Niles Eldredge - The Darwin Blogs

The Darwin Blogs – August 3, 2006.

The Darwin Exhibition #7. The Voyage of the Beagle, Part 3. Darwin’s Clues.

Late in life, in his Autobiography, Darwin reminisced about his experiences on the Beagle, and what it was that he saw in the natural world that led him to become an evolutionist (or, more accurately in the terminology of the day, a transmutationist):

"During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that of the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the Continent; and thirdly, by the South American character of most of the productions of the Galapagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of the islands appearing to be very old geologically."

In notebooks, letters—and indeed, in the very first sentence of On the Origin of Species—Darwin had been making pretty much the same remark, starting not long after he returned to England in the Fall of 1836. But, in pointing to three distinct kinds of observations that led him to his transformational views, Darwin's passage in his Autobiography is his most complete account of his critical Beagle experiences and observations.

I think of these three sets of observations as patterns: Darwin saw examples of (1) newly appearing species replacing extinct species in time; (2) similar species replacing one another in space ("in proceeding southwards over the Continent"); and (3) on a microcosmic level, he saw similar species replacing one another on different islands in the Galapagos Archipelago. He had examples of each category—but then proceeded to generalize them, taking his one or two examples and seeing that they reflected general phenomena the world over: replacement patterns of species in space and time.

Historians rightly are wary of taking someone's reminiscences as gospel: people's memory plays tricks on them, and often they (whether consciously or not) distort the stories they tell of themselves. Worse, in historians' eyes, is the pitfall of writing "Whig history": of knowing the outcome, then turning around and interpreting prior events as inevitably leading up to the known outcome.

Both are valid caveats. But something led Darwin to break with his mentors, the Reverend Adam Sedgwick and especially the Reverend John Stevens Henslow, over the issue of transformationalism. And Darwin's repeated insistence that it was replacement patterns of species over time and space at the very least points out the direction in which to look as one pores over his geological and biological notes written while on the Beagle.

Darwin got lucky pretty early on in the voyage: in October, 1832—only about nine months into the journey—the Beagle was at Bahia Blanca, at the mouth of the La Plata River in what is now Argentina. There, at two places (Monte Hermoso and Punta Alta), Darwin discovered the bones of several species of fossil mammals. As he excitedly wrote in his letter to Henslow of October 26th, 1832:

"I have been very lucky with fossil bones; I have fragments of at least 6 distinct animals; as many of them are teeth I trust, shattered & rolled as they have been, they will be

recognised. I have paid all the attention, I am capable of, to their geological site, but of course it is too long a story for here.---- 1st. the Tarsi & Metatarsi very perfect of a Cavia: 2nd the upper jaw & head of some very large animal, with 4 square hollow molars.----& the head greatly produced in front.---- I at first thought it belonged either to the Megalonyx or Megatherium.---- In confirmation, of this, in the same formation I found a large surface of the osseous polygonal plates, which "late observations" show belong to the Megatherium.---- Immediately I saw them I thought they must belong to an enormous Armadillo, living species of which genus are so abundant here:3d The lower jaw of some large animal, which from the molar teeth, I should think belonged to the Edentata: 4th. some large molar teeth, which in some respects would seem to belong to an enormous Rodentia; 5th, also some smaller teeth belonging to the same order: &c &c."

So here, along with other intriguing finds, was Darwin's extinct armadillo-like animal that he talked about in his Autobiography. Darwin was aware that some naturalists had speculated that the "osseous polygonal plates" actually belonged to the extinct giant ground sloth that had already been described in the scientific literature. The speculations turned out to be groundless: Darwin had been right all along: he was seeing the carapace of glyptodonts—which, as Richard Owen showed in his 1840 monograph on Darwin's Beagle fossil mammals, really are extinct forms of armadillos.

These fossil discoveries of course did not instantly transform the young naturalist into a transformationalist. But it did obviously set him thinking. He had already read volume 1 of Charles Lyell's Principles of Geology. Later, he received volumes 2 & 3. Lyell's volume 2 was a diatribe against Lamarck—and the idea of transmutation in general. To come to transmutation, Darwin had to refute Lyell—no small task, especially for a young man who was a devoted fan of Lyell's way of doing geology. As Darwin recalled in his Autobiography:

"I had brought with me the first volume of Lyell's Principles of Geology (Lyell 1830--3), which I studied attentively; and this book was of the highest service to me in many ways. The very first place which I examined, namely St. Jago in the Cape Verde islands, showed me clearly the wonderful superiority of Lyell's manner of treating geology, compared with that of any other author, whose works I had with me or ever afterwards read."

But Charles Darwin was his own man—and his geological notes as the trip progressed reflect his first doubts over one of Lyell's firm convictions: that external, primarily physical, causes account for the extinction of species. Geologists and paleontologists had already accepted extinction as a reality, and were prone to attribute extinction to natural causes. Lyell in particular cited physical environmental events as the main cause of extinction—as do I and most other paleontologists of the modern era.

But Darwin couldn't see physical causation in the disappearances of his large fossil mammals. He thought the evidence clearly showed that the environment—semi-arid scrubland habitat—had been the same for his fossil fauna as it was for the living inhabitants of Patagonia. (The possibility that the arrival of humans may have been the cause of the extinction of these species was simply not considered a possibility then). Something else must be at work, driving these species extinct. Darwin's speculations along these lines eventually played a role in his embrace of transmutational views.

The second great clue Darwin encountered—the replacement of closely allied species geographically over the southern reaches of South America—also came to him early in the Beagle voyage. Darwin was particularly struck by the geographic relationship of the two species of ostrich-like rheas. The common rhea is replaced by a smaller, southern form—that for a while was known as "Darwin's rhea" (until it became clear that the species had already been named by the French). The two species actually meet, but do not interbreed, at the Rio Negro. Darwin didn't actually see the second species until he found himself munching on one at Christmas dinner, 1833.

Why, Darwin came to wonder, do similar species replace one another geographically? In addition to his rhea examples, Darwin began to document further examples—for the most part also involving birds. But it was the rheas that were uppermost in his mind—as his later, post-Beagle writings make very clear. Eventually, the rheas came to stand as the archetypical example of divergent evolution from a common ancestral species—the standard picture of gradual evolutionary divergence still present in textbooks on evolution today.

We must remember that Darwin came to the Beagle already familiar with transmutationalism. He had read his grandfather's book Zoonomia, and been harangued by Robert Grant on the virtues of Lamarck's ideas. And from his experiences with the creationist John Stevens Henslow, he learned about the importance of variation. And presumably from Henslow as well, he had developed a keen sense of what we would today call "phylogenetics." For back in the 1830s, it was perfectly natural and acceptable to entertain creationist views on the appearance of new species, but at the same time to think in terms of relatedness of closely similar species belonging to natural groups—like genera and families. In a sense, it was possible to be a phylogeneticist without being an evolutionist—something highly unlikely in today's world (though not entirely unheard of).

And Darwin was also accustomed to thinking in terms of historical processes of change—specifically, in the origin of geological features. He was trained to make observations that would allow him to account for the geological history of a region—something he began doing at St. Jago, the very first stop on the Beagle voyage. He learned this from his creationist geological mentor Adam Sedgwick—and from reading the creationist Charles Lyell's books while on the Beagle.

So Darwin thought about replacement of species strictly in phylogenetic terms: extinct armadillo-like species are replaced by modern armadillo species. One kind of rhea replaces another rhea species geographically. Creationists like Charles Lyell spoke about the extinction and replacement of species—but downplayed the phylogenetic connections, preferring to emphasize ecological patterns: at one point in his Principles of Geology, Lyell writes that a new species of carnivore cannot be created unless there is a suitable prey species already in existence. True enough—but Lyell was deliberating pointing away from the signal Darwin homed in on from the very first: species of the same natural group replace one another in space and time.

And there is one thing more that Darwin seems uniquely to have brought to the discussion—while still on the Beagle voyage: he saw the importance of focusing on examples of extinction and appearance of new species involving species only found in a particular portion of the world. Armadillos are edentate mammals—and only found in the Americas; likewise for rheas and Darwin's other examples of geographic replacement. And, as the quotation from his Autobiography that begins this blog makes perfectly clear, his third pattern—specifically involving the Galapagos—makes it clear that his replacement patterns from island to island also involve closely allied forms—that are, moreover, closely allied to species from mainland South America.

We'll complete this saga of Darwin and his three clues leading him to transmutation while on the Beagle, in the next blog—when we consider his experiences before, during and after his visit to the Galapagos Islands.

Niles Eldredge

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