Great Minds Think Alike
Sean Carroll tells the remarkable story of the other father of evolution by natural selection: Alfred Russel Wallace.
The search for the origins of species, both in general and of specific kinds of creatures, has entailed a series of truly epic adventures over the past 200 years. Throughout 2009, the 200th anniversary of the birth of Charles Darwin, the world is marking the achievements of our greatest naturalist and the leader of a far-reaching scientific revolution.
Darwin's great voyage and works are well known, and rightly so. But the making of the theory of evolution and its early growth and acceptance also owe a considerable debt to Alfred Russel Wallace, who undertook two long voyages under yet more difficult conditions, and independently reached very similar conclusions to Darwin.
Wallace's dramatic story and scientific contributions are generally much less well known. The writer C.W. Ceram described adventure as "a mixture of spirit and deed," and I think no naturalist's experiences better fit that definition than Alfred Wallace's. I will highlight some of his adventures and discoveries, show how he developed similar ideas as Darwin, and offer a glimpse into the very warm relationship that emerged between the two great naturalists.
The two men had a number of traits in common. Both were eager to escape England and to explore the glories of the Tropics. Both did so as young men—Darwin was 22 when he boarded the HMS Beagle, and Wallace was 25 when he first left England. Above all, they were each prodigious collectors, and through collecting they developed an appreciation for the variety each species exhibited. From this very hard-earned knowledge, they evolved from collectors into scientists, who asked not just what creatures existed in a given place but how they came to be, and these questions led each man to unique and shared discoveries.
Deep into Amazonia
In 1847, Wallace proposed to his friend and fellow collector Henry Walter Bates that they travel to the Amazon. His principal motivation was to build a personal natural history collection. But Wallace was also well-read on scientific topics of the day. Unlike Darwin, who did not set out with any intent of gathering evidence for or against any great idea, Wallace suggested to Bates that on their journey they could "gather facts toward solving the problem of the origin of species." That problem, circa 1847, pivoted on a potent question: Were species immutable and specially created by God, or changeable and the product of natural processes?
Wallace and Bates were self-taught amateurs who did not have the family financial resources that Darwin had, or the connections with academia, or berths on a British naval vessel. They had to make their way to the Amazon on a commercial trading ship and then cover their expenses by shipping prized specimens back to England for sale.
They arrived in Para on the northeast coast of Brazil in May 1848. After a time exploring the region they split up, with Wallace heading up the main trunk of the Amazon and then up the Rio Negro and its largest branch, the Rio dos Uaupés. By 1852, after four years of arduous travel and collecting, he was 2,000 miles upriver from the Atlantic Ocean, farther than any European had ever gone.
But he was spent. Physical exertion, poor nutrition, and tropical diseases had weakened him to a state in which he feared that if he did not turn back he would die in the jungle. In addition to many preserved specimens that he had with him and stored downriver, Wallace had accumulated a large menagerie of live animals—monkeys, macaws, parrots, and a toucan—that he hoped to take all the way to the London Zoo. Their upkeep was draining him of what little energy he had remaining.
Wallace headed back downriver to Para. He found a ship headed for England, the brig Helen, boarded it with 34 live animals and many boxes of specimens and notes, and set sail for home.
Lost at sea
Four weeks into the journey and about 700 miles east of Bermuda, the Captain came to Wallace's cabin and said, "I'm afraid the ship's on fire; come and see what you think of it." Wallace followed the Captain to the hold and saw smoke pouring out of it.
The crew could not douse the smoldering blaze. The Captain ordered down the lifeboats. Wallace went to his hot, smoky cabin and salvaged a small tin box and threw in some drawings, some notes, and a diary. He grabbed a line to lower himself into a lifeboat, slipped, and seared his hands on the rope. His pain was compounded when his injured hands hit the saltwater. Once in the lifeboat, he discovered it was leaking.
Wallace watched his animals perish and the Helen burn, along with all of his specimens.
And now everything was gone, and I had not one specimen to illustrate the unknown lands I had trod....
And so there he was, lying on his back in a leaky lifeboat in the middle of the Atlantic. Day after day passed in the open boats. Wallace was blistered by sunburn, parched with thirst, soaked by sea spray, exhausted from constantly bailing water, and near starvation. At last, on the tenth day, they were picked up.
Aboard his rescue ship, Wallace began a letter to a friend in Brazil detailing his ordeal and the magnitude of his loss:
"How many weary days and weeks had I passed, upheld only by the fond hope of bringing home many new and beautiful forms from those wild regions; every one of which would be endeared to me by the recollections they would call up... And now everything was gone, and I had not one specimen to illustrate the unknown lands I had trod...."
Wallace wrote to his friend that "fifty times" on the voyage home he had sworn to himself "if I once reached England, never to trust myself more on the ocean." If he had held to that promise, his story would end here and few would have ever heard of Alfred Wallace again. But, as he wrote to his friend, "good resolutions soon fade...." Wallace decided that, despite his loss and near death, he would voyage again.
To the Malay Archipelago
Wallace's lust for exploration and collecting was not satisfied, nor was his interest in the origin of species. That mystery was still unsolved as far as the scientific world knew in 1852. Though Darwin had reached his conclusions many years earlier, his ideas were known to only a few intimates, and Wallace was not one of them.
Wallace began to ponder his next destination. He had to collect quarry that would fetch good prices, but he ruled out a return to the Amazon. He began thinking about the Malay Archipelago, the vast group of islands between Southeast Asia and Australia [see map]. Other than those on the island of Java, the animals and plants of the region were unknown. Enough fragments of natural history were emerging from the Dutch settlements there to convince Wallace that it offered both rich pickings and good facilities for a traveler.
The islands span more than 4,000 miles from east to west and 1,300 miles from north to south, an area almost as large as the entire continent of South America. Covered in tropical forest, the islands might appear similar, but some held different treasures, and discovering and explaining the differences would put Wallace, literally, on the map.
Wallace arrived in Singapore in April 1854 and set out to explore the country. He would encounter altogether different treasures, and dangers, than those on the Amazon. For example, there were tigers roaming about Singapore; they killed on average one resident a day. Wallace occasionally heard their roars, and in typical British understatement he noted that "it was rather nervous work hunting for insects ... when one of these savage animals might be lurking close by...."
Germinating ideas
Unflustered by such concerns, Wallace followed a daily routine. Up at 5:30 a.m., he started the day with a cold bath and hot coffee. He sorted out the previous day's collection and then set out again into the forest with his gear. He carried a net, a large collecting box hung on a strap over his shoulder, pliers for handling bees and wasps, and two sizes of specimen bottles for large and small insects, attached by strings around his neck and plugged with corks. On some days, he carried a rifle.
I naturally expected to meet with some of these birds again; but during a stay there of three months I never saw one of them....
Despite some reputation for ferocity, the native tribesmen in parts of the archipelago he visited shared their knowledge of the forest with Wallace and helped him to find what he was after. He stalked the islands' most beautiful and prized natural riches—orangutans, monkeys, spectacular birds of paradise, and enormous, brilliantly colored butterflies. Wallace mused that:
"Nature seems to have taken every precaution that these, her choicest treasures, may not lose value by being too easily obtained. First, we find an open harbourless, inhospitable coast, exposed to the full swell of the Pacific Ocean; next, a rugged and mountainous country, covered with dense forests, offering [in] its swamps and precipices and serrated ridges an almost impossible barrier to the central regions; and lastly, a race of the most savage and ruthless character...."
Wallace was paying close attention to the diversity of species he found, the variety among the individuals of each species, and where he found them. These were the practical concerns of a paid collector but also the catalysts of his transformation into a scientist.
For example, while pursuing beautiful birdwing butterflies, which were coveted for their large wingspan and rich coloration, Wallace noticed that different birdwing types were restricted to particular islands. These butterflies signaled to him just what the birds of the Galapagos archipelago signaled to Darwin—that species change.
While Darwin was keeping quiet about evolution, Wallace was thinking out loud, putting his thoughts on paper and firing them off to magazines and journals in England. Some of these were short field notes; others revealed bigger ideas. But Wallace had none of the concerns that restrained Darwin. He had a reputation to make, and nothing to lose.
A law of nature
In 1855, while waiting out the wet season in Sarawak, on Borneo, Wallace wove together threads of geology and natural history to propose a new law: Every species has come into existence coincident both in space and time with a pre-existing closely allied species.
Wallace thought that species were connected like "a branching tree." He was proposing that new species come from old species as new twigs grow from older branches. This bold idea refuted the then-dominant doctrine of special creation—that each species was specially created, in one moment, to fit the land it inhabited. Moreover, Wallace used some of the very arguments that Darwin had agonized over for almost two decades but had not yet published.
Wallace supported his "Sarawak Law" with all sorts of observations on the distribution of species, especially those on islands. For example, the Galapagos, he wrote, "which contain little groups of plants and animals peculiar to themselves, but most nearly allied to those of South America, have not hitherto received any, even a conjectural explanation." Wallace was referring to Darwin's observations, which had not been explained.
Wallace pointed out that families of butterflies, birds, and various plants are confined to certain regions. He had noticed when he was in the Amazon that some species of monkeys were confined to one side of the river. "They could not be as they are," he wrote, "had no law regulated their creation and dispersion." By "dispersion," Wallace meant that the extent to which a species could spread out over the land was constrained by features of the land—rivers, mountain ranges, and so forth.
Almost no one read or noticed the paper when it first appeared. Wallace heard nothing from England about his Law, except for some grumblings that he should focus on collecting and not theorizing.
Drawing a line
Wallace went island-hopping quite often. He made 96 journeys totaling about 14,000 miles and visited some of the same islands several times over the span of eight years. Often the availability, or unavailability, of a boat determined his path. One day in May 1856, he took a Chinese schooner from Singapore to Bali, which he had no intention of visiting, but he figured he could find a way from there to Lombok and then on to Makassar on the island of Sulawesi. This accidental detour would give Wallace the most important discovery of his expedition.
On Bali, Wallace found kinds of birds as on the other islands he had visited to the west, including a weaver, a woodpecker, a thrush, a starling—nothing too exciting. But then, "crossing over to Lombok, separated from Bali by a strait less than twenty miles wide, I naturally expected to meet with some of these birds again; but during a stay there of three months I never saw one of them...." Instead, Wallace found a completely different assortment: white cockatoos, three species of honey-suckers, a loud bird the locals called a "Quaich-Quaich," and a really strange bird called a megapode ("big foot") that used its big feet to make very large mounds for its eggs. None of these groups were known on the western islands of Java, Sumatra, or Borneo.
Now here was a puzzle. What constraint prevented the spread of these species from island to island? Surely, birds could cover a 20-mile strait with little trouble.
The life of wild animals is a struggle for existence.
Wallace described the mystery in a letter to Bates. He theorized that there was some kind of invisible "boundary line" between Bali and Lombok. Traveling farther east to Flores and Timor, the Aru Islands, and New Guinea, the changeover in bird life was very clear. All the families of birds that were common on Sumatra, Java, and Borneo were absent from Aru, New Guinea, or Australia, and vice versa. The differences in mammals among the western and eastern islands were just as striking. On the large western islands there were monkeys, tigers, and rhinoceri. But on Aru there were no primates or carnivores. All the native mammals were marsupials—kangaroos, cuscus, and the like.
That line between Bali and Lombok was real, and it signified something very profound to Wallace. He put his thoughts to paper again. Wallace pointed out that under the doctrine of special creation, one would expect to find similar animals in countries with similar climates, and dissimilar animals in countries with dissimilar climates. This is not at all what he saw.
Comparing Borneo (in the west) and New Guinea (in the east), he wrote, "[I]t would be difficult to point out two [lands] more exactly resembling each other in climate and physical features." But their birds and mammals were entirely different. Comparing New Guinea and Australia, he wrote, "we can scarcely find a stronger contrast than in their physical conditions ... the one enjoying perpetual moisture, the other with alternations of excessive drought." Wallace reasoned, "If kangaroos are especially adapted to the dry plains and open woods of Australia, there must be some other reason for their introduction into the dense damp forests of New Guinea...." In the tropical forests of the eastern islands, tree kangaroos occupied the habitat occupied by monkeys in the west.
Wallace reasoned further that "some other law has regulated the distribution of existing species." That law, Wallace suggested, was the "Sarawak Law" he had proposed two years earlier. Again Wallace relied on geology to make his case. He surmised that New Guinea, Australia, and Aru must have been connected at some time in the past and so share similar sets of birds and mammals. Similarly, Wallace deduced that the western islands had once been part of Asia and so share the fauna of tropical Asia—monkeys, tigers, etc.
Wallace was right. He had linked the question of the origin of species to how species were distributed, and he had defined a dividing line between the fauna of Asia and Australia. His discovery would forever after be known as the "Wallace Line" and Wallace himself as the founder of biogeography, the science dealing with the geographical distribution of plants and animals.
Meeting of the minds
For Wallace the question, then, was not if species evolved but how? Baking in a malarial fever on the volcanic island of Ternate in early 1858, the answers came to him.
Alternating between hot and cold fits, Wallace had nothing to do but "think over subjects then particularly interesting to me." Wrapped in a blanket on a 88°F day, he thought of the English economist Thomas Malthus's essay on population, which he had read some years earlier. It occurred to him that the diseases, accidents, and famine that Malthus argued check the growth of human populations act on animals, too. He thought about how animals breed much more rapidly than humans and, if left unchecked, would overcrowd the world very quickly. But all of his experience revealed that animal populations were limited. "The life of wild animals is," Wallace concluded, "a struggle for existence [my italics—watch for more below, and for why I highlight them]." Wallace continued: "The full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring." Finding food and escaping danger ruled animal lives, and the weakest would be weeded out.
Wallace the great collector was intimately familiar with the variety of individuals of a species. "Perhaps all the variations ... must have some definite effect, however slight, in the habits of or capacities of the individuals ... a variety having slightly increased powers ... must inevitably acquire a superiority in numbers."
Wallace wrote the paper out in its entirety in just a few nights. He entitled it "On the Tendency of Varieties to Depart Indefinitely From the Original Type." Wallace's paper was just a sketch, conceived in a dilapidated house on an earthquake-ravaged island during bouts of fever, 10,000 miles from the center of science in England. Wallace did not send it directly to a journal; he wanted others to look at it first. So he sent it to a naturalist with whom he had begun a correspondence—Charles Darwin.
Darwin received Wallace's paper sometime in June 1858. He was shocked when he read it. The reason for that shock is especially clear when one considers what Darwin had recently written in drafts of two chapters for a large book he was working on and compares it with the language of Wallace's paper.
I know not how or to whom to express fully my admiration of Darwin's book.
In February 1857, Darwin had composed Chapter 5 of his book and entitled it "The Struggle for Existence as Bearing on Natural Selection" [again, my italics]. The next month he had completed Chapter 6, in which he explained that:
"All Nature ... is at war. ... The struggle very often falls on the egg & seed, or on the seedling... any variation, however infinitely slight, if it did promote during any part of life even in the slightest degree, the welfare of the being, such variation would tend to be preserved or selected."
Neither author was aware of the other's thoughts and writing. How can we explain the remarkable similarities in language—"the struggle for existence" and "slight variation"?
Great minds think alike.
Both men had seen nature up close and understood it was a battlefield. Both men had collected enough specimens of individual species to appreciate that species were variable. Both men had seen slightly different species restricted to particular islands and concluded that species change. Both men had read and recognized the relevance of Malthus's essay on populations. Confronted with similar evidence, they had reached very similar conclusions.
Nonetheless, Darwin, more than 20 years after his first insights into species formation, feared that "all of my originality, whatever it may amount to, will be smashed."
"A little proud"
What happened thereafter is still a subject of debate among scholars. The facts are that Wallace had asked Darwin to forward the manuscript to the geologist Sir Charles Lyell, which Darwin did. Lyell and J. D. Hooker, the eminent botanist, were intimates of Darwin, to whom he had divulged his theory of natural selection and much of the argument supporting it. Lyell and Hooker took the initiative to arrange for Wallace's paper, and a brief sketch from Darwin on his theory, to be read together at an upcoming meeting of the Linnaean Society and to be published together.
Was Wallace robbed of his individual right to glory? Was the arrangement of joint publication fair? (Wallace was not informed of it until after the fact.) It was Darwin who had coined the term "natural selection," and he had shared his 1842 sketch, at least privately, with other scientists.
It is true that Darwin's name and works are far better known than Wallace's today. But consider Wallace's perspective on the matter. While still in the Malay Archipelago, he received a copy of the Origin of Species from Darwin. He read it over and over. Then he disclosed his reactions in a private letter to his longtime friend Bates:
"I know not how or to whom to express fully my admiration of Darwin's book. ...I do honestly believe that with however much patience I had worked up & experimented on the subject I could never have approached the completeness of the book,—its overwhelming argument, & its admirable tone & spirit. ... Mr. Darwin has created a new science & a new Philosophy, & I believe that never has such a complete illustration of a branch of human knowledge, been due to the labours and researches of a single man."
Not in this letter nor for the rest of his long life—he lived to 90—did Wallace utter a word of regret, envy, or resentment.
Perhaps for Wallace it was simply a matter of being accepted. He was, up until 1858, an outsider to the circle of eminent scientists who led the new revolution in thought. When he heard that Lyell and Hooker had made complimentary remarks about his paper, he wrote his oldest friend and school-fellow that "I am a little proud...." Wallace did not need or seek to be the center of the circle; he just wanted to be let inside. That, and more, he surely earned.
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