The chief exponent of this view was the Manx naturalist Edward Forbes (1815–1854), a brilliant and creative thinker who is now best known, somewhat unfairly, as someone who was consistently wrong in his theorizing. Perhaps the biggest of Forbes’s blunders was his advocacy of the existence of vast former continents or continental bridges that have disappeared without a trace. To be fair, there are elements of truth in the idea. Forbes published a lengthy memoir in 1846 aiming to explain striking disjunctions in species’ ranges, like the similarities in the arctic and alpine flora of Scotland, Scandinavia, and Switzerland. How did plant species of the arctic regions, certain saxifrages, say, also end up high in the Alps, separated by vast stretches of inhospitable habitat? His explanation was recent subsidence of large expanses of land during the recent glacial period, followed by uplift. During subsidence the ocean covers continental areas, and any higher regions—the tops of ancient mountains of Wales and Scotland as well as the younger but still very old Alps—become islands in a chill sea. In such a cold climate the northern species easily disperse southward on island stepping stones. As the climate warmed, Forbes suggested, and the seas receded by uplift of the land, we are left with disjunct populations: the arctic species can only persist at the highest elevations in the Alps, so they are isolated there far from the main range of the species which occurs at lower elevations much farther north.
This model was far from unreasonable, and proved extremely influential. In spirit it is not so far off the modern explanation—the cool climate of glacial periods did indeed play a role in creating such disjunctions, but not in the context of a subsided and flooded European continent. Darwin put his finger on this idea as early as 1842, in the brief sketch of his species theory. He opened his discussion of geographical distribution stressing the importance of barriers but then immediately turned to alpine plants, proposing precisely the same kind of climatic shifts as Forbes, explaining the ebb and flow of high-latitude plants according to cycles of climate. Darwin’s model did not involve wholesale subsidence and uplift of the continent, however, but simply the natural climactic push and pull of species according to the dictates of their environmental needs. During cold periods high-latitude species are pushed south and high-elevation species pushed down. As climate warms and alpine ice shrinks, “arctic fauna would take place of ice, and an inundation of plants from different temperate countries [would] seize the lowlands, leaving islands of arctic forms.”8 Darwin elaborated on this in his longer species Essay of 1844, and dedicated a whole section of the Origin to the idea. There he gave Forbes full credit, though he was disappointed that he hadn’t published it first: “I was forestalled in only one important point, which my vanity has always made me regret,” he wrote in his Autobiography years later. “Namely, the explanation by means of the Glacial period of the presence of the same species of plants and of some few animals on distant mountain summits and in the arctic regions. This view pleased me so much that I wrote it out in extenso, and I believe that it was read by Hooker some years before E. Forbes published his celebrated memoir on the subject. In the very few points in which we differed, I still think that I was in the right.”9
Some aspects of Forbes’ model stand today, but not his idea of uplift and subsidence. That aspect of his model is problematic enough, but Forbes’ real blunder comes in a related part of his 1846 memoir. Having nicely explained the alpine and arctic disjunctions, Forbes turned to the problem of continental-island relationships: How did some species of the mainland also end up far out to sea on islands like Madeira or the Azores? “There are three modes in which an isolated area may become peopled by animals and plants,” he wrote. “1st. By special creation within that area. 2nd. By transport to it. 3rd. By migration before isolation.”10 Forbes reasoned against the first by pointing out that with a few exceptions the flora and fauna of the isolated areas he was considering are identical with continental species, implying that special creation of the same species in several locations did not make sense. He dismissed the second as insufficient, arguing that a few groups of high mobility might be accounted for in this way, but if dispersal by wind and water explains the sizable number of identical species in continental and island areas, shouldn’t this have made these areas completely similar? He thought it most plausible that colonization from “neighbouring lands” prior to isolation of the islands made sense for his model of movement of arctic species, and Forbes reasoned that there must also have been some sort of intervening land, or at least a scattering of island stepping stones, between the continent and these distant islands.
From a modern perspective we would agree with Forbes’ commitment to the idea of single rather than multiple centers of origin for species. We also recognize that sea level rises and falls with glacial cycles, exposing continental shelves whenever it falls low enough and thereby unites the mainland with any outlying islands sitting on the shelf. When sea level rises, the lowlands are flooded and the higher areas once again become islands—termed, appropriately enough, land-bridge islands. (Note that seemingly isolated habitat islands like mountaintops are basically land-bridge islands too—climatic cycles open and close overland migration corridors.) Flora and fauna are free to move over the continuous land areas, and when sea level rises lots of those same species become marooned on those portions that become islands.
This is a very different dynamic from truly oceanic islands that never had any connection with a continent. In those cases, colonization is largely a matter of chance as well as factors like distance from the nearest mainland, and prevailing winds and currents. We expect lots of shared species between land-bridge islands and the mainland since continental species repeatedly migrate there as intervening land comes and goes. The last time that happened was during the last glacial maximum a mere 12,000–15,000 years ago. Forbes was right to be struck by the similarities he was seeing, and to argue that chance dispersal was unlikely given that degree of similarity. However, he took the idea of connecting continental land bridges to an extreme, proposing vast extensions of the European continent stretching far out to sea, in some cases across the entire Atlantic. Forbes saw the Atlantic islands like the Azores, Canaries, Madeira, Iceland, and others as remnants of a former continental mass, and inevitably this idea of an ancient continent in the Atlantic connecting western Europe, north Africa, and its outlying islands with North America, became associated with the mythical lost continent of Atlantis. To Darwin’s shock quite a few naturalists were taken with this idea, and some began postulating land bridges and disintegrated continents with abandon, which drove Darwin to distraction.
He, too, argued against multiple centers of origin, and agreed with Forbes about the idea of climatic fluctuations influencing the movement of species. This became known as the “dual flux” idea—climate and species shift in concert as a result of geological processes. But Darwin saw little evidence that the sea had flooded the European continent so far inland as to make islands of the Alps, and no evidence that vast continental areas once occupied the abyssal sea—certainly not in recent geological time. Climate-induced shifts in the distribution of species, not uplift, explained how species got to mountaintops, Darwin maintained. And as for those islands, he was convinced that the powers of dispersal were far underrated by Forbes and his followers.
Getting There
Although Darwin was a supporter of the Lyellian idea of slow uplift and subsidence of large areas of earth’s crust, he felt strongly that continental extensions of the kind that Forbes and others were proposing was unnecessary. Even worse than unnecessary—he felt the idea was deeply flawed, maybe spurred by his own growing commitment to Forbes’ self-rejected hypothesis that an isolated area might become “peopled” by animals and plants “by transport to it.” Very soon after becoming convinced of evolution in spring of 1837, Darwin realized the importance of barriers, and in the 1842 Sketch reflected on how “new forms” could arise on islands. “Here the geologist calls in creationists,” he concluded—clearly
not an explanation he found satisfactory. His preferred explanation is clear: “Discuss one or more centres of creation: allude strongly to facilities of dispersal and amount of geological change.”11
While the Sketch says little about dispersal abilities beyond a mention of icebergs, currents, and storms, his 1844 Essay goes into great detail. Let us not be too hasty in concluding that island species had to be created in situ, Darwin urges, “Considering our ignorance of the many strange chances of diffusion.”12 Or, for that matter, that they required land bridges or stepping stones to get there. Yes, stepping stones, which he called “intermediate spots,” could play a role, and must have in some cases, but his imagination was already running wild with scenarios for long-distance transport by floods and currents, whirlwinds and hurricanes: dispersal by birds, rafting quadrupeds carrying seeds in their stomachs or adhering to their fur, floating trees with seeds wedged in root masses, insects with seeds or eggs stuck to their legs, icebergs, and more.
Forbes published his continental extension idea in 1846, just two years after Darwin’s essay. Fellow naturalists proceeded to discover evidence for ancient continents and continental bridges in virtually every ocean basin. Darwin’s friend Joseph Dalton Hooker invoked a vast land mass in the southern ocean as the most likely explanation for the distribution of what we now call Gondwanan flora, distributed among the southern continents and circumantarctic islands. Hooker was not wrong outright—there was a southern continent, Gondwanaland, but it consisted of a union of existing continents, not a different landmass that had since sunk into the ocean depths. An appreciation of this would not come for more than a century, so in the meantime Darwin got more and more exasperated, especially with talk of Atlantis. Chagrined, he fired off a testy note to Hooker:
I got so wrath about the Atlantic continent, more especially from a note from Woodward . . . who does not seem to doubt that every island in Pacific & Atlantic are the remains of continents, submerged within period of existing species; that I fairly exploded & wrote to Lyell to protest & summed up all the continents created of late years by Forbes, (the head sinner!) yourself, Wollaston, & Woodward & a pretty nice little extension of land they make altogether! I am fairly rabid on the question & therefore, if not wrong already, am pretty sure to become so.13 (emphases Darwin’s)
As “rabid” as he was on the subject, he concluded the letter on a characteristically humorous note: “I must try & cease being rabid & try to feel humble, & allow you all to make continents, as easily as a Cook does pancakes.” That same humor, only thinly masking his frustration with the wanton raising and sinking of continents to explain every biogeographical puzzle, is on display in a letter to Lyell, the geological authority of the day. Darwin took Lyell to task for encouraging the continental extension nonsense. He declared that his “blood gets hot with passion & runs cold alternately at the geological strides which many of your disciples are taking.”
Here, poor Forbes [recently deceased] made a continent to N. America & another (or the same) to the Gulf weed [Sargasso Sea].— Hooker makes one from New Zealand to S. America & round the world to Kerguelen Land. Here is Wollaston speaking of Madeira & P. Santo “as the sure & certain witnesses” of a former continent . . . why not extend a continent to every island in the Pacific and Atlantic Oceans! And all this within the existence of recent species! If you do not stop this, if there be a lower region for the punishment of geologists, I believe, my great master, you will go there. Why your disciples in a slow & creeping manner beat all the old Catastrophists who ever lived.— You will live to be the great chief of the Catastrophists!14
“Don’t answer this, I did it to ease myself,” he concluded. Darwin may have been half-joking with Lyell, but only half. How much simpler was long-distance dispersal of individuals, or their seeds, eggs, or spores! Improbable as it was that, aided by wing or wave, propagules from a mainland could make it to distant islands, mere specks in the ocean, in the fullness of geological time even the odd and highly improbable event is bound to happen. All it takes is one successful colonization. To Darwin such an idea was not only a simpler explanation, but it was also testable.
In fact, by this time Darwin had already embarked upon an ambitious program of experiments. As with Darwin’s earlier barnacle studies, these experiments were prompted by Hooker, who threw down the gauntlet some years before by maintaining that mere dispersal could not explain the similarities among the flora of Tierra del Fuego, Tasmania, and New Zealand: “I fear that they neither belong to transportable species or orders or present any facilities for transport,”15 he wrote to Darwin in June of 1847, but reassured him with a promise of “an honest investigation.” Darwin was to be disappointed.
Hooker had served as assistant surgeon and naturalist on the Ross expedition to Antarctica and the subantarctic islands aboard HMS Erebus between 1839 and 1843 (the same HMS Erebus later lost on the ill-fated Franklin arctic expedition—the wreckage of which was only discovered in 2014 by the Canadian government). Over the next 15 years he produced his monumental three-volume Flora Antarctica: The Botany of the Antarctic Voyage. By 1853, when he completed the third volume—the Flora Novae-Zealandiae—Hooker had carefully weighed the evidence for and against continental extensionism, and the extensionists won his support. The obstacles to Darwinian dispersal were many: these scattered plant species are hardly abundant enough to produce many seeds for chance transport, he thought, and were unsuited for transoceanic travel, having “feeble vitality” and “soft or brittle integuments.” And what were the chances of seeds germinating anyway after floating in frigid saltwater for days on end? Take the Kerguelen Islands, an archipelago of the remote Antarctic ocean: “seedlings are extremely rare,” Hooker pointed out. “The seeds, if not eaten by birds, either rot on the ground or are washed away . . . if such mortality attends them in their own island, the chances [of survival] must be small indeed for a solitary individual, after being transported perhaps thousands of miles, to some spot where the available soil is pre-occupied.”16 No, he concluded, the chances of successful oceanic dispersal are remote; we must look elsewhere for an explanation for dispersal. The idea of movement across long-sunken land bridges seemed like the only reasonable alternative.
Darwin and Hooker had argued back and forth over the matter for a couple of years when Darwin decided to try to show Hooker the error of his views. In March 1855 he started what he called his “seed-salting experiments,” testing seed flotation and viability with homemade saltwater. He put seeds of various species—cabbage, cress, radishes, lettuce, carrots, onions—into a cellar tank with saltwater made with melted snow, to ensure the water was ocean cold. He then tried a variant using small open bottles kept in the shade outdoors. Both were successful: “These after immersion for exactly one week, have all germinated, which I did not in the least expect (& thought how you would sneer at me),” he crowed.17 Darwin impishly told Hooker that he initially held back on telling him of the success of this experiment in hopes that the skeptical Hooker would say he would eat all the plants Darwin could raise after immersion. Hooker would have had quite a nice salad—everything but the cabbage seeds did well after saltwater immersion. Darwin was elated that his data showed that long-distance dispersal by ocean currents is indeed possible. He then planted the seeds to prove they not only could travel many miles by sea, but would succeed in growing once they came upon land. Hooker was impressed.
About the same time that Darwin published the pictured request for information in the weekly Gardeners’ Chronicle, he followed up with a short report of his own results. This caught the attention of his long-time botanical correspondent Rev. Miles Berkeley, who offered to try his hand at the seed immersion experiments. Between the two of them they collected lots of valuable data, with Berkeley reporting successfully floating and germinating seeds of 53 species.
Darwin’s first letter on the effects of salt water on seeds, published in the Gardeners’ Chronicle for April 14, 1855 (no. 15, p. 242). He published sever
al open queries in this magazine and others over the years, asking readers for information or urging them to replicate his experiments and share their results—a nineteenth-century form of crowd-sourcing.
Meanwhile, annoyance with his friend Thomas Wollaston’s notion of a continental bridge linking Europe with the Azores, out in the Atlantic, undoubtedly led Darwin to undertake a set of “seed-salting” experiments especially with Azoran species. In the summer of 1855 he wrote to enlist the help of Henslow and his kids in gathering up the seeds of plants near their home in Hitcham that were also found in the Azores so he could test their germination rate after immersion in saltwater. He would reward the children with a few shillings for their help. “The experiment seems to me worth trying; what do you think?”18 he ventured to Henslow. By that fall Henslow and daughters provided seeds from 22 Azoran species, and Darwin delightedly paid up, sending the money on to Henslow’s “good little Botanists.”
Darwin’s sense of humor and humility often come through in the many letters he exchanged with Henslow, Hooker, and others over his “seed-salting” experiments. But as he tested more and more species, he couldn’t hide his pleasure at proving Hooker wrong time and again. It became a sport with his kids, who delighted in every experiment that rebutted Hooker. Then came Hooker’s observation that sent Darwin back to the drawing board: many seeds did not float on the saltwater. He overlooked the fact that most sank to the tank’s bottom, taking with them his spirits. “As for showing your satisfaction in confounding my experiments,” he wrote Hooker, “I assure you I am quite enough confounded—those horrid seeds, which, as you truly observe if they sink they won’t float. . . . The bore is if the confounded seeds will sink, I have been taking all this trouble in salting the ungrateful rascals for nothing.”19
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