because, after decades of cogitation and delay, the writing process was so hurried and painful. He wanted to title it An Abstract of an Essay on the Origin of Species and Varieties Through Natural Selection, but his publisher persuaded him to accept something at least marginally more snappy. It appeared in November 1859, titled On the Origin of Species by Means of Natural Selection et cetera, and was a sellout success immediately.
Five more editions went to print during Darwin's lifetime. Almost inarguably, it's the most significant single scientific book ever published. After 150 years, people still venerate it, people still deplore it, and The Origin of Species continues to exert an extraordinary influence-though, unfortunately, not many people actually read it.
And the forgotten clues that led him to his theory are still largely forgotten. Anyway, they're omitted from the mythic account. Scholars still dispute the significance of those extinct and living Argentine creatures, especially the ground sloths and glyptodonts, the tree sloths and armadillos and rheas. Evidence is mixed, even among the various comments on the matter left behind by Darwin himself. The most telling of those comments, in my view, is one so conspicuously placed that it tends to get overlooked. It comprises the first two sentences of The Origin of Species, beginning the book on a nostalgic note. It says:
"When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species.?…"
The finches of the Galápagos make their appearance about 400 pages later.