The Tangled Tree

by David Quammen

  Hitchcock read Lyell’s work promptly as the first three volumes came out, from 1830 to 1833, and found it all discomforting. He was no young-earth creationist himself; he acknowledged that volcanism and erosion were continuing processes. But he worried that Lyell’s view of the planet would “exclude a Deity from its creation and government.” In an article on deluges, comparing the biblical with the geological records, Hitchcock wrote cattily: “We know nothing of Mr. Lyell’s religious creed. But there is something in such an ambiguous mode of treating of scriptural subjects that reminds us of infidel cunning and duplicity.” Lyell was a dutiful Anglican, not an infidel, at least when he authored Principles of Geology, but Hitchcock seems to have sensed, maybe better than Lyell himself, that his work would nudge some readers toward godless, materialistic ideas.

  One of those so nudged was Charles Darwin, who read Lyell’s three volumes aboard the Beagle and followed their influence, not just toward uniformitarianism in geology but eventually (because Lyell described Lamarck’s ideas, without endorsing them) toward a theory of evolution. So although Hitchcock was wrong about Lyell’s supposed “cunning and duplicity,” he was right about Principles of Geology taking readers—one crucial reader, anyway—onto a slippery slope.

  In 1840, seven years after Lyell’s third volume appeared, Hitchcock published his own Elementary Geology, and with it that Paleontological Chart of Lombardy poplars, included as a hand-colored, foldout figure presenting his two nonevolutionary trees of life. The chart showed changes in Earth’s flora and fauna over geological time, with this or that group of plants or animals waxing or waning in diversity and abundance, but not much branching of one from another. The cause of those changes, Hitchcock explained in his text, was God’s direct agency, adding and subtracting creatures, improving and perfecting the world as a long-term project. The major groups were present all along, according to this slightly tortured schema, but new species manifesting “a higher organization” had been inserted along the way, until at last Earth was ready for “more perfect” kinds of creatures, “the most generally perfect of all with man at their head.” The gradual introduction of “higher races,” he wrote, “is perfectly explained by the changing condition of the earth which being adapted for more perfect races Divine Wisdom introduced them.” These were special creations by the Deity, appropriate as environments changed. God wasn’t rethinking the planet’s fauna and flora, just adjusting them to newly available niches. If that doesn’t quite make sense, don’t blame Charles Lyell or me.

  Hitchcock’s Elementary Geology was a hit. Between 1840 and the late 1850s, it went through thirty editions, to which he made minor revisions of language and data. Throughout all those editions, the trees figure remained—unchanged except for color adjustments. Then something happened. As a consequence of that something, or else by improbable coincidence, the thirty-first edition of Hitchcock’s book, in 1860, contained a notable difference. An omission. No trees.

  What happened was that in 1859 Charles Darwin published On the Origin of Species. His book also contained a tree, but one with dangerous new meaning.

  6

  By that point, Darwin had incubated his theory in secret for half a lifetime. After sketching his little tree into the B notebook in 1837, he had continued reading, gathering facts, pondering patterns, trying out phrases, brainstorming fervidly for another sixteen months in a series of such notebooks, labeled “C” and “D” and “E,” like a man pushing puzzle pieces around on a table. Then suddenly, in November 1838, as recorded in the E notebook, he solved the puzzle of how species must evolve. Combining three pieces in his mind, he hit upon an explanatory mechanism for evolution.

  The first piece was hereditary continuity. Offspring tend to resemble their parents and grandparents, providing a stable background of similarity throughout time. The second factor, a countertrend to the first, was that variation does occur. Offspring don’t precisely resemble their parents. Brown eyes, blue eyes, taller, shorter, differences of hair color or nose shape among humans; wing markings in a butterfly, beak size in a bird, length of neck in a giraffe. Reproduction is inexact. Likewise, siblings, as well as parents and offspring, differ from one another. Darwin saw that these two pieces, heredity and variation, stand together in some sort of dynamic tension.

  The third puzzle piece, which he had begun considering just recently, having been alerted to it by his eclectic reading, was that population growth always tends to outrun the available means of subsistence. Earth is always getting too full of life. One female cat may give birth to five kittens; one rabbit may deliver eight bunnies; one salmon may lay a thousand eggs. If all those offspring were to survive, and reproduce in their turns, there would soon be a very great lot of cats and bunnies and salmon. Whatever the litter size, whatever the lifetime fecundity, whatever the kind of organism, including humans, we all tend to multiply by geometric progression, not just by arithmetic increase—that is, more like 2, 4, 8, 16 than like 2, 3, 4, 5. Meanwhile, living space and food supply don’t increase nearly so quickly, if at all. Habitat doesn’t replicate itself. Places get crowded. Creatures go hungry. They struggle. The result is competition and deprivation and misery, winners and losers, unsuccessful efforts to breed and, for the less fortunate individuals, early death. Many are called, but few are chosen. The book that awakened Darwin to this reality was An Essay on the Principle of Population, by a severely logical clergyman and scholar named Thomas Malthus.

  Malthus’s gloomy treatise was first published in 1798. It went through six editions in the next three decades and influenced British policy on welfare. (It argued against the relatively easy charity of the contemporary Poor Laws, which were soon changed.) Darwin read it in early autumn 1838–“for amusement,” as he recalled later. Seldom is amusement more productive. He came away with the population piece, combined that with his two other pieces, and scribbled an entry in his D notebook about “the warring of the species as inference from Malthus.” Yes, this “warring” applied not just to humans, Darwin realized, but also to other creatures. Competition was fierce, and opportunities were finite. “One may say there is a force like a hundred thousand wedges,” Darwin wrote, all trying to “force every kind of adapted structure” into the gaps in the economy of nature. “The final cause of all this wedgings,” he added, “must be to sort out proper structure & adapt it to change.” By “final cause,” he essentially meant final result: the struggle yielded well-adapted forms. That was the essence, though still inchoate and crudely stated.

  Darwin seemed to leave Malthus behind as he finished the D notebook, but returned to him soon in the next. That one, labeled E, begun in October 1838, was bound in rust-brown leather, with a metal clasp. It’s one of the true relics in the history of biology. In its earlier pages, Darwin ruminated further about “the grand crush of population” and alluded repeatedly to what he now called “my theory.” He was growing more confident and clear. Then, on or soon after November 27, with his usual clipped grammar and eccentric punctuation, he wrote:

  Three principles, will account for all

  1. Grandchildren, like, grandfathers

  2. Tendency to small change . . . especially with physical change

  3. Great fertility in proportion to support of parents

  Inheritance, variation, overpopulation. He saw how they fit. Put those three together and turn the crank: you’ll get differential survival, based on something or other. Based on what? Based on which variations turn out to be most advantageous. And those variations will tend to be inherited. The result will be gradual transmutation of heritable forms, and adaptation to circumstances, by a process of selective culling. Eventually he gave the crank a name: natural selection.

  Twenty years passed after the E notebook entry. The world heard nothing about natural selection.

  7

  It was a perplexingly long delay, almost two decades, between the writing of those four lines in his secret E notebook and the first public announcement of Darwin’s
theory. Longer still, twenty-one years, to publication of the theory in book form—On the Origin of Species appeared in November 1859. The reasons for that delay, which were both scientific and personal, both anxious and tactical, have been minutely examined in other works (including some of mine). We can skip over them here except to note that, when Darwin finally went public with his theory, it was because a younger naturalist had forced his hand by coming forward with the same idea.

  Alfred Russel Wallace, after four years of fieldwork in the Amazon and four more in the Malay Archipelago, had hit upon the notion of natural selection (framed in his own language, not that pair of words) and written it up in a short paper. As recounted by Wallace long afterward, the idea came during a layover in his collecting travels through the northern Moluccas. He suffered a bout of fever (maybe malarial), and, amidst it, he had this extraordinary insight. Variation plus overpopulation, minus the unsuccessful variants, would yield heritable adaptation. When the fever broke, and the sweat dried, and the dreamy brainstorm still seemed cogent, Wallace composed his manuscript and then tried to get it considered.

  But he was a poor man’s son, working his way through the tropics by selling decorative specimens—bird skins, butterflies, pretty beetles—not a gentleman traveler as Darwin had been on the Beagle. Wallace wasn’t well educated or well connected. He knew almost nobody in the higher circles of British or European science, and almost nobody in those circles knew him—not face-to-face and not as a peer, anyway. He was a collector of dried creatures for pay, a natural-history tradesman. There was class stratification in science as in every other part of Victorian British society. But he had published a few earlier papers in a respectable journal, and one of those papers had drawn favorable attention from Charles Lyell, the great geologist. Oh, and Wallace knew one other famous man, not personally but as a sort of pen pal, who had spoken generously to him in a letter: Charles Darwin.

  It was now February 1858. Hardly anyone at that point recognized Darwin for what he was—an evolutionary theorist, in secret—and though Lyell was among that small group who did, as a close friend and confidant, Alfred Wallace certainly wasn’t. Charles Darwin to him was just a conventionally eminent naturalist, author of the Beagle chronicle and other safe books, including several on the taxonomy of barnacles. But a Dutch mail boat would soon stop at the port of Ternate, in the Moluccas, where Wallace had fetched up. He was excited by his own discovery, if it was a discovery, and eager to share this dangerous hypothesis with the scientific world. So he packed up his paper with a cover letter and mailed the packet to Mr. Darwin, hoping that Darwin might find it worthy. If so, maybe Darwin would share it with Mr. Lyell, who might help get it published.

  The packet reached Darwin, probably on June 18, 1858, and hit him like a galloping ox. He felt crushed, scooped, ruined—but also honor-bound to grant Wallace’s request, passing the paper on toward publication. It would mean, Darwin knew, letting the younger man take all the credit for this epochal idea he himself had incubated for twenty years but was not yet quite ready to publish. Despite that, he did send the Wallace paper along to Lyell—communicating yelps of his own anguish along with it. Lyell took not just the paper but also the hint. Along with another of Darwin’s close scientific allies, the botanist Joseph Hooker, Lyell talked Darwin back from despair, suggested a posture of sensible fairness rather than self-abnegating honor, and brokered a compromise of shared credit. The result was a clumsily conjoined presentation—a pastiche of Wallace’s paper plus excerpts from Darwin’s unpublished writings—before a British scientific club, the Linnean Society, in the summer of 1858. Lyell and Hooker offered an introductory note, and then simply watched and listened. Proxies read the works aloud, with neither of the authors present. (Darwin was at home, where his youngest son had just died of scarlet fever; Wallace was still out in the far boonies of the Malay Archipelago.) This joint presentation made almost no impression on anyone, not even the few dozen Linnean members in attendance, because the night was hot, the language was obscure, the logic was elliptical, and the big meaning didn’t jump forth.

  Seventeen months later, Darwin published On the Origin of Species. That 1859 book, not the 1858 paper or excerpts, launched the Darwinian revolution. It was only an abridged and hasty abstract of the much longer (and more tedious) book on natural selection that Darwin had been writing for years, but The Origin was just enough, in the right form, at the right time. It presented the theory as “one long argument,” not just a bare syllogism, and with oodles of data but not many footnotes. It was plainspoken, and readable by any literate person. It became a bestseller and went into multiple editions. It converted a generation of scientists to the idea of evolution (though not to natural selection as the prime mechanism). It was translated and embraced in other countries, especially Germany. That’s why Darwin is still history’s most venerated biologist and Alfred Russel Wallace is a cherished underdog, famous for being eclipsed, to the relatively small subset of people who have heard of him.

  The crux of the “one long argument” comes in chapter 4 of The Origin, titled “Natural Selection,” in which Darwin describes the central mechanism of his theory. It’s the same combination of three principles that he had scratched into his notebook two decades earlier, plus the turned crank. “Natural selection,” he wrote in the book, “leads to divergence of character and to much extinction of the less improved and intermediate forms of life.” Lineages change over time, he stated. You could see that in the fossil record. Different creatures adapt to different niches, different ways of life, and thereby diversify into distinct forms and behaviors. Transitional stages disappear. Then he wrote: “The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth.”

  8

  Darwin explored the tree simile in one extended paragraph, ending that chapter of The Origin. “The green and budding twigs may represent existing species,” he wrote. From there he worked backward: woody twigs and small branches as recently extinct forms; competition between branches for space and for light; big limbs dividing into branches, then those into lesser branches; all ascending and spreading from a single great trunk. “As buds give rise by growth to fresh buds,” Darwin wrote, and those buds grow to be twigs, and those twigs grow to be branches, some vigorous, some feeble, some thriving, some dying, “so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.” There’s a nice word: ramifications.

  It’s especially good in this context because, while the literal definition is “a structure formed of branches,” from the Latin ramus, of course the looser definition is “implications.” Darwin’s tree certainly had implications.

  Furthermore his book, like Edward Hitchcock’s, included a treelike illustration. This was the only illustration, the only graphic image of any sort, in the first edition of The Origin. It appeared between pages 116 and 117, amid his discussion of how lineages diverge over time. A foldout, again like Hitchcock’s, but published in simple black and white. It was a schematic figure, not an artfully drawn tree, not even so lively as the little sketch in his notebook long ago. Darwin called it a diagram. It showed hypothetical lineages, proceeding upward through evolutionary time and diverging—that is, dotted lines, rising vertically and branching laterally. Darwin was no artist, but, even lacking such talent, he could have laid out this diagram with a pencil and a ruler. In its draft version, as sent to the lithographer, he probably had. But it made the arboreal point.

  Darwin’s diagram of divergence, from On the Origin of Species, 1859.

  Each increment of vertical distance on the ruled page, Darwin explained, stood for a thousand generations of inheritance. Deep time. Eleven major lineages began the ascent. Eight of those came to dead ends—meaning, they went extinct. Trilobites, ammonites, ichthyosaurs, and plesiosaurs had
all suffered such ends, leaving no descendants of any sort. One lineage rose through the eons without splitting, without tilting, like a beanstalk—meaning that it persisted through time, unchanged. That’s much the way horseshoe crabs, sometimes called living fossils, have survived relatively unchanged (at least externally, so far as fossilization can show) over 450 million years. The other two lineages, dominating the diagram, branched often and spread horizontally—as well as climbed vertically. Their branching and horizontal spread represented the exploration of different niches by newly evolved forms. So there it all was: evolution and the origins of diversity.

  Back in Massachusetts, Edward Hitchcock read Darwin’s book, and it stuck in his craw. This wasn’t his first exposure to the idea of transmutation (he knew of Lamarck’s work and some other wild speculations), but it was the latest statement of that idea, the most concrete and logical, and therefore the most dangerously persuasive. Like some other pious scientists who chose to see God’s hand acting directly in the fossil record—Louis Agassiz at Harvard, François Jules Pictet in Geneva, and Adam Sedgwick, who had been Darwin’s mentor in geology at Cambridge—Hitchcock wasn’t pleased.

  Into the 1860 edition of his Elementary Geology, he inserted his rejoinder to Darwin’s book, based mainly on proof by authority. He noted that Pictet saw no evidence for transmutation in the fossil record of fishes. Agassiz said that the resemblances among animals derive from—where?—the mind of the Creator. “It is well to take heed to the opinions of such masters in science,” Hitchcock wrote, “when so many, with Darwin at their head, are inclined to adopt the doctrine of gradual transmutation in species.”