In 1801, the French naturalist Jean-Baptiste Lamarck proposed that species could change in response to environmental conditions. Giraffes, for instance, had developed their fantastic necks to browse on the upper branches of trees. Lamarck mistakenly thought such traits could be acquired by one generation and passed on to the next. He is ridiculed, to this day, for suggesting that giraffes got their longer necks basically by wanting them (though the word he used, some scholars contend, is more accurately translated as "needing"). But his was the first real theory of evolution. If he had merely suggested that competition for treetop foliage could gradually put short-necked giraffes at a disadvantage, we might now be talking about Lamarckian, rather than Darwinian, evolution.
By the 1840s, evolutionary ideas had broken out of the scientific community and into heated public debate. The sensation of 1845 was the anonymous tract Vestiges of the Natural History of Creation, and it set both Darwin and Wallace on career paths that would converge in that fateful 1858 mail delivery. Vestiges deftly wove evolutionary ideas into a sweeping history of the cosmos, beginning in some primordial "fire-mist." The author, later revealed to be the Edinburgh journalist and publisher Robert Chambers, argued that humans had arisen from monkeys and apes, but he also appealed to ordinary readers with the uplifting message that evolution was about progress and improvement.
Vestiges quickly became a popular hit, a rose-tinted 2001: A Space Odyssey of its day. Prince Albert read it aloud to Queen Victoria at Buckingham Palace, and it was the talk of every gentlemen's club and social soiree, according to James A. Secord, author of Victorian Sensation. Jocular types greeted each other on the street with phrases like, "Well, son of a cabbage, whither art thou progressing?" Others took evolution more seriously. On a museum visit, Florence Nightingale noticed that small flightless birds of the modern genus Apteryx had vestigial wings like those of the giant moa, an extinct bird that had recently been discovered. One species ran into another, she remarked, much "as Vestiges would have it."
Clergymen railed from the pulpit against such thinking. But scientists, too, hated Vestiges for its loose speculation and careless use of facts. One indignant geologist set out to stamp "with an iron heel upon the head of the filthy abortion, and put an end to its crawlings." In Cambridge, at a meeting of the British Association for the Advancement of Science, an astronomer criticized the book's failure to explain how evolution might have occurred; Vestiges, in his view, was about as miraculous as the biblical account of Creation. (During this attack, the author, still anonymous, sat in the front row, probably trying not to squirm.) Even Darwin disliked what he called "that strange unphilosophical, but capitally-written book." He confided to a friend that the author's "geology strikes me as bad, & his zoology far worse."
Darwin had begun to develop his own theory of evolution seven years earlier, in 1838, while reading the demographer T. R. Malthus on factors limiting human population growth. It dawned on him that, among animals, hunger, predation and other "checks" on population could provide "a force like a hundred thousand wedges," thrusting out weaker individuals and creating gaps where better-adapted individuals could thrive. By 1844, he had expanded this idea into a manuscript of more than 200 pages.
But Vestiges heightened Darwin's characteristic caution. He hesitated to publish partly because radicals were taking up evolutionary theory as a way to undermine the idea of a divinely ordained social hierarchy. Darwin himself sat comfortably in the upper ranks of that hierarchy; he had inherited wealth, and his closest colleagues were other gentlemen naturalists, including the clergy. Admitting transmutationist beliefs in these circles, Darwin had written to his friend Hooker, would be like "confessing a murder." But beyond that, he also hesitated because the abuse being heaped onto Vestiges drove home the need for detailed evidence. Darwin, at age 37, backed away from theorizing and settled down to describing the minute differences within one invertebrate group: the barnacles. He would spend the next eight years at it, at some peril to his sanity.
Wallace was more receptive to Vestiges. He was just 22 when the controversy raged. He also came from a downwardly mobile family and had a penchant for progressive political causes. But Vestiges led him to the same conclusion about what needed to be done next. "I do not consider it as a hasty generalization," Wallace wrote to a friend, "but rather as an ingenious speculation" in need of more facts and further research. Later he added, "I begin to feel rather dissatisfied with a mere local collection.... I should like to take some one family to study thoroughly—principally with a view to the theory of the origin of species." In April 1848, having saved £100 from his wages as a railroad surveyor, he and a fellow collector sailed for the Amazon. From then on, Wallace and Darwin were asking the same fundamental questions.
Ideas that seem obvious in retrospect are anything but in real life. As Wallace collected on both sides of the Amazon, he began to think about the distribution of species and whether geographic barriers, such as a river, could be a key to their formation. Traveling on HMS Beagle as a young naturalist, Darwin had also wondered about species distribution in the Galápagos Islands. But pinning down the details was tedious work. As he sorted through the barnacles of the world in 1850, Darwin muttered darkly about "this confounded variation." Two years later, still tangled up in taxonomic minutiae, he exclaimed, "I hate a Barnacle as no man ever did before."
Wallace was returning from the Amazon in 1852, after four years of hard collecting, when his ship caught fire and sank, taking down drawings, notes, journals and what he told a friend were "hundreds of new and beautiful species." But Wallace was as optimistic as Darwin was cautious, and soon headed off on another collecting expedition, to the islands of Southeast Asia. In 1856, he published his first paper on evolution, focusing on the island distribution of closely related species—but leaving out the critical issue of how one species might have evolved from its neighbors. Alarmed, Darwin's friends urged him to get on with his book.
By now, the two men were corresponding. Wallace sent specimens; Darwin replied with encouragement. He also gently warned Wallace off: "This summer will make the 20th year (!) since I opened my first-note-book" on the species question, he wrote, adding that it might take two more years to go to press. Events threatened to bypass them both. In England, a furious debate erupted about whether there were significant structural differences between the brains of humans and gorillas, a species discovered by science only ten years earlier. Other researchers had lately found the fossil remains of brutal-looking humans, the Neanderthals, in Europe itself.