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.
1.1 Great minds think alike: Alfred Russel Wallace (left) and Charles Darwin independently saw that the distributions of plants and animals are evidence of evolution. Both also came up with the theory of natural selection.
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.
Incidentally, Wallace would ultimately spend much more time than Darwin did studying geographic distributions. Wallace would write a two-volume work on animal biogeography and another book on island life, and he became known for delineating faunal regions with borders representing barriers to dispersal. Fittingly, the most famous of these borders, deep water running between various islands in the Malay Archipelago and separating the Asian and Australian regions, has become known as Wallace’s Line. Throughout his life, Wallace was considered a kind of poor man’s Darwin, both literally and figuratively, but in this one arena he didn’t have to play second fiddle: he ended up being dubbed the “father of biogeography.”
THE REVOLUTION THAT WASN’T
Darwin and Wallace had another belief in common, namely, that the continents had remained more or less fixed through deep time.4 This assumption influenced biogeographic explanations. If, for instance, Africa and South America had always been where they are now, then taxonomic groups found in both places, such as monkeys and ratite birds, must have either crossed the Atlantic Ocean, traveling over the water or on a now-sunken land bridge, or taken the long way round through the northern continents. This belief in the permanence of the continents was by no means universal; for instance, one popular theory in the late nineteenth and early twentieth centuries held that the Earth’s crust cycled through massive changes, with the continents of one era sinking to become the ocean basins of the next.5 However, there was almost complete agreement on one point: the continents, whether permanent or transient, did not move sideways to any great extent.
This is not to say that the notion of continents moving horizontally hadn’t been raised. In fact, by the beginning of the twentieth century, the idea of continental drift had a long, albeit mostly obscure, history. Way back in 1596, the Flemish cartographer Abraham Ortelius, seeing the jigsaw puzzle fit between the continents on opposite sides of the Atlantic, had suggested that those landmasses had once been joined and had drifted apart. Nothing much came of Ortelius’s idea, but in 1858, the same year that Wallace sent his paper on natural selection to Darwin, a French geographer named Antonio Snider-Pelligrini came up with a new version of continental drift. Anticipating later thought, Snider-Pelligrini suggested that all the continents had been joined together during the Carboniferous period, basing this inference on identical plant fossils found in Europe and North America as well as the long-recognized South America–Africa fit. He also suggested that what had driven the continents apart was material erupting from the Earth’s interior. Again, not many people took notice, but from about that time on, this apparently batty idea of continental drift kept cropping up, even if hardly anyone believed it. In an 1898 book and a 1910 paper, Frank Bursley Taylor, an amateur American geologist, presented a more complex scenario for continental movement that included the separation of South America and Africa along the Mid-Atlantic Ridge and the creation of mountains at the forward edges of moving landmasses. Taylor also proposed a mechanism for drift, a combination of tidal forces and a speeding up of the Earth’s rotation caused by the capture of a comet during the Cretaceous. That comet, according to Taylor, had become the moon. Taylor’s ideas didn’t make much of an impact either, doomed perhaps by a lack of evidence and the catastrophic nature of his moon-capture scenario, which didn’t sit well with the uniformitarian views of most geologists. Other continental drift proposals met with a similar lack of interest or worse.
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bsp; Much of what Snider-Pelligrini, Taylor, and others said would turn out to be true. Fossil similarities and the fit of South America to Africa really were evidence of the former attachment of continents, the mechanism for continental drift did involve material erupting from the interior of the Earth, and the Mid-Atlantic Ridge really was the line along which the Atlantic had opened up. But none of these early advocates of continental movement had presented much reason to believe their proposals. It would take a scientist very focused on the task to make a strong case for continental drift . . . and get shot down as well.
1.2 Alfred Wegener in 1910, the year he started thinking about continental drift.
That scientist was a quiet, intense man named Alfred Wegener, the son of a minister, born in Berlin in 1880. Wegener’s doctoral dissertation was in astronomy, but he was an eclectic researcher and, before his work on continental drift, he was best known as a meteorologist. He was clearly a bold character, both intellectually and physically, with, as one colleague noted, “fine features and penetrating blue-gray eyes.” He seemed almost a stereotype of a scientist, albeit a positive one—focused, serious, and uncompromising.
Wegener’s life reads like an adventure story. In his mid-twenties, he and his brother set a world record by floating in a hot-air balloon for fifty-two hours. Several years later, on a meteorological expedition, he and another scientist made a seven-hundred-mile trek on foot across the Greenland icecap. Running low on food, they had just killed their dog and were about to eat it when a group of Inuits arrived on the scene and helped them find their way to a settlement. Even the writing of his book on continental drift was connected to a physical ordeal: he finished it while recovering from a bullet wound in the neck suffered in battle during the First World War.
The Monkey's Voyage Page 4
