Discover Darwin’s plants

For Charles Darwin, it was his extensive collection of flora across the four islands he visited that gave him a more powerful insight into the origin of new species.

It was September 1835 when Darwin stepped ashore on his first Galapagos island – San Cristobal. Just as today’s visitors observe, the landscape near the coast is a harsh, arid zone inhabited by only the toughest of species. “Nothing could be less inviting than the first appearance,” Darwin wrote in his Journal of Researches. “A broken field of black basaltic lava, thrown into the most rugged waves”, was “covered by stunted, sun-burnt brushwood, which shows little signs of life,” and the scrub, from a distance, appeared “as leafless as our trees during winter.”

Yet in the five weeks that HMS Beagle spent in the Archipelago, Darwin amassed a spectacular collection of Galapagos plants, estimated to comprise around one quarter of all species known today, and these specimens would play a pivotal role in his thinking.

It was only after the Beagle had sailed from Galapagos that Darwin’s thoughts on “transmutation” of species began to take a coherent form. If two populations of the same species became isolated, the struggle for survival and reproduction in the face of slightly different pressures might lead each population in different directions. This process of “natural selection”, Darwin argued, would inevitably lead to evolutionary change and eventually to the origin of new species.

“I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted,” he wrote in his Journal. In 1837, Darwin’s bird specimens, particularly the Galapagos mockingbirds, alerted him to the possibility that the Galapagos flora and fauna would be allied with the productions of South America, that there would be an extraordinarily high level of novelty and that most new species would be confined to a single island. But his conservative collection of birds (he only ever collected one voucher specimen for each species) and his failure to record the island of origin for many of these specimens meant that the Galapagos birds could not furnish him with the robust evidence he needed to test whether different islands were, in fact, “differently tenanted”.

Darwin’s collection of Galapagos plants, however, was much more thorough. “From my ignorance in botany, I collected more blindly in this department of natural history than in any other,” he wrote. In fact, Darwin was not wholly ignorant and was fairly sure that the Galapagos plants looked a lot like those from mainland South America, but he did not make assumptions about whether a plant on one island was the same as that on another. He just collected pretty much everything he came across, with a view to getting the specimens described by an expert back home.

The task fell to up-and-coming botanist Joseph Dalton Hooker. At first, Hooker did not anticipate a huge amount of novelty in Darwin’s Galapagos plant collection. “The species will no doubt be peculiar, but they may not form peculiar genera of more than one or two species.” In other words, Hooker expected that most of the Galapagos plants would turn out to be species already described on the South American continent or elsewhere.

Darwin remained hopeful. “I hope the Galapagos plants…will turn out more interesting than you expect,” he replied, urging Hooker to pay particular attention to the differences between islands.

Within days of getting stuck into Darwin’s collection, Hooker was in for a real surprise. “The Galapagos plants are far more extensive in number of species than I could have supposed,” he wrote to Darwin in great excitement. Although Darwin’s Galapagos plants resembled the continental vegetation, almost half of the flora appeared to be endemic, found only in Galapagos. Even more interestingly, Hooker noted that almost every species of flowering plant and fern that he described was confined to just one island. This “most strange fact,” he wrote, “quite overturns all our preconceived notions of species radiating from a centre.”

Why would this Archipelago be home to so much novelty and why would each endemic species appear to have its own dedicated island? The dispersal of seeds – floating on oceans or air-lifted by birds – simply could not account for this remarkable distribution. The only rational explanation was that each island had somehow set the stage for the origin of its own unique flora.

Darwin was over the moon. “I cannot tell you how delighted & astonished I am at the results of your examination,” he wrote to Hooker, “how wonderfully they support my assertion on the differences in the animals of the different islands.” Then, in wrapping up this letter, Darwin confided that “I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable.”

Hooker’s work reached Darwin just in time for him to weave them into the second edition of his Journal of Researches, published in 1845. The chapter on Galapagos ballooned to accommodate the new material and Darwin became more confident and expansive in his conclusions. “Reviewing the facts here given,” he wrote, “one is astonished at the amount of creative force, if such an expression may be used, displayed on these small, barren, and rocky islands.”

And it was in direct response to Hooker’s findings that Darwin reached this poetic conclusion: “Both in space and time, we seem to be brought somewhat near to that great fact – that mystery of mysteries – the first appearance of new beings on this earth.”