A few days later Darwin made his last major inland journey, traveling again into the Andes. At the summits of the mountains that surrounded the Uspallata Pass, Darwin could recognize the same layers of rocks that he had seen months before in the low, flat plains to the east—rocks that had originally been formed by ocean sediments. He found a forest of petrified wood at the pass still standing upright, much like fossils he had found in Patagonia.
“These trees,” he wrote to his sister Susan, “are covered by other sandstones and streams of lava to the thickness of several thousand feet. These rocks have been deposited beneath water; yet it is clear the spot where the trees grew must once have been above the level of the sea, so that it is certain the land must have been depressed by at least as many thousand feet as the superincumbent subaqueous deposits are thick.”
With earthquakes and volcanoes rumbling through his memory, Darwin concluded that the Andes were a recent creation. Once these 14,000-foot peaks had been as flat as the pampas to the east. The giant mammals whose fossils he had found there had wandered these places as well. And then this land had sunk underwater and then risen up again, as pressure from below had jacked it up. Darwin realized that mountains might be younger than mammals, and that they might still be rising under his feet.
With the coastal survey finished, the Beagle sailed north to Lima and then westward, leaving South America altogether. After the blasting winds of Tierra del Fuego and the hollow cold of the Andes, Darwin was looking forward to the tropics. The first stop would be at a peculiar cluster of islands called the Galápagos.
The Galápagos Islands have a reputation as the place where Darwin’s theory of evolution was born, but Darwin would not realize their significance until nearly two years after his visit. When he arrived at the Galápagos Islands, he was still thinking more about geology than biology, and he looked forward to visiting a place where he could see new land being created as Lyell had proposed.
Darwin first set foot on Chatham Island (now known as San Cristobál), a raw volcanic heap still untamed by plants and soil. Ugly iguanas and countless crabs greeted him. “The natural history of this archipelago is very remarkable,” Darwin later wrote. “It seems to be a little world within itself.” A world, he meant, unlike the greater one. Here there were enormous tortoises, with shells 7 feet in diameter, that fed on prickly pears and didn’t care if Darwin rode on their backs. Here lived not one but two hideous species of iguana—one that stayed on the islands and another that dove into the ocean to eat seaweed. The birds of the Galápagos were so tranquil that Darwin could walk up to them without their taking flight.
Darwin dutifully added the birds to his collection, making only a few notes about them. Some had big beaks, good for crushing large seeds, while others had beaks shaped like needle-nose pliers, for grabbing seeds that were small and hard to reach. Judging from their beaks, Darwin guessed that some were wrens, others finches, warblers, and blackbirds. He did not, however, bother to note which islands the birds came from. He assumed that the birds were South American species that had colonized the islands at some point.
Only after Darwin had finished collecting his animals did he realize that he should have been more careful. Shortly before the Beagle left the islands, he met the director of the penal colony on Charles Island (Santa María), an Englishman named Nicholas Lawson. Lawson used tortoise shells for flower pots in his garden, and he had noticed that the tortoises of each island were so distinct from one another that he could tell where they came from by the shape of the flares and flanges on their shells. The tortoises on each island, in other words, were a unique variety, or perhaps even a unique species. The plants of the islands, Darwin learned, were likewise distinct.
Perhaps the birds were as well, but since he hadn’t marked where most of his birds had come from, Darwin couldn’t know. It would not be until he returned to England that Darwin would finally sort out his birds, and only then would he begin to work out the way in which life changes from one form to another.
When the Beagle was finished at the Galápagos, it set sail across the glassy Pacific. It traveled quickly, reaching Tahiti in three weeks, New Zealand in another four, and Australia in only two. As it crossed the Indian Ocean, the Beagle’s objective was to map coral reefs. Coral reefs are living geography, produced by colonies of tiny polyps as they build external skeletons. The polyps can live only near the surface of the ocean because, as marine biologists would later discover, they depend on photosynthetic algae that live inside their tissues. As the Beagle passed reefs in the Indian Ocean, Darwin wondered how they formed such perfect circles, sometimes around islands and sometimes around nothing but water. And how was it that the reefs could always be found close to the surface of the water, which was exactly where they needed to be in order to get enough sunlight to grow?
In Principles of Geology Darwin had read Lyell’s hypothesis on corals: they formed only on the tops of submerged volcanic craters. For once, Darwin thought that Lyell was wrong. The crater hypothesis was ugly and awkward, since it would require that every reef sit precisely on a crater that just happened to be lurking close to the surface of the ocean. Darwin came up with another explanation.
If the Andes were rising, according to Lyell’s geology, Darwin reasoned that some other part of the planet must be sinking. That could well be happening in places such as the Indian Ocean. Corals might form in the shallow waters around new islands or along the coasts of mainlands, which then began to sink. As the land began to subside underwater, the corals would sink with them. But they might not be lost, Darwin reasoned, because new corals could grow on top of the reefs as the ground sank. While the old corals died in the darkness, the reef would survive. After a while the island itself might completely erode away, but the reef would have maintained itself near the water’s surface.
Every coral reef that the Beagle surveyed fit somewhere along this sequence. At Keeling Islands (Cocos Islands), the surveyors on the Beagle found that on the seaward edge of the reef there was a sharp drop to the ocean floor. When they scraped off corals from near the bottom, they found that they were dead. It was all as Darwin had predicted.
Darwin was no longer a mere disciple of Lyell, but a mature, independent thinker. He had used Lyell’s principles to formulate a better explanation for coral reefs than Lyell’s own, and he had worked out how he could test it. Darwin was learning how to study history—in this case, the history of life on Earth—scientifically. He could not replay thousands of years and watch coral reefs grow, but if history had unfolded the way he proposed, he could test his predictions. “We get at one glance an insight into the system by which the surface of the land has been broken up, in a manner somewhat similar but certainly far less perfect, to what a geologist would have done who had lived his 10,000 years, and kept a record of the passing changes,” he later wrote.
The planet might look changeless, but Darwin was learning to see it on a scale of millions of years. And from that perspective it was a quivering ball, rising and collapsing, its skin tearing itself apart. Darwin was also learning how life could change on the same scale as well. Given enough time, coral reefs could keep from drowning as the ocean floor fell out from under them. They could build giant fortresses, founded on the skeletons of their ancestors.