You could see the history of this evolution, Geoffroy claimed, if you looked at how embryos develop today. German scientists were discovering how embryos changed in a matter of days from one bizarre form to another, often bearing no resemblance to the adult forms of the animals. The researchers carefully catalogued their fleeting parts and shapes, and the longer they looked, the more order they claimed to see in the confusion. They were impressed in particular by the way an embryo began as a simple form and gradually grew more complex. They even claimed that each increasingly complex form added a new stage to its development.
Lorenz Oken, one of the German scientists, explained the process this way: “During its development, the animal passes through all stages of the animal kingdom, rising up as it takes on new organs. The foetus is a representation of all animal classes in time.” At first it was just a tube, like a worm. Then it developed a liver and a vascular system and became a mollusk. With a heart, and a penis, it became a snail. When it sprouted limbs, it became an insect. When it developed bones, it became a fish; muscles, a reptile; and so on, up to mankind. “Man is the summit, the crown of nature’s development,” Oken announced.
Geoffroy proposed that embryos didn’t just climb the scale of nature; they replayed history. The ancestors of humans really were fish, and the evidence that we sport gill slits at an early stage in our development proved it.
As Geoffroy argued for evolution, European explorers were discovering new species that he claimed fit in perfectly with his theory. The platypus of Australia, for example, was a mammal, but it had a ducklike bill and laid eggs, inspiring Geoffroy to call it a transitional form between mammals and reptiles. In Brazil explorers found lungfish that could breath air through lungs, representing a link between vertebrates in the ocean and on land.
In England, the leading scientists denounced Geoffroy just as they had denounced Lamarck. Adam Sedgwick, the devout Cambridge geologist, declared the work of the two Frenchmen to be “gross (and dare I say it, filthy) views of physiology.” But while British scientists generally abhorred evolution, the job of attacking it head-on in the 1830s was left to one man: a brilliant young anatomist named Richard Owen.
Owen was often the first British anatomist to study new species such as the lungfish and the platypus, and he took these opportunities to strike down Geoffroy’s claims. Owen showed that platypuses actually secrete milk, a hallmark of mammals. And lungfish, while they might have lungs, didn’t appear to have nostrils, which all land vertebrates have. That, for Owen, was enough to relegate them to being ordinary fish.
Yet Owen himself was not content simply to say that God created life, and that its design reflected His goodness. Owen wanted to uncover the natural mechanisms of creation. He abhorred Geoffroy’s wild speculations on evolution, but he was too good a naturalist to deny that he was right about some things. The similarities between species, and the ways in which they could be arranged into a series of transformations, were too obvious to be denied.
Owen decided that Geoffroy had just lurched too far in his interpretation of the evidence. Owen knew, for example, that Geoffroy’s notions about how embryos formed had been supplanted by new research. A Prussian scientist named Karl von Baer had shown that life was not a simple ladder, with more advanced embryos recapitulating the development of more primitive ones. In the earliest stages of embryos, vertebrates did look like one another, but only because they were just a handful of cells. As time passed they grew more distinctive. Fishes, birds, reptiles, and mammals all have limbs, and they all initially form limb buds as embryos. But in time, those buds turn into fins, hands, hooves, wings, and the other kinds of limbs unique to certain kinds of vertebrates. One kind does not form from another. “A linear arrangement of animals in order of perfection,” von Baer wrote, “is impossible.”
Owen’s ambition was to tie together the work of von Baer, Geoffroy, and all the other great biologists of his day into one grand theory of life. He wanted to fight against evolution, but he wanted to do so by finding laws of nature that could account for the evidence of fossils and embryos.
He met Darwin for the first time three weeks after the Beagle’s return. They both came to dinner at Lyell’s house, where Darwin regaled the party with his stories about the earthquake in Chile. After dinner, Lyell introduced the two young men (Owen was only five years older than Darwin). They got along well, and Darwin recognized in Owen someone famous enough to bring his fossil mammals to national attention. He asked Owen that night if he would examine them. Owen said that he would be happy to do so. They would give him a chance to test his ideas against fossils that no one had seen before.
He had no way of knowing that Darwin would one day render him a fossil as well.
Four months after the Beagle’s return, Darwin began to hear back from his experts about his collection of fossils and carcasses. At first they did nothing but confuse him. Owen had inspected the fossil mammals and announced that they were gigantic variations on the animals that still lived in South America. The rodents were the size of hippos, the anteaters the size of horses. Why, Darwin wondered, was there a continuity between extinct animals and the ones alive on the same spot on Earth? Could the living animals have descended, in a modified form, from the fossil ones?
Darwin had given his Galápagos birds to lames Gould, one of Britain’s leading ornithologists. He hadn’t thought much of them when he had collected them, and when he heard Gould speak about them at a meeting of the Zoological Society, he regretted his carelessness. Judging from their beaks, Darwin had identified many of his birds as finches, wrens, and blackbirds. But Gould announced that they were all finches. They simply had wrenlike or blackbird-like beaks, which allowed them to eat particular kinds of foods.
And later, when Darwin paid a visit to Gould’s office, Gould showed that he had made an even more grievous mistake. Darwin hadn’t noted exactly which island he had gotten most of his birds from, because it seemed unimportant at the time. He had happened to note that three mockingbirds had come from three different islands, and Gould showed him that the mockingbirds belonged to three new—and separate—species.
Darwin wondered why there would be three different species of mockingbirds so close to one another. And were separate species of finches living on separate islands as well? Darwin contacted FitzRoy and asked for some of the birds collected by the Beagle crew to be sent to Gould. Fortunately, the crewmen had done a better job than Darwin, jotting down which islands they had shot their birds on. And just like the mockingbirds, the finches of different islands belonged to separate species.
Something, Darwin realized, was very wrong. Why should there be so many unique species on these similar islands? He opened his notebooks and tried to figure out an explanation for the finches of the Galápagos. To the people around him, he seemed unchanged as he went on with his geological work, writing about coral reefs and rising plains and the shape of volcano cones. But in private he was obsessed with an extraordinary thought. Perhaps the finches had not been created in their current form. Perhaps they had evolved.