Charles Darwin (Credit: AP)
Léon Croizat sat at his desk, writing . . . and seething. For Croizat, an Italian botanist living in Venezuela, writing seemed as natural as breathing, and nearly as constant—from 1952 to 1962, his especially prolific period, he published four technical biology books totaling close to 6,000 pages—and when he wrote, he was often thinking about Charles Darwin. And when he thought about Darwin, he seethed. It was not about religion—Croizat was as complete an evolutionist as Darwin had been. However, in Croizat’s eyes, Darwin had gotten almost everything about evolution wrong. To begin with, Croizat believed that natural selection was a trivial part of evolution, not its main driving force. More than anything, though, he hated Darwin’s views of historical biogeography, of the means by which living things had acquired their particular distributions on the Earth.
Croizat had a grand vision, a unified theory of the geography of life. It boiled down to this: the distributions of groups, from orchids to earthworms to armadillos, all reflected the dynamic climatic and geologic history of the planet itself. Sea levels rose to inundate land bridges; ocean basins opened, dividing continents; island arcs plowed into continental margins. These changes in the configurations of landmasses and oceans left an indelible imprint on life. In fact, that imprint was so unmistakable that one could use the distributions of living things to reveal the history of the Earth. Find out where the orchids and the worms and the armadillos live, and the arrangements of the continents through time also would be revealed.
Croizat gave his theory a name befitting its all-encompassing nature, its power to explain the distributions of living things over the entire planet. He called it panbiogeography. He also provided a memorable phrase, probably the most memorable one in the history of the discipline, five words that captured the essence of his worldview: “Earth and life evolve together.”
Croizat’s panbiogeography ran counter to an idea that had a long history among biologists and naturalists, namely, that the discontinuous distributions of species and higher taxa often were the result of chance dispersal, of unpredictable, long-distance jumps. To the extent that such dispersal was common, it meant that distributions did not reflect Earth history. Terrestrial organisms, for instance, could move even among landmasses that were widely separated. To Croizat, this was lunacy, mere storytelling founded on absurdly improbable events. Beyond that, it robbed biogeography of any kind of generality, because a different story might apply to every taxonomic group. Perhaps snails had reached the Hawaiian Islands attached to the feathers of a bird, spiders by using long silk strands to float on storm winds, and bean trees as seeds embedded in a raft of vegetation. And perhaps ants and termites and bumblebees had not reached those islands simply because, well, because they had not. This view of biogeographic history was pure chaos, the antithesis of unification. And where did this pabulum come from? It came from Charles Darwin. To most biologists, Darwin was like a secular saint, even a deity, but to Croizat he was a fool and worse—he was the unthinking dilettante who had come up with an unsupportable view of the geographic history of life and somehow convinced almost everyone that he was right. A hundred years after publication of "The Origin of Species," in which Darwin had presented his ideas on chance dispersal, the field of biogeography was still laboring under the delusions of the “master.”
Croizat thought it was time for this long, anti-intellectual chapter to come to an end, and that he, of course, would be the one to end it.
At Down House, his country home in Kent, Charles Darwin worried about the implausibility of long-distance dispersal, especially dispersal over water. He thought it was a problem for his theory of evolution. How could the same species, or two species that were closely related by descent, turn up in regions separated by seas or oceans? For that matter, how did many species find their way to oceanic islands, which were separated from everywhere by ocean barriers? Darwin had all kinds of reasons to believe that species were connected by descent, but he thought this problem—the problem of related groups living in areas divided by large bodies of water—could be a sticking point for skeptical readers. In these cases, it almost seemed as if creation were a better explanation than evolution. Wasn’t it easier to imagine that God had created the same or related species in these widely separated places than to envision all manner of animals and plants making absurdly long ocean voyages? Could iguanas really have rafted from South America to the Galápagos on their own? Could beech-tree seeds have floated from Australia to New Zealand?
Darwin was aware of the other natural (as opposed to divine) explanation for such distributions, that is, the existence of former land connections. However, over time he had come to view the easy use of such explanations as little better than invoking the supernatural. It seemed like cheating, pulling something out of thin air, or, more precisely, conjuring up land out of the deep, unfathomable ocean. He and his close friend, the botanist Joseph Hooker, had a running argument about the subject. Like Darwin, Hooker had taken a long ocean voyage, as a naturalist aboard the HMS Erebus and the HMS Terror, and, seeing obvious similarities among the floras of various southern lands, he had suggested that plants had moved across now-sunken land bridges. Darwin did believe that lands had risen and fallen—he had seen evidence of rising land in the Chilean Andes and of subsidence in the coral islands of the Pacific—but he didn’t like using land-bridge explanations in specific cases when there was no geological evidence to back them up. In an 1855 letter to Hooker he wrote, “It shocks my philosophy to create land, without some other & independent evidence” (that is, other than distributions of organisms). Hooker, for his part, was equally skeptical about some of Darwin’s ideas on the dispersal of plants and animals across water. He especially didn’t like Darwin’s penchant for suggesting transport on icebergs. The Erebus and the Terror had journeyed to Antarctica, crashing their way through ice floes, and Hooker had seen his share of icebergs. He had the impression that not many living things caught rides on them.
The issue of oceanic dispersal was important enough to Darwin that, from 1854 to 1856—while he was still waffling over how to present his evolutionary ideas publicly—he conducted a whole series of experiments at Down House to figure out whether plant seeds and other propagules could possibly cross large water barriers. He put the seeds of eighty-seven kinds of plants in bottles filled with salt water for weeks and months, then planted the seeds to see if they were still viable. He dangled the disembodied feet of a duck in an aquarium to see if hatchling freshwater snails would cling to them. Knowing that some fish would eat plant seeds, he forced seeds into the stomachs of fish, fed the fish to eagles, storks, and pelicans, and then tried to germinate the seeds he retrieved from the birds’ droppings.
The experiments convinced him that long-distance oceanic dispersal was a lot more likely than one might think. Many kinds of seeds survived after being immersed in salt water for 28 days, and a few survived for 137 days. The young snails did climb up onto the duck’s feet, suggesting they could hitch a ride to wherever a duck might fly (although the distance would be limited to the time it takes a tiny snail to dry up). Some of the seeds from the eagle, stork, and pelican droppings germinated, indicating another possible means of transport by birds. Careful, as always— Darwin was nothing if not a careful thinker—he reasoned that seeds on their own wouldn’t make it very far because they would sink. So he also collected dry branches with fruits attached and dropped these into salt water to see how long they could remain afloat. Combining the results of these floating-branch experiments with the seed-viability numbers and estimates of the speed of ocean currents, he calculated that seeds of 14 percent of plant species could travel at least 924 miles and still germinate at the end of the trip.
Darwin wrote quite a few letters to Hooker describing these results and, like most experimentalists, he seemed to take pleasure in conveying the difficulties of the work. “It is quite surprising that the Radishes shd [should] have grown, for the salt-water was putrid to an extent, which I cd [could] not have thought credible had I not smelt it myself,” he wrote in one letter. He also enjoyed what sounds like a self-effacing, Victorian version of trash-talking at Hooker’s expense: “When I wrote last, I was going to triumph over you, for my experiment had in a slight degree succeeded, but this with infinite baseness I did not tell in hopes that you would say that you would eat all the plants, which I could raise after immersion.” Eventually, he changed Hooker’s mind on the subject. At one point, Hooker even conceded that “I am more reconciled to Iceberg transport than I was.” Darwin had won a round for dispersal explanations.
* * *
It was not as if Darwin were the first person to think about oceanic dispersal. In fact, some 250 years earlier, in the late 1500s, there had been a surge of interest in both overwater dispersal and land bridges. What brought on this early attention to the geography of living things was, oddly enough, a shift from allegorical to literal interpretations of the Bible. In particular, taking the story of Noah’s Ark at face value meant that all the animals in the world, two by two, must have ended up in a crowd on the top of Mount Ararat after the Flood. This meant that somehow animals had repopulated the world from that single spot, which in turn required them to cross oceans. How had they done it? One theory was that transoceanic journeys had been made in stepping-stone fashion, with the animals swimming from island to island. Another was that animals had traveled as cargo on boats (the same boats with which people had repopulated the world). A third had animals crossing from the Old World to the New World on the lost continent of Atlantis.
An English historian of science named Janet Browne has argued persuasively that these biblically motivated ideas about the colonization of the world by animals (plants weren’t part of the Ark story) mark the beginnings of scientific thinking about the distributions of living things on the Earth. They also may represent early inklings of the dispersal-vicariance debate: the stepping-stone and cargo ideas are obviously about long-distance dispersal, and the notion of Atlantis as a land bridge looks like a vicariance hypothesis, with the continuous ranges of animal “kinds” being split into Old and New World portions by the drowning of the lost continent.
Still, if these ideas represented the beginnings of biogeography, it was a case of rational or semirational thinking being piled on a foundation of myth. To my mind, modern biogeography—that is, a science that would be instantly recognizable to, say, a grad student poring over evolutionary trees generated from monkey DNA sequences—began with Darwin and his putrid seed bottles, disembodied duck’s feet, and eagle droppings. To be precise, it began with two assumptions about the history of life that led Darwin to perform those experiments.
I alluded to one of these assumptions above, the obvious one, the idea of evolution itself, which Darwin had come to accept as fact in 1837, not long after returning from his voyage on HMS Beagle. More specifically, the notion that each species originated in a single place, having evolved from some other species, and the related premise that similar species had evolved from a common and localized ancestor, meant that disjunct distributions had to be explained by natural movements of organisms. If a species originated in one place but ended up in a second place, across a sea or ocean, some kind of explanation was needed. In essence, Darwin was faced with the problem of Mount Ararat all over again, only this time without ancient people hauling animals all over the world on boats. He needed natural explanations for such distributions, and that meant either land bridges or his favored mechanism, oceanic dispersal.
Darwin’s second key assumption was that the Earth was enormously old, an idea he may have picked up from reading Charles Lyell’s "Principles of Geology." Lyell, following the eighteenth-century Scottish geologist James Hutton, had argued for geological uniformitarianism, the theory that the features of the Earth had been generated by processes people could still observe, such as erosion and vulcanism, acting at relatively constant rates. From uniformitarianism, it followed that some features required an awfully long time to reach their present form: the Grand Canyon, for instance, must have been created over eons, as the Colorado River carved its way down into the rock, inch by inch, year by year. This in turn meant that the Earth itself must be exceedingly old. Exactly how old was a matter of much questionable conjecture, but the planet was clearly many millions of years old, not just a few thousand, as biblical literalists believed. The acceptance of this incomprehensibly long history—what the writer John McPhee would later call “deep time”—meant that the processes and events that influenced the distributions of living things had had a very long time to operate. Of course, the age of a single species or group of closely related species did not extend all the way back to the origin of the Earth, but such groups might still be many thousands or even millions of years old. This realization was critical to Darwin’s belief in the importance of dispersal, because, although he had shown that long-distance colonization over water was possible, he was not arguing that it happened frequently. The dispersal of seeds or birds or insects across the Atlantic or to the Galápagos or Hawaii would be rare, at best, so long stretches of time were required to account for the observed distributions.
In short, what Darwin had begun, as an outgrowth of trying to prove the truth of evolution, was the new science of historical biogeography. Soon, he would have company in this new field.
In 1855, while Darwin was fiddling with his saltwater seed bottles and duck’s feet at his home in Kent, Alfred Russel Wallace, who was then thirty-two (fourteen years younger than Darwin), was collecting natural history specimens, especially beetles, in the independent kingdom of Sarawak on the island of Borneo. Unlike Darwin the gentleman, Wallace was from a working-class family and had toiled at various other jobs before deciding to earn a living as a collector. He had already spent four years in the Amazon—a trip that ended with the loss of most of his specimens and notes in a shipboard fire and a subsequent week and a half spent aboard a lifeboat in the Atlantic—and now he was on what would ultimately be an eight-year sojourn in the Malay Archipelago.
Wallace is sometimes remembered as just that guy who pushed Darwin to publish his theory of natural selection by coming up with the same idea years after Darwin did. But Wallace was a thinker of great scope and depth in his own right. Like Darwin, he seems to have been an honest and generous man, but he may have been more ambitious than his older colleague, or, at least, had ambitions less tempered by caution. In his mid-twenties, while planning his Amazon trip, he was already hoping to gather facts “towards solving the problem of the origin of species.” It was in Sarawak that he made his first big step toward that goal. With the rainy season holding up his collecting, he had some time on his hands and made good use of it, writing a theoretical paper called, somewhat cryptically, “On the Law Which Has Regulated the Introduction of New Species.” He wrote the paper in February 1855 and shipped it off to England, where it was published later that year in Annals & Magazine of Natural History.
The key observation in Wallace’s paper was that close taxonomic connection went hand in hand with close geographic association. For instance, within a widespread taxonomic family, species in the same genus tended to be found in the same geographic area, or at least near each other, whereas species in different genera often were not geographically close to each other. To take a nonrandom example, the garter snakes I study make up the genus Thamnophis, a group confined to North America, but there are other genera within the same snake family on every continent except Antarctica. Something analogous could be seen in the fossil record: within a family, for example, genera from the same time period tended to be more alike than those from different periods.
Such observations did not originate with Wallace, but the conclusion—the law—he drew from them was radical. “Every species,” he wrote, “has come into existence coincident both in space and time with a pre-existing closely allied species” (italics in original). What he was implying was that species evolved from other species; similar species were associated in space and time because they arose from a common ancestral species that lived in that same area. For emphasis, he included the statement of his “law,” italicized, at both the beginning and the end of the paper, but unfortunately the message was still a bit cryptic. He never quite came out and said species A gave rise to species B. Some readers got the evolutionary message; others didn’t. Charles Lyell was so impressed by Wallace’s arguments that he started thinking much more seriously about whether species evolved from other species (although it wasn’t until ten years after publication of "The Origin of Species" that he finally conceded that they did). Weirdly, Darwin read Wallace’s paper and, at least initially, didn’t see it as either interesting or evolutionary. In the margins of his copy, he wrote, “nothing very new,” and, “It all seems creation with him.” His misreading of the “Sarawak paper,” as it came to be known, is especially odd, since he was already making virtually the same arguments for evolution based on the geographic proximity of similar species. It is hard not to think that Darwin, worrying that someone would scoop him, subconsciously distorted Wallace’s paper into something that didn’t overlap much with his own thinking and, therefore, didn’t threaten him.
Three years later, the parallel thinking of Darwin and Wallace would become unmistakable, and part of the lore of scientific history when Wallace, holed up with a malarial fever on the island of Ternate in the Moluccas (Maluku Islands), flashed upon the survival of the fittest as the mechanism for evolution. Within a few days, he had written a paper on the subject and, in what must be one of the most bizarre coincidences in the history of science, sent the manuscript to just one person, a man he barely knew, Charles Darwin. This time, Darwin got the point and nearly had a conniption. Had Wallace instead submitted the manuscript to a journal, we might now talk of Wallace’s theory of natural selection. Instead, after some behind-the-scenes machinations by Darwin’s friends Lyell and Hooker, papers by both Darwin and Wallace were read at a meeting of the Linnean Society on July 1, 1858. Darwin, finally spurred to action twenty years after he first thought of natural selection, quickly wrote "On the Origin of Species by Means of Natural Selection," the “abstract” of a much longer planned work that was never finished. And the rest is the Darwinian Revolution.
But that is part of another (and frequently told) story. The point I want to make here is that, even before Wallace’s fevered “Eureka!” moment about the survival of the fittest, he and Darwin already shared a common intellectual path. Both men had a profound interest in geographic distributions, and that interest had been critical to both of them in recognizing that species evolve from other species. Both had accepted the notion that the Earth and the life upon it have a history extending many millions of years into the past. For biogeography, what all of this meant was that these two men were trying to explain the distributions of living things within a new framework, a new set of assumptions about the nature of the world. It was the framework of descent through deep time.
In this new framework, some answers to biogeographic questions were no longer legitimate. Take the work of Edward Forbes, a contemporary of Darwin’s, who, during the 1840s and 1850s (he died in 1854) studied geographic distributions of European species, both terrestrial and marine. Forbes was, in Thomas Henry Huxley’s words, “an acute and subtle thinker,” and, like Darwin and Wallace, he was interested in general explanations for the similarities of species found in different geographic areas. But he was not an evolutionist, and that made all the difference.
Finding molluscs in the Aegean Sea that were similar but not identical to those off the coast of Scotland, he thought there must have been separate centers of creation in these regions. According to Forbes, God had seen fit to create nearly identical shelled creatures in two places because the environments were nearly identical. This idea of separate creations of similar (or even the same) species in different regions was popular at the time, but it obviously wasn’t the way Darwin and Wallace would have interpreted the same facts. They would have seen the evolutionary connections of these similar species and wondered where their ancestors had lived, and how the descendants had ended up in different areas. On the flip side, in this new worldview, some answers now seemed more reasonable than they had before. Oceanic dispersal events that were exceedingly unlikely over short periods, for example, might become probable given “deep time.” Basically, some very wrong assumptions—creation in various forms and the notion of a young Earth—had been replaced with the right ones. In short, before they became linked as the independent discoverers of natural selection, Darwin and Wallace had already become the first modern historical biogeographers.
In terms of published work, the landmark for this new biogeography was "The Origin of Species" and, in particular, its two chapters on geographic distribution. Like so much in "The Origin," reading those chapters is almost a jaw-dropping experience; even now, more than 150 years later, they could serve as a useful introduction to biogeography (although, as we will see, some modern researchers, following Croizat, view those same chapters as worse than useless). It must have been a revelation for naturalists reading these arguments for the first time, the jumble of disconnected facts of distribution suddenly all making sense, as if the discordant notes of an orchestra tuning its instruments had coalesced all at once into a symphony. It was all about species arising from other species and then moving about the Earth, limited by their powers of dispersal. Suddenly it was clear why island species are usually similar to those on nearby continents; why animals that cannot easily cross large ocean barriers, such as frogs and mammals, are missing from remote islands; why regions with distinct floras and faunas are separated by barriers to dispersal such as deep waters or deserts; why similar environments in widely separated parts of the world are populated by taxonomically distant species. And, in case people doubted the possibility of some of the ocean journeys that must have taken place, Darwin included a discussion of means of dispersal. The seed experiments were in there, and the duck’s feet and the pelican droppings. The idea of transport on icebergs got more than its fair share of space.
I think of the publication of "The Origin" as the death knell for the era of simply making stuff up about how God had ordered the distributions of living things. Now a legitimate study could not rest on one person’s idiosyncratic view of the Divine. Now things had to make sense in materialistic terms. It was, among many other things, the beginning of thinking about distributions broken up by oceans in modern terms.
Excerpted with permission from "The Monkey's Voyage" by Alan de Queiroz. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2014.