Darwin came up with a new experiment, testing whether seeds, flowers, and even whole plants bearing flowers and seed pods and other fruits would float in seawater and germinate. These experiments ran all through the summer of 1855 and into the fall. By October his enthusiasm seemed to be waning, as he wrote Hooker: “I am sick of the job . . . NB. capsicum & celery seed have come up after 137 days immersion.”20 Hooker became as fascinated as he was amused by Darwin’s experiments, and duly provided him with all sorts of seeds and fruits from Kew Gardens. While the problem of the sinking seeds seemed at first to ruin Darwin’s argument for long-distance dispersal by ocean currents, he kept at it and the eventual results surprised him as much as Hooker. “I have almost finished my floating experiments on salt-water,” he wrote Hooker two years later, in April 1857. “72/94 sunk under 10 days—seven plants, however, floated on average 67 days each.— I then dried all these (with in each case, with pods, bits of twig & few leaves) & 62/94 sunk under 10 days, so that generally the drying had no great effect, but . . . sometimes it had great effect.”21 Dried asparagus was one, floating about 23 days when fresh but up to 86 days dried, and still “germinated excellently.” By his calculations, ocean currents could have carried that asparagus over 2800 miles. He concluded that about a tenth of all plants would float an average of 30 days dried, and with an average current of 33 miles per day they could easily make it across the ocean. By now Hooker received Darwin’s latest results as eagerly as did the Darwin children. But although Darwin’s seed-salting experiments were a great success, it turned out he was being overly optimistic about plant flotation in that letter to Hooker. He was eventually forced to acknowledge that, on balance, being ensconced in fruits is a hindrance, not a help, to seed flotation.
What he wasn’t ready to share yet was the results of his varied spin-off experiments, some of which were suggested by his children. One son came up with an imaginative experiment recounted in yet another letter to Hooker: “I must tell you another of my profound experiments!” Eight-year-old Franky suggested that if a seed-eating bird was killed over the ocean (by, say, lightning or hail), it could drift for some time. “No sooner said, than done,” says Darwin. An unlucky pigeon from his dovecote “floated for 30 days in salt water with seeds in crop & they have grown splendidly.” Anticipating the ever-skeptical Hooker’s reply, Darwin headed him off: “You will say gulls & dog-fish &c would eat up the [carcass], & so they would 999 out of a thousand, but one might escape: I have seen dead land bird in sea-drift.”22 Fish, however, were not so obliging. Darwin envisioned that fish might eat seeds washed out to sea, then either swim great distances or themselves be eaten by a bird that would then carry the fish, seeds in stomach, to distant shores where they would be voided and grow wonderfully. The letter to his cousin Fox quoted at the beginning of this chapter gives you an idea of how the seed-eating fish experiments went: not well.
He persevered, and as with so many of his research projects, recruited all manner of assistants to aid him in his seed-dispersal obsession, from friends and family to colleagues, acquaintances, and fellow naturalists around the world. In early 1857, at Darwin’s suggestion, the keeper of the aquarium at the Zoological Society’s gardens tried feeding wheat seeds to minnows. It was a success, the keeper reported, and what’s more: “a Fellow of the Society who is a great Angler told me, that he has taken Barbel and dissected them and found a quantity of Wheat in them . . . he says he has caught them near the water mills where the wheat has been spilt into the river.”23 So some fish would eat floating seeds after all. Darwin’s nephew Edmund Langton, son of Emma’s sister Charlotte and her husband Charles Langton, obligingly tried this experiment at the family’s home in Sussex, but without the zookeeper’s success. He planned on doing it again on a warmer day, thinking the weather might have been the problem, he promised his uncle. On another front, discovering that he could get dozens of plants to germinate from as little as 2 tablespoons of pond mud, Darwin thought that herons, ducks, and other water birds would carry seeds far and wide on their muddy feet. He implored his college friend Thomas Eyton, who studied ducks and other waterfowl, to wash the feet of water birds at his Shropshire estate and send him the dirty water. Eyton was ever happy to oblige Darwin, despite disagreeing with his friend’s evolutionary ideas.
Nature the Careful Gardener
Darwin had been thinking about the implications of experiments such as these for a long time. A decade earlier, in the mid-1840s, he recorded several dispersal-related studies in his Questions and Experiments (Q & E) notebook: “shoot tame duck on pond with Duck-weed—coots—waterhens—examine dog, which has swum . . . every kind of seed must be distributed.” And: “Spread sheets of Paper covered with some sticky stuff in flat places & see whether wind, on windy day, will drift many seeds . . . Have paper ruled in squares to facilitate investigation—Capital in middle of ploughed field—on hills.”24 It’s not clear if Darwin followed up on these ideas, but there is another experiment in this notebook that he definitely pursued later: one looking at whether predators could disperse seeds within the prey they consumed. In October of 1856 he took some birds with seeds in their crops to the Zoological Society and fed them to eagles and owls. The zoo keeper retrieved the resulting pellets for him and kindly separated out the seeds. Darwin then planted them and reported the results in his experiment book: “Nov. 13: Pellet from Snowy owl from Bird with seeds 18 hours in stomach: ([December]. Germinated 5. Oats. 1 Wheat. 1. Hemp 2. Millets).”25 So birds of prey could indeed disperse seeds carried within the birds they had eaten.
The experiment book is the most thorough record of Darwin’s experiments in that period, and the entries make for fascinating reading, full, as it is, with his research on fish, rats, pigeons, pelicans, storks, eagles, and even guinea pigs. Darwin was nothing if not thorough in his investigations. These notes were never intended for others to read, but we are exceedingly fortunate to have them, as they are a portal into Darwin’s working method.
His creativity wasn’t limited to the varied ways that seeds are transported, either. Imagining ponds and lakes as islands on the land, he assumed they were colonized by aquatic plants and animals through accidental transport of one kind or another. As with so many natural processes, he imagined that something mundane, regular, and dependable would do the trick. What about waterfowl as a dispersal agent? There are untold millions of ducks and geese, and in the course of their lives they must visit innumerable ponds and lakes, inadvertently carrying all manner of seeds, algae, insects, eggs, snails, and more. Little by little, over the decades and centuries, pond dwellers surely hopscotch from pond to pond by the collective activity of waterfowl. Logical, but he needed to find evidence. It turned out that little was actually known about the propensity for waterfowl to accidentally carry stowaways. So, besides checking the muddy feet of herons and other wading birds for seeds, in June of 1856 Darwin turned his eye to the most obvious candidate for carriage by waterfowl: ubiquitous duckweed, the minute plant often mistaken for algae, found the world over covering the surface of lakes and ponds like tiny green confetti. Minute duckweed (the most common being genus Lemna) is the smallest of the flowering plants, lacking stems and leaves. If any aquatic species is going to get carried by ducks, surely this is the one. In what was surely a comical sight, he simply dunked a couple of ducks upside-down in a pan of duckweed-covered water and carefully counted how many stuck. And indeed, duckweed lived up to its name: “June 17th it does stick to feathers of Ducks,”26 he reported in his experiment book. We might have expected as much, but nothing should be taken for granted in science.
That was an easy experiment with a positive result. But Darwin needed to prove that more than plants and seeds could be carried from one place to another. He set up experiments aimed at determining how well aquatic snails survive out of freshwater and immersed in saltwater, “tormented,” he said, by the question of how land snails got to remote oceanic islands. In one experiment he immersed snails in saltwater for
a week. To his relief and delight, he wrote Hooker, in each of two trials the snails revived. “I feel as if a thousand pound weight was taken off my back.”27 Darwin only needed to show that survival was possible in conditions similar to natural ones. He didn’t need to have a large percentage of the mollusks survive, just show that some could survive an extended period exposed to saltwater.
Darwin did not fare as well with frog eggs—nor did Emma. In one experiment, he laid sheets of damp paper covered with frog eggs up and down the hallways adjacent to his study to see how long they would remain viable. After 3 days they dried up on the paper and did not revive when placed in water. This gives some insight into the curious family life of the Darwins at Down House—and how the experiments were a perpetual presence in the household. We have no record of how Emma felt about her home being turned into a field station, but chances are she not only took it in stride, she was experimentising too.
Failure did not make Darwin think his idea was wrong. Instead, he enlisted help. Puzzling over the dispersal of fish led him to write Edinburgh surgeon and naturalist John Davy, brother of the celebrated chemist Sir Humphry Davy, to do experiments on the viability of fish eggs out of water. Davy obliged by undertaking a series of experiments exploring how long fertilized salmon eggs remained alive when exposed to air and water (fresh and brackish) for different periods of time and at different temperatures. He concluded that moisture is critical for the survival of fish eggs during transit, which, incidentally, is not the only way freshwater fish might be dispersed: Davy told Darwin of an account read before the Royal Society of Edinburgh of a fingerling char that survived for 72 hours barely covered with water, but small enough that a bird could have carried it on its feet or stuck to its feathers. Davy concluded that his results confirmed Darwin’s hypothesis of freshwater fish dispersal, and would shed light on the geographical distribution of migratory fish such as salmon. Darwin was delighted, and passed on Davy’s report to the Royal Society.28 But he needed to show the possibility of dispersal for more species, if he were going to make an ironclad case.
March and April of 1857 saw Darwin doing some of the oddest experiments yet at Down House. Darwin had an aquarium that he jokingly called his “snailery,” with a thriving colony of two or three species of freshwater snails. Into it, he placed a pair of dried duck’s feet. He wanted to see if any snails would climb aboard, but more than that he wondered how long, once on the feet, the snails could survive out of water. The first time he tried the experiment, a small Planorbis (a freshwater air-breathing snail that looks a bit like a miniature ammonite) held fast, surviving some 20 hours on the mantel once the foot was removed from the snailery. He tried again, this time with Lymnaea snails, removing the feet to a small jar where they could be kept a bit damp. He repeated the experiment, and both times found the young snails survived 17–20 hours out of water. Freshwater snails clearly could endure a journey out of water hanging on to the feet of waterfowl, especially considering that ducks tend to close their feet when flying and the webbing inside remains damp. He reported his duck-foot findings in the Origin, pointing out that in 20 hours “a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point.”29
Unio clam adhering to a duck’s foot. From Darwin (1878), p. 121.
Nor did that end the subject for him. He was still at it a decade later, when he put a dried goose’s foot on the ground in a field and counted the snails and slugs that climbed on a day or so later. (They dropped off in about 5 hours.) And over a decade after that, in 1878, Darwin published a short letter in the journal Nature reporting an odd account of long-distance dispersal from a correspondent in Massachusetts who had shot a teal on the wing and found a living freshwater clam firmly clamped to its middle toe. “It would have undoubtedly been transplanted to some pond or river, perhaps miles from its original home, had the bird not been shot, and might then have propagated its kind,” Darwin’s correspondent wrote.30 Nature readers followed suit, with many people writing to Darwin with similar accounts. He compiled and published them with his own further observations in an article in Nature in early April 1882—his last publication to appear in his lifetime, as he died later that month.31 Thus, Darwin accumulated many rich observations—more than he could ever witness himself—over decades without leaving home.
Unexpected Means of Migration
Improbable as any given dispersal event is, Darwin was convinced that species have a remarkable tendency to get moved around in the fullness of time, whether through habitat corridors opened and closed by cycling climate and the rise and fall of land and sea, or by means of wind, water, and wing. He wanted to be the fly in the ointment to the continental extensionists, and although he lambasted his friend Charles Lyell over encouraging such explanations, Lyell himself was intrigued by Darwin’s novel approach to arguing against such views: observation and experiment. In fact, inspired by Darwin, Lyell collected dispersal information of his own. April 1856 found him visiting Down House, making notes about Darwin’s latest results with his seed flotation experiments, and the next month he had news for Darwin. A friend of a friend had found a freshwater limpet attached to a water beetle. “Here is a new light as to the way by which these sedentary mollusks may get transported from one river basin to another,” he wrote Darwin. “How far can an Hydrobius [water beetle] fly with a favourable gale?”32 Like Darwin’s ducks, this beetle carried a mollusk on the wing. Lyell followed this report of water beetle transportation services with another, this time one found bearing the egg sac of a water spider under its wings. “What unexpected means of migration will in time be found out,” he marveled.
Indeed, the “chance dispersal” view that Darwin championed would eventually become dominant in the field of biogeography, though even a century later it would have its detractors, some of whom dismissed transoceanic dispersal as “a science of the improbable, the rare, the mysterious, and the miraculous” (as famously quipped by ichthyologist Gareth Nelson).33 Such sniping was not uncommon in the 1970s, in a reaction against the difficulty of testing dispersal scenarios in natural circumstances, and the relevance of the new science of plate tectonics and continental drift to explain geographical distribution. Ironically, there is a kernel of truth behind the old idea of continental extensionism: rearranged and sometime contiguous continents, not long-sunken land bridges, explain the distribution of some groups. But chance long-distance dispersal has never gone away. Improbable and rare as such events are, they are far from mysterious, and certainly not miraculous.
A multitude of cases like Lyell’s water beetles have been documented, and the even rarer—or at least harder to show—phenomenon of long-distance “rafting” has been convincingly shown as well. For example, two back-to-back hurricanes had tracked through the Caribbean just weeks apart in the fall of 1995. Immediately after, a species of iguana known not to live on a particular island were documented there and shown to be in breeding condition. Herpetologist Ellen Censky and colleagues made a strong case for the iguanas having come from an island further west, traveling to their new home via huge mats of uprooted trees and other debris.34 But it is the unprecedented treasure trove of genetic evidence made possible through molecular techniques that has brought long-distance dispersal back to the fore,35 showing that in many cases relationships between groups separated by even vast expanses of ocean are best explained by oceanic dispersal, not continental drift. Certainly dispersal is the rule in regard to populating remote volcanic islands never connected with land. A gourd species (Sicyos villosus) collected by Darwin himself provides a nice example. In 1835 Darwin collected specimens of this gourd on Floreana Island in the Galápagos (and it has never been seen there since, perhaps having been grazed to extinction by feral livestock). A DNA-based analysis conducted 175 years later by a team of German and British botanists revealed that its closest relatives are found in North America and Mexico, whi
le a second Galápagos gourd species likely descends from a species in Peru and Ecuador.36 Ancestors of the two species floated to the islands at different times, perhaps thousands or even millions of years apart.
Darwin’s notion of the power of dispersal to explain geographical distribution was largely correct—“Nature, like a careful gardener, thus takes her seeds from a bed of a particular nature, and drops them in another equally well fitted for them,” as he put it in the Origin.37 Darwin’s view has not always been borne out, but his tenacity and creativity in testing his ideas nonetheless provides a powerful lesson in the pursuit of scientific knowledge. As with so many of Darwin’s pursuits, his long adventure collecting evidence of dispersal, carrying out quirky experiments in his homespun fashion, shows that as often as not all it takes to probe nature’s secrets is a bit of creativity and resourcefulness.
Experimentising: Getting Around
I. Sink or Swim: Seeds in a Pickle
Darwin’s “seed salting” experiments were aimed at determining, first, how long seeds might retain their vitality after exposure to saltwater, and second, how long they can float versus sink in saltwater. He performed several versions of this experiment using seeds from many species, including garden vegetables, common weeds, and tropical plants.
A. Materials
Saltwater: prepare anywhere from 1 to 10 gallons (3.8 to 38 liters) of saltwater, depending on the scale of your experiment. We’re interested in approximating seawater. Like Darwin, you can readily make artificial seawater with commercially available salt and mineral preparations. You can purchase your salt mixture from any of the numerous online aquarium supply retailers (Instant Ocean® and Natural Sea Aquarium Salt Mix from Oceanic Systems, Inc. are less expensive brands), or from a local pet shop or aquarium supply store. Or, make your own with this recipe:
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