Niles Eldredge - The Darwin Blogs

The Darwin Blogs – August 9, 2006.

The Darwin Exhibition #7. The Voyage of the Beagle Part 4. Darwin’s Clues (cont’d)

Ask anyone what comes to mind first about Darwin’s voyage on HMS Beagle and you’ll likely hear the word “Galapagos.” The Galapagos archipelago sits astride the equator some 600 miles west of the Ecuadorian coast. Nowadays it is the wealthiest part of Ecuador—though the ecotourism and rapacious commercial fishing underlying its new-found wealth, along with the rapid rise in immigrant human population that has come along with them, now threatens to trash this, the single most important place in the history of evolutionary thinking.

But critical as his Galapagos experiences truly were to the development of Darwin’s evolutionary thinking, the patterns he saw there were not the first clues that alerted him to evolution that he encountered on the voyage. After all, he didn’t get to the Galapagos until September of 1835—nearly three years after the Beagle had set sail from England. And, as we saw in my previous blog, Darwin had had plenty to chew over with his collections of fossils mammals and his speculations on the extinction of species; and his observations on the replacement of species by “allied forms” as he traveled down the reaches of southern South America.

As we have seen, Darwin, perhaps more than any other naturalist of his day, saw the replacement of species in time (new species replacing extinct ones—like living armadillos replacing extinct ones) or in space (like one species of rhea replacing another one in southern Patagonia) in what we would today call “phylogenetic” terms: Darwin saw replacement of species of like kind, by what were often called “allied” forms, species belonging to the same “natural group.” This mode of thinking went back at least as far as the mid-eighteenth century, when Linnaeus presented his classification scheme—going so far, for example, to name the human species (Homo sapiens) and classifying us with the great apes and other Primates. Yet all this while still denying evolution: it was possible to think in terms of “natural groups” of “allied forms” without being a transmutationist—a position Darwin himself seemed to have adopted before even setting sail on the Beagle—to judge from his observations made in his earliest notes while on the journey.

Yet, as we also saw in the last blog, Darwin’s insistence on thinking phylogenetically when it came to analyzing patterns of replacement of species in space and time was by no means typical of his fellow naturalists. Charles Lyell preferred to think of replacement of extinct species by newly created species in ecological terms: for example, a new species of carnivore could not be created until its prey species was already there in place. And the great French anatomist Cuvier, despite his keen ability to see the relationships between, for example, living elephant species and extinct fossil mammoths, thought of replacement of extinct species by modern ones mostly in terms of the migration of pre-existing faunas from one part of the world to another.

And that is why Darwin limited his analytic gaze to species in groups only known from South America: Darwin focused on the replacement of one species by another in time or in space belonging to natural groups that are found nowhere else in the world. In effect, Darwin imposed an experimental control on the entire phenomenon of the replacement of species—eliminating the possibility that new species had simply come from somewhere else in the world.

And Darwin had begun, however cautiously and cryptically, to consider an explanation for the replacement of species in terms of natural law. Deaths of species—extinction—had already been explained in terms of natural law: Lyell, for example, always thought in terms of a physical cause of the extinction of any given species. Darwin began to take the next logical step and consider the appearances of new species taking the place of extinct ones, also in terms of natural law. Historians M.J.E. Hodge and Sandra Herbert have both remarked particularly on a brief essay Darwin wrote on the Beagle—simply but tellingly entitling it “February 1835.” In this essay, Darwin takes issue with a number of points Lyell raises in Vol. 2 of his Principles of Geology—which was written as a diatribe against transmutationalism, and specifically against the ideas of Lamarck.

In February 1835, Darwin makes the analogy between the birth and deaths of individuals and the births and deaths of species—agreeing in this instance with Lyell that it is plausible to think of species’ births and deaths. He also makes it clear that his focus is phylogenetic and, further, restricted to groups of allied species that are themselves restricted to South America. I agree with M.J.E. Hodge that it is impossible to conclude definitively that Darwin was a transmutationist when he wrote this little essay. But, on the other hand, one cannot categorically reject that possibility either.

It was later that year that Darwin reached the Galapagos. Much has been made of his overlooking what years later came to be known as “Darwin’s finches.” So named by the ornithologist David Lack, these birds have become most famous through the work of Peter and Rosemary Grant. They have now become the textbook example not only of natural selection in the wild, but also of speciation and adaptive radiation.

Also unknown to Darwin while on the Galapagos is the important story told by the plants.

Originally Darwin had sent his plants to Henslow for study—but his old mentor had moved on in life, and never really worked seriously on Darwin’s Galapagos plant collection. That task had to await the ministrations of Joseph Hooker, son of the head of Kew Gardens in London. It wasn’t until sometime in early 1844 that Darwin began getting reports from Hooker about the patterns of plant differentiation on the various different islands in the Galapagos.

But that didn’t mean that Darwin completely missed the basic evolutionary pattern seen in so many of the species found on the Galapagos archipelago. Before the trip was over, Darwin enunciated clearly in his notes the gist of this pattern: there are differences from island to island between species “closely allied” to mainland South American species—and these differences are often so minor that it is impossible to say whether they represent different “varieties” of the same species living on separate islands—or whether they are indeed different, albeit very similar, separate species. But the differences, however, are there: all the birds of one type will have a certain color pattern, or eye color on one island—different from those found on other islands.

The birds Darwin focused on in particular were the mockingbirds—very similar to those he had seen in various parts of mainland South America. Mockingbirds are mimic thrushes—and restricted to the New World. During the final months of the trip, Darwin was writing up his notes while Captain FitzRoy made one last dash across the Atlantic to make some more nautical observations in Brazil (evidently much to everyone else on board’s dismay). When he got to the Galapagos mockingbirds, Darwin also threw in some thoughts about the Galapagos tortoises—and reflected as well on earlier observations he had made on the foxes of the Falkland Islands. I’ll let Darwin recount the Galapagos pattern in his own words—as written in his Ornithological Notes:

“These birds are closely allied to the Thenca of Chile…I have specimens from four of the larger islands…The specimens from Chatham and Albemarle Isd appear to be the same; but the other two are different. In each Isld. each kind is exclusively found; habits of all are indistinguishable. When I recollect, the fact that the form of the body, shape of scales & general size, the Spaniards can at once pronounce, from which Island any Tortoise may be brought. When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure and filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware, is the constant /asserted difference—between the wolf-like Fox of East and West Falkland Islds.”

In the Darwin exhibition, we juxtaposed a specimen of an extinct, armadillo-like glyptodont with taxidermied specimens of two of the nine different species of armadillos still living in South America; with them, we also showed a taxidermied specimen of a living three-toed sloth alongside the skull of an extinct giant ground sloth. (The fossils are casts of actual specimens—rendered in fiberglass so kids can touch them). We said that these are examples of the replacements of extinct species by modern ones that helped lead Darwin to his evolutionary ideas.

We also juxtaposed taxidermied specimens of the greater and lesser rheas—and said again that the rheas were an example of the sort of replacement patterns of closely similar species that Darwin saw as he traveled down the South American continent—another pattern that eventually led Darwin to his evolutionary ideas.

With the Galapagos pattern, we showed four taxidermied tortoises (emphasizing in particular the difference between “saddle-backed” and dome-shaped carapaces). And we showed a sample of mockingbirds from various different islands.

But most thrilling of all, just as the visitor is about to exit the Beagle section of the exhibition to enter the next section (London, with its very different look, feel and storyline), we have a case with a page from these very Ornithological Notes. It never fails to make the hair stand up on the back of my neck when I read the sentence, in Darwin’s own hand, that follows on from his comments on “the constant/asserted difference—between the wolf-like Fox of East and West Falkland Islds.”:

First writing the more definitive “for such facts undermine that stability of Species,” Darwin went back, probably immediately, and inserted “would,” in my opinion a precautionary insertion of ambiguity in case anyone—including the Captain, might read his notes. Darwin’s secrecy over evolution, I feel sure, started while he was on the Beagle.

And I agree wholeheartedly with Darwin’s granddaughter Nora Barlow (who was the first to publish Darwin’s Ornithological Notes), and with David Kohn and his colleagues who have recently re-examined the meaning of this passage, that Darwin reveals himself to be a thorough-going transmutationist by the time he wrote these words around July of 1836.

To me, seeing his patterns of geographic replacement on a micro-scale, with differences so slight that it is hard to say whether different forms on different islands are varieties of the same species, or in fact closely similar yet truly separate, different species was the pattern that erased any lingering doubt about evolution in Darwin’s mind. The same ingredients are there as were in the earlier two replacements of species in space and in time: comparison of species/varieties that are clearly “closely allied” (the phylogenetic component); and the emphasis on species that are native strictly to that part of the world.

The Beagle arrived back in England in October, 1836. Soon thereafter, Darwin writes his first definitive version of an evolutionary theory. The theory is based on his Beagle experiences—and in my opinion reflects thoughts he had already long since entertained while still on the Beagle. It was the safety of home, and not the opinion of various experts that did indeed confirm his conclusions, that for the first time allowed himself a measure of freedom to record on paper (albeit still intended for his own eyes only) his evolutionary thoughts.

Niles Eldredge

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