Charles Darwin's Beagle Voyage, Fossil

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Journal of the History of Biology (2010) 43:363–399 Springer 2009 DOI 10.1007/s10739-009-9189-9

Charles Darwin’s Beagle Voyage, Fossil Vertebrate Succession, and ‘‘The Gradual Birth & Death of Species’’

PAUL D. BRINKMAN North Carolina Museum of Natural Sciences 11 W. Jones Street Raleigh, NC 27601-1029 USA E-mail: [email protected]

Abstract. The prevailing view among historians of science holds that Charles Darwin became a convinced transmutationist only in the early spring of 1837, after his Beagle collections had been examined by expert British naturalists. With respect to the fossil vertebrate evidence, some historians believe that Darwin was incapable of seeing or understanding the transmutationist implications of his specimens without the help of Richard Owen. There is ample evidence, however, that he clearly recognized the similarities between several of the fossil vertebrates he collected and some of the extant fauna of South America before he returned to Britain. These comparisons, recorded in his correspondence, his diary and his notebooks during the voyage, were instances of a phenomenon that he later called the ‘‘law of the succession of types.’’ Moreover, on the Beagle, he was following a geological research agenda outlined in the second volume of Charles Lyell’s Principles of Geology, which implies that paleontological data alone could provide an insight into the laws which govern the appearance of new species. Since Darwin claims in On the Origin of Species that fossil vertebrate succession was one of the key lines of evidence that led him to question the ﬁxity of species, it seems certain that he was seriously contemplating transmutation during the Beagle voyage. If so, historians of science need to reconsider both the role of Britain’s expert naturalists and the importance of the fossil vertebrate evidence in the development of Darwin’s ideas on transmutation.

Keywords: Darwin, transmutation, fossil vertebrate succession, Beagle, agouti, Megatherium, armadillo, nineteenth century

What an immense ﬁeld for reﬂection is opened to the mind of the philosopher, by the survey of the discoveries to which fossil osteology has conducted us! Edward Pidgeon, 18301

1 Pidgeon, 1830, p. 39. 364 PAUL D. BRINKMAN

Introduction

Though it is well known that Charles Darwin made an important collection of vertebrate fossils in South America while serving on HMS Beagle,2 relatively little serious attention has been given to the meaning and significance that these specimens held for him during the course of the voyage. Among scholars who have written about the role of the fossil vertebrate evidence in the development of Darwin’s thinking about the mutability of species there are two competing hypotheses. One early conversion hypothesis holds that he was immediately and profoundly impressed by the discovery of fossils in South America, which closely resembled the local, extant fauna, and that this – together with other evidence accumulated during the voyage – led him to question the fixity of species while still on the Beagle. Darwin himself first advanced this idea. In at least two publications, he claimed that he first recognized the phenomenon of fossil vertebrate succession during the voyage. The first sentence of On the Origin of Species, for example, reads: When on board HMS Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to throw some light on the origin of species.3 In an autobiographical sketch written for his family and published by his son in 1887, Darwin wrote: 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 on 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 ancient in a geological sense. It was evident that such facts as these, as well as many others, could only

2 See Darwin 1839; Moorehead, 1969; Keynes, 2003; Simpson, 1984, pp. 23–39. 3 Darwin, 1859,p.1. CHARLES DARWIN’S BEAGLE VOYAGE 365

be explained on the supposition that species gradually become modified; and the subject haunted me.4 This hypothesis was later promoted by his son, Francis Darwin, his granddaughter, Nora Barlow, the biologist Cyril D. Darlington, and others. British Geologist John W. Judd made the strongest case for the importance of the role of the fossil vertebrate evidence in the early development of Darwin’s evolutionary views.5 This view has since fallen into disfavor, however, and a late conversion hypothesis has emerged as the new consensus view among historians of science. Some of Darwin’s own remarks would seem to support a late conversion. In an 1877 letter to German naturalist Otto Zacharias, for example, he wrote: When I was on board the Beagle, I believed in the permanence of species but, as far as I can remember, vague doubts occasionally flitted across my mind. On my return home in the autumn of 1836, I immediately began to prepare my journal for publication, and then saw how many facts indicated the common descent of species so that in July 1837, I opened a note-book to record any facts which might bear on the question. But I did not become convinced that species were mutable until, I think, two or three years had elapsed.6 An oft-quoted journal entry from 1837 seems to pin down the very month of Darwin’s conversion: In July opened first note-book on Transmutation of Species. Had been greatly struck from about the month of previous March on character of South American fossils and species on Galapagos Archipelago. These facts origin (especially latter) of all my views.7

4 Darwin, 1959[1887], Vol. I, p. 67. Darwin made a similar claim in a number of letters, also, including C. Darwin to L. Jenyns, 25 (Nov. 1844), in: Burkhardt and Smith, 1987, p. 84; and, C. Darwin to C. Lyell, (Dec.) 27 (1859), in Burkhardt and Smith, 1991, p. 455. 5 See Darwin, 1909, p. xiii; Barlow, 1946, pp. 166–167; Darlington, 1959, p. 316; Judd, 1909, pp. 351–353; Judd, 1911. Obviously, early twentieth century scholars did not enjoy the easy access to Darwin’s manuscript materials that we have today. Con- sequently, their claims about Darwin’s conversion must be taken with a grain of salt. Francis Darwin and John Judd, on the other hand, had access to Darwin himself. Judd (1909, p. 337, footnote 1) noted that he and Darwin, late in the latter’s life, met peri- odically for ‘‘geology talks.’’ 6 Letter, C. Darwin to O. Zacharias, 1877. This quotation appears in numerous places, including Darwin, 1893, p. 175. 7 Quoted in de Beer, 1959,p.1. 366 PAUL D. BRINKMAN

The late conversion hypothesis draws considerable support from these remarks – especially the latter – which suggest that Darwin did not question the fixity of species until after the voyage. But because he contradicted himself in several places on this question, advocates of a late conversion cannot depend solely on Darwin’s own words. Instead, a number of scholars have argued that Darwin could not have become a convinced transmutationist without the aid of London’s expert naturalists. With respect to the fossil vertebrate evidence, two principal reasons are most commonly given to show why Darwin could not have appreciated the evolutionary implications of his fossils during the voyage. First, Darwin collected fossils for geological rather than zoological purposes, and he made little or no attempt to compare his fossil vertebrate discoveries to the living fauna of South America.8 Second, Darwin was not a competent enough comparative anatomist to notice or understand the anatomical similarities between the fossil and extant faunas of South America. This tradition emphasizes Darwin’s insuffi- cient experience as a naturalist, his lack of formal anatomical training, and his dependence on the expertise of the British scientific community. As early as 1888, Thomas Henry Huxley argued that Darwin’s ideas about evolution took shape after his return from the voyage: While at sea, [Darwin] diligently collected, studied, and made copious notes…. But with no previous training in dissection, hardly any power of drawing, and next to no knowledge of comparative anatomy, his occupation with work of this kind – notwithstanding all his zeal and industry – resulted, for the most part, in a vast accumulation of useless manuscript. …[U]ntil the relations of the existing with the extinct species … were determined with some exactness, they afforded but an unsafe foundation for speculation. It was not possible that this determination should have been effected before the return of the ‘‘Beagle’’ to England.9 Likewise, historian Sandra Herbert singled out the phrase ‘‘if one species altered’’ in the ‘‘Red Notebook’’ as Darwin’s first substantive speculation on transmutation to which she assigned a date no earlier than the end of January, 1837, after the voyage. ‘‘The factual basis for a

8 Gertrude Himmelfarb, who took an extremely dim view of Darwin’s work as a naturalist, argued that his ‘‘geological enterprise was kept quite distinct from the zoological.’’ See Himmelfarb, 1959, pp. 109–110. The quotation appears on p. 109. 9 Huxley, 1896[1888], pp. 271–275. Huxley, who was a champion of the profes- sionalization of British science, was careful not to give too much credit to Darwin, a gentleman-naturalist trained for the clergy (although see Desmond, 1997, p. 563 for a different interpretation). CHARLES DARWIN’S BEAGLE VOYAGE 367 transmutationist view’’ appears in Darwin’s Beagle-era notes, Herbert conceded, but judgments of fact on species and varieties required the input of professional zoologists for verification. Thus, it was only after his return to England that Darwin’s speculations on the stability of species could be tied definitively to his observations of nature.10 According to this view, Darwin needed the collaboration of London’s scientific experts, especially the comparative anatomist Richard Owen, who made the requisite study and classification of the fossil vertebrate portion of the Beagle collections, before he could interpret any of his data as evidence of transmutation. Vertebrate paleontologist George Gaylord Simpson, in summarizing the modern understanding of the phylogenetic relationships of Darwin’s fossil taxa, argued unequivocally that ‘‘Darwin’s collections of fossil mammals could not and did not lead directly to evolutionary conclusions. …[N]one of [his] fossils could reasonably be considered as ancestral, in an evolutionary sense, to any living species.’’11 Further- more, Michael Ghiselin argued that Darwin’s claim in the first sentence of Origin (quoted above) is a ‘‘dialectical maneuver’’ intended to stress the inductive nature of his discovery.12 In other words, readers are meant to understand that Darwin’s ideas were derived from an accumulation of facts found in nature, not mere speculation. A landmark paper by Frank Sulloway, published in 1982, argued persuasively that Darwin became a convert to transmutation in March, 1837, after consultations with Owen, John Gould, and others about the nature of his Beagle specimens. Though Sulloway placed most of his emphasis on the significance of Darwin’s extant Galapagos collections, his argument implied that Darwin’s views on the fossil vertebrate evidence remained pre-evolutionary until after the Beagle’s return. In fact, Sulloway made fairly short work of the fossil vertebrate evidence, arguing in a footnote that Darwin’s misidentifications and his failure to distinguish between several different forms confused the evolutionary implications of the paleontological evidence.13

10 Herbert, 1974, p. 236; Herbert, 1980. 11 Simpson, 1984, p. 36. 12 Ghiselin, 1984[1969], pp. 34–35. 13 Sulloway, 1982. The footnote appears on p. 353 and directs readers to pp. 88–92 of Sulloway’s unpublished thesis, ‘‘Charles Darwin and the Voyage of the Beagle (1831– 1836).’’ There Sulloway argued that it would have been ‘‘highly unlikely’’ that Darwin could have derived his ‘‘law of succession’’ from the fossil evidence without expert help. He added that ‘‘it was as a geologist and not as a zoologist that [Darwin] considered his fossils of value.’’ See Sulloway, 1969, especially p. 92. 368 PAUL D. BRINKMAN

In summary, the majority view among historians of science, based primarily on the weight of the biogeographical evidence provided by the Galapagos specimens, and following Sulloway, holds that Darwin did not become a convinced transmutationist until early 1837, after his return from the Beagle voyage. This now orthodox view can be found in many recent Darwin biographies and histories of biology.14 A biography by Adrian Desmond and James Moore, for example, describes the crucial roles played by Owen and geologist Charles Lyell in helping Darwin to understand the importance of his fossil vertebrate evidence: Lyell … in his [1837] presidential address to the Geological Society … drew out the conclusions from Owen’s ﬁndings: that fossil faunas are closely related to their living replacements. … Darwin came down to hear the talk [which] brought home the real importance of his fossils for the ﬁrst time. He sensed the close relationship between extinct megatheriums and glyptodons and the modern sloths and armadillos. Darwin had never expected this;on the voyage he assumed that he had found European and African mastodons and rhinos, not exclusive South American species. It pulled him up sharp, causing him to ask the key question: why is present and past life on any one spot so closely related [emphasis added]?15 This paper will argue that Darwin did not wait for consultations with Lyell and Owen before asking himself this key question. First of all, books in the Beagle library alerted him to a number of suggestive examples of fossil vertebrate succession – all from places he visited during the voyage. More importantly, he made his own collection of fossil vertebrates in southern South America, which, whether by design or by coincidence, put him in a position to test the idea of succession.

14 This literature is enormous. A few examples from my bookcase include: Mayr, 1982, pp. 408–409; Bowler, 1984, p. 154; Bowlby, 1990, p. 208 (who conceded that Darwin’s doubts about the ﬁxity of species developed during the voyage); Browne, 1995, p. 360; Herbert, 2005, p. 320; Quammen, 2006, p. 27; Browne, 2006, pp. 40–42. David Oldroyd provided a helpful review of the conversion literature up to 1982. He pointed out that the importance of the role played by Darwin’s fossil mammal collection is now regarded as less important than it formerly was, and that near consensus has been reached, placing his conversion after the voyage. See Oldroyd, 1984, especially pp. 360– 361. For a pair of recent counterexamples that argue for an earlier conversion, see Eldredge, 2006, pp. 41–44; Eldredge, 2008. Likewise, Kohn et al., 2005, argued that Darwin ‘‘crosses the threshold between creation and evolution’’ during the last leg of the Beagle voyage, although his new way of seeing species could only be settled by expert naturalists (p. 645). 15 Desmond and Moore, 1994[1991], p. 210. CHARLES DARWIN’S BEAGLE VOYAGE 369

There is evidence enough in his correspondence, his Beagle diary, and his specimen lists and notebooks to suggest that the patterns of resemblance between certain extinct fossil vertebrates he collected and some of the extant, endemic fauna of South America did not escape Darwin’s notice. On the contrary, his fossil observations during the Beagle voyage mark the starting point of an investigation that would later culminate with Darwin’s ‘‘law of the succession of types.’’16 Lyell’s Principles of Geology – which Darwin studied and used as a kind of ﬁeld manual – argues that fossils are uniquely suited for deriving the laws which govern the appearance of new species, so it seems certain that Darwin wondered why ‘‘present and past life on any one spot [was] so closely related’’ during the voyage. Darwin pointed to fossil vertebrate succession as one of the two or three key lines of evidence that per- suaded him of the validity of transmutation. Thus, the timing of his earliest thinking about fossil succession bears on the history of the development of his revolutionary ideas about evolution.

Fossil Vertebrate Succession in the Literature

Young, experienced in several branches of natural history, and full of enthusiasm and ambition for science, Darwin was an ideal choice to serve as unofficial naturalist on the Beagle. Darwin’s Cambridge mentor John Stevens Henslow, who had recommended him for the voyage, believed he was well-suited for the natural science enterprise of the expedition: collecting specimens, observing, and taking copious notes. Darwin was an avid hunter and beetlist. He had learned a great deal of marine invertebrate zoology and microscopy from Robert Grant, who once ‘‘burst forth in high admiration of Lamarck and his views on evolution’’ while they were collecting together.17 From Henslow’s lec- tures, Darwin learned systematic botany. In his research, Henslow emphasized the importance of delineating natural variation within species, and Darwin embarked on the Beagle with this broader concept of species firmly in mind.18 Most importantly, he took a crucial geological field excursion through Wales with Adam Sedgwick shortly

16 Darwin, 1839, pp. 209–210. 17 C. Darwin, quoted in Egerton, 1976, p. 454. Egerton also noted that Darwin read his grandfather Erasmus Darwin’s book, Zoonomia, with its ‘‘evolutionary concepts,’’ before going off to study in Edinburgh in 1826. An anonymous reviewer of this paper remarked that ‘‘no grandson of Erasmus Darwin was going to be closed minded on the subject of development.’’ 18 Kohn et al., 2005. 370 PAUL D. BRINKMAN before the voyage and was, according to historian James Secord, ‘‘one of the best-trained men of his age in Great Britain’’ in the geological sciences.19 Whatever shortcomings he had as a naturalist would be mitigated by the availability of ‘‘plenty of Books’’ in the Beagle library.20 He made liberal use of these books while his ship was at sea. By the time Darwin sailed, a number of reports of instances of fossil vertebrate succession had already graced the scientific literature, and at least three of these appeared in books he is known to have read on the Beagle. For example, one of the earliest known English accounts of fossil vertebrates in South America appears in a book entitled: A Description of Patagonia, and the Adjoining Parts of South America which was penned by Jesuit priest Thomas Falkner and published in London in 1774.21 Falkner’s book describes the many and varied observations made by the author during his extensive travels in southern South America, including a brief but important notice of fossil vertebrates. Falkner wrote: On the banks of the River Carcarania, or Tercero, about three or four leagues before it enters into the Parana, are found great numbers of bones, of an extraordinary bigness, which seem hu- man…. I myself found the shell of an animal, composed of little hexagonal bones, each bone an inch in diameter at least; and the shell was near three yards over. It seemed in all respects, except … size, to be the upper part of the shell of the armadillo…. These things are well known to all who live in these countries; otherwise, I should not have dared to write them.22 Darwin first read Falkner’s Patagonia sometime on or before 9 July 1832, while at sea, headed for Montevideo.23 The Beagle would shortly be sailing for Patagonia, and Darwin was busy preparing himself for the experience. In a letter to his sister Susan he wrote:

19 Secord, 1991, p. 133. On Darwin’s training and experience, see also Ruse, 1999[1979], pp. 32–35. 20 Henslow, quoted in Desmond and Moore, 1994[1991], p. 101. 21 Falkner, 1935[1774]. 22 Falkner, 1935[1774], pp. 54–55. According to the introduction by Arthur E. S. Neumann (p. viii), Falkner may have written a four-volume manuscript on his natural history observations in the Americas. The location of this manuscript is not recorded. Falkner’s name is consistently misspelled as Falconer in Darwin’s Journal and Remarks (1839). See Simpson, 1984, pp. 21–22, for some interesting confusion on Simpson’s part regarding Falkner’s identity. 23 A brief entry for 9 (July 1832), ‘‘Rio Corcovado.— Bones. [Falconer],’’ in Dar- win’s Rio notebook (EH1.10; English Heritage 88202330), p. 47b, establishes this date. CHARLES DARWIN’S BEAGLE VOYAGE 371

After laying in fresh water at M[onte] Video, we sail for Rio Negro [Patagonia].- Comparatively near as this is to the civilized world, yet the whole coast & interior country is totally unknown.- Falcners [sic] account, inaccurate as it must be, is the only one.24 Falkner’s account is particularly interesting because of his explicit comparison of the fossil remains he found to the living armadillo. He was able to make this evaluation despite his lack of training in comparative anatomy. Darwin, who visited the locality described by Falk- ner later in the voyage, would make the same comparison when he collected similar specimens (see below). Much later in Origin, Darwin would write: ‘‘In South America, a [closely allied] relationship is man- ifest, even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of La Plata [emphasis added].’’25 Another example of fossil vertebrate succession in the scientific literature comes from the renowned French comparative anatomist Georges Cuvier, who named and described the skeleton of another new fossil vertebrate from South America, Megatherium americanum, in 1796. The colossal and relatively complete specimen represented, according to Cu- vier, a new genus of edentate closely related to the humble sloths and armadillos of South America.26 This astonishing conclusion generated considerable excitement in the British scientific community. In 1833, geologist Roderick Murchison called it the most important comparison in Cuvier’s four volume work, Recherche sur les ossemens fossiles (1812).27 According to Sandra Herbert, ‘‘substantial portions of Cuvier’s summary views were available’’ in Edward Griffith’s English translation, The Ani- mal Kingdom, that was in the Beagle library.28 A supplementary volume, The Fossil Remains of the Animal Kingdom,byEdwardPidgeon,includesa lengthy treatment of Cuvier’s Megatherium, with a full page reconstruction of the mounted Madrid skeleton on which Cuvier’s description was indirectly based (see Figure 1). As for Megatherium, Pidgeon claimed that: [I]ts examination proves that it had more analogy to the sloths than to any other beings, especially in regard to the system of dentition

24 Letter, C. Darwin to S. Darwin, 14 July–7 August (1832). See Burkhardt and Smith, 1985, p. 248. 25 Darwin, 1859, p. 339. 26 See Rudwick, 1997, pp. 27–32, for an English translation of Cuvier’s description. 27 See Winslow, 1975, p. 351, note 4. 28 Herbert, 2005, p. 303. See also Herbert, 1995, p. 28, for a discussion of the ‘‘rich pool of ideas,’’ including the notion of ‘‘universal descent,’’ in Griﬃth’s text. 372 PAUL D. BRINKMAN

Figure 1. A complete reconstruction of Megatherium from a book in the Beagle library (from Pidgeon, 1830, p. 132).

… and the composition of the extremities. …The general forms of the head of the megatherium resemble considerably that of the bradypi [tree sloths]. But the most striking trait of resemblance consists in a long descending apophysis, ﬂattened, and situated at the basis of the zygomatic arch.29 Darwin read this text during the Beagle voyage.30 Additionally, a short article on Megatherium, written by French geologist Louis-Constant Prevost but based on Cuvier’s work, appears in Bory de Saint-Vincent’s Dictionnaire classique d’histoire naturelle, in which the fossil genus is characterized as a ‘‘cousin’’ of the tree sloths, partly on the basis of the oddly similar structure of their zygomatic arches.31 Darwin made pro- liﬁc use of all the reference materials on the Beagle, but was especially

29 Pidgeon, 1830, pp. 131–133. 30 Darwin quoted a phrase from this volume, ‘‘recent observations,’’ with reference to the alleged link between Megatherium and some ‘‘osseous polygonal plates’’ he collected (see below), in his geological diary (DAR 32.1, p. 66), which suggests that he definitely had access to it during the Beagle voyage. The phrase ‘‘late observations’’ in his letter to Henslow (see below) is a paraphrase of Pidgeon’s text. 31 Prevost, 1822–1831, Vol. 10, p. 309. Darwin likely borrowed his phrase ‘‘osseous polygonal plates’’ (see below) from Prevost’s ‘‘plaques polygons ossifiees.’’ CHARLES DARWIN’S BEAGLE VOYAGE 373 dependent on this one, writing: ‘‘I am well off in books, the Dic: Class: is most useful.’’32 A third example of succession appears in Lyell’s Principles of Geol- ogy. Comparative anatomists William Clift (in London) and Joseph Pentland (then working in Cuvier’s laboratory in Paris) each received a collection of co-mingled and unidentified fossil mammal bones recovered from a number of caves in Australia. Both examined the remains and referred some of them to extinct and others to living representatives of Australia’s endemic marsupial fauna. These results, together with a clear statement about fossil vertebrate succession written by Darwin’s former professor, Robert Jameson, appeared in a series of brief articles in the Edinburgh New Philosophical Journal in 1831–1832.33 Lyell then incorporated these facts into Volume III of his Principles. In discussing the significance of Australia’s fossil kangaroos and wombats he wrote: These facts are full of interest, for they prove that the peculiar type of organization which now characterizes the marsupial tribes has prevailed from a remote period in Australia, and that in that continent … many species of mammalia have become extinct. It also appears … that land quadrupeds, far exceeding in magnitude the wild species now inhabiting New Holland, have, at some former period, existed in that country.34 Darwin was known to have had Lyell’s Volume III in his possession by at least July, 1834, and his enthusiasm for the geologist’s work is leg- endary. There are two important points to be taken from this literature. First, though the general idea of fossil vertebrate succession was not yet in wide circulation in the early 1830s, some of the facts on which this generalization would later be based were already well known. Darwin was familiar with these suggestive facts from at least four sources in the Beagle’s library. Lyell discussed a sweeping array of these facts – including new ones brought to light during the Beagle voyage – in his presidential address to the Geological Society of London in 1837 which Darwin attended as his invited guest.35 Darwin himself would then be the first to generalize these facts into his ‘‘law of the succession of types’’

32 Letter, C. Darwin to J. S. Henslow, 18 May–16 June 1832. See Burkhardt and Smith, 1985, p. 237. 33 Pentland, 1831–1832; Jameson, 1831. William Clift’s report appears in Jameson’s article. 34 Lyell, 1830–1833, Vol. III, p. 144. 35 Desmond and Moore, 1994[1991], p. 210. See also Lyell, 1837. 374 PAUL D. BRINKMAN in 1839. Owen commented about succession in 1840, in his description of Darwin’s Beagle fossils, and then made a much fuller and clearer statement on this phenomenon in 1844 (without crediting Darwin or citing Lyell).36 Second, at least some of the resemblance – at a super- ﬁcial level – between recently extinct and extant taxa of South America was so apparent that it was not necessary to be an expert comparative anatomist to notice it. Nor was it necessary to have access to a vast collection of comparative material at one of the museums of metro- politan Europe. Professional expertise and museum access were necessary for veriﬁcation of specimens, but not for initial recognition.37 Falkner, an amateur naturalist working on the periphery of science, recognized the obvious resemblance between the strange fossil shells and the living armadillos. So did Darwin.

Darwin’s Vertebrate Fossils

Darwin made a significant collection of vertebrate fossils during his travels in South America. He recognized that many of his specimens were: (1) new to science; and, (2) extinct. Though his interest in fossils was mainly geological, he made an effort to identify them and he speculated on their ecology – or, what Darwin might have called their place in the economy of nature – and the cause of their extinction. In trying to identify his specimens, he compared them to descriptions and figures of vertebrate fossils and other animals in the Beagle literature, and – more important – to certain representatives of the living fauna of South America. In a few cases, he clearly recognized a similarity. Several of these specimens corroborated the comparison he read in Falkner’s book, while others reinforced the idea that recent fossil remains bear a close resemblance to their living counterparts.

36 Owen, 1840, p. 110; Owen, 1844, p. 240. To be fair, Owen drew some of the conclusions that led to Darwin’s generalization in the first place, and then he assumed a leading role in discussing the subject comprehensively in the 1840s. Thus, he might have felt that he did not owe any debt of acknowledgement to Darwin or Lyell. Interestingly, Nicolaas Rupke argues that this could be the work that convinced a reluctant Owen of the fact of transmutation (see Rupke, 1994, pp. 223–224). Nevertheless, it appears that Darwin was the first to publish this idea in law-like form in his Journal and Remarks (1839). 37 I am grateful to historian Richard Bellon, who helped me see this idea more clearly. See also Camerini, 1997 (especially p. 360) for a brief discussion of the tension between museum and field work. CHARLES DARWIN’S BEAGLE VOYAGE 375

Darwin’s earliest fossil vertebrate discoveries in South America oc- curred in late September 1832, at Punta Alta, a rocky outcrop on the beach near Bahia Blanca, a struggling frontier village and fort on the coast southwest of Buenos Aires. While the Beagle rested at anchor in the bay, he went ashore and found a cache of fossils. His specimens from this locality were several, and included a partial skull, which he tentatively identified as ‘‘allied to the Rhinoceros;’’ a disassociated mandible, bearing a single tooth; and fragments of a bony shell.38 In early October, Capt. Robert FitzRoy landed a party about 20 miles east of Punta Alta with instructions to build a cairn at Monte Hermoso. FitzRoy credits a member of the shore party with the discovery of ‘‘many curious fossils in some low cliffs under the mount.’’39 Darwin later accompanied the captain to this site. There, he had the good for- tune to collect ‘‘some well preserved fossil bones of two or three sorts of Gnawing animals. –One of them must have much resembled the Agouti but it is smaller.’’ He also noted the local geology.40 In November, Darwin wrote a long letter to Henslow, summarizing his fossil vertebrate discoveries to date. He craved his mentor’s ap- proval, hinting, ‘‘If it interests you sufficiently to unpack them, I shall be very curious to hear something about [my fossils].’’ In his letter, he described six fossil vertebrate specimens in some detail, although the following excerpt pertains only to the first three: 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 recognized….—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 ei- ther 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’ (what are they?) show belong to the Megatherium.41 Darwin also wanted to impress Henslow with his willingness to go against the grain of current European scientific thinking. Naturalists in

38 Keynes, 2001[1988], pp. 107–109. 39 FitzRoy, 1839, p. 112. 40 See Keynes, 2001[1988], pp. 110–111. 41 Letter, C. Darwin to J. S. Henslow (ca. 26 October–24 November) (1832). See Burkhardt and Smith, 1985, pp. 279–281. The phrase ‘‘late observations’’ is a paraphrase of Pidgeon’s ‘‘recent observations’’ (see note 30). 376 PAUL D. BRINKMAN

Europe were then convinced that these enormous fossil shells, or ‘‘osseous polygonal plates,’’ as Darwin calls them, belonged to Meg- atherium. Cuvier had come to this conclusion – which would later be shown to be incorrect – on the basis of a report he had received of a fossil that he never had the opportunity to examine personally.42 Dar- win, on the other hand, was skeptical about this conclusion, asking Henslow for an account of the ‘‘late observations’’ on which it was founded. He had drawn a tentative conclusion, which he shared with Henslow, based on his own observations and bolstered by Falkner: ‘‘— Immediately I saw [the osseous polygonal plates] I thought they must belong to an enormous Armadillo, living species of which genus are so abundant here.’’43 Likewise, in his ﬁeld notebook, on the day of his ﬁrst Punta Alta fossil discovery, Darwin wrote: ‘‘there is a horizontal bed of earth containing much fewer shells—but armadillo.’’ Later in the same notebook he observed: ‘‘Megatherium like Armadillo case.’’44 In his geology notes Darwin was much more explicit about the resemblance between the osseous plates he collected and the cases of living armadillos. He even took the trouble to speculate about how these cases might have entered the fossil record: At Punta Alta the only organic remain I found in the Tosca (excepting mere particles of shell) was a most singular one: it consisted in an extent of about 3 feet by 2 covered with thick osseous polygonal plates; forming together a tessellated work: it resembles the case of Armadillo on a grand scale: these plates were double. or an interval of few inches between them.—With it was only a fragment of joint of extremity.—At present the case of the dead Armadillos are oftener found separate from the body. than connected with any part.—In this case the envelope of the great animal would easily be carried by the water. & by the pressure of the Tosca would be doubled up as described.45

42 See Huxley, 1865, pp. 31–43; Rudwick, 1997, 25–32; Herbert, 2005, pp. 303–306 for a history of the misunderstood ‘‘Megatherium’’ hide. 43 Burkhardt and Smith, 1985, pp. 279–281. Historian Paul H. Barrett argued that Darwin was reluctant to theorize about fossils, even in his personal notebooks, after being reprimanded for doing so by Adam Sedgwick during their geological tour of Wales, just before the Beagle voyage (see Barrett, 1974, p. 150). That he was now willing to speculate so openly to Henslow indicates something about how much Darwin had matured by this point as a naturalist. It also demonstrates how much Darwin held Henslow in high esteem. 44 See Barlow, 1946, pp. 166–167. 45 CUL-DAR 32.65-66. Portions of this same text are quoted and discussed in Her- bert, 2005, pp. 304–305. CHARLES DARWIN’S BEAGLE VOYAGE 377

As for Cuvier’s identification of these remains with Megatherium, Darwin remained skeptical, but he hedged his bets, noting: It is stated. that ‘recent observations’ show the Megatherium had such an envelope; it certainly is probable it belongs to some of the animals the bones of which are so abundant in the gravel.46 Darwin found similar remains elsewhere in South America, but he was inconsistent in his identifications. He referred to them variously in his diary and notebooks as ‘‘Megatherium hide,’’ ‘‘armour,’’ ‘‘armadillo like case,’’ ‘‘shell,’’ ‘‘plates,’’ and, very often, ‘‘Paludo’’ or ‘‘Paludas,’’ which are bastardizations of peluda [hairy], a Spanish word for hairy armadillos of the genus Chaetophractus. The osseous polygonal plates Darwin collected pertain not to Meg- atherium, but to a member of a related family of extinct, armadillo-like animals now called Glyptodontidae (Figure 2). Due to Cuvier’s error, confusion reigned in Europe for a number of years on the identity of these remains. Bits and pieces of fossil armor sent to Europe for identification in the 1820s and 1830s were often referred to Megatherium. By 1836, however, a number of naturalists had concluded that the armored ‘‘animal of South America is not the Megatherium and is allied to the Armadillos.’’47 Strangely, William Buckland, in his widely read Bridge- water Treatise of that same year, continued to associate the fossil armor with Cuvier’s Megatherium. In 1837, Peter Wilhelm Lund established the new genus Hoplophorus for a specimen of the armor and some associated bones, which, he wrote, were ‘‘closely allied to, if not identical with, the ‘Dasypus’ [armadillo].’’48 Owen then established his own new genus, Glyptodon, based on similar remains, in 1838. The upshot of all this is that Darwin’s hunch about the obvious relationship between the fossil armor and the living armadillos was correct (though whether he was correct or not does not matter for the purposes of the argument in this paper). This example goes to show that European savants were every bit as capable of muddling the fossil evidence as Darwin was. In the case of the dermal armor, at least, it proved advantageous to be a field naturalist in South America, where the fossils were preserved in prolific numbers and the living comparative materials were scurrying around underfoot. Most of the naturalists ‘‘on the ground,’’ Darwin included, noticed the similarity.

46 CUL-DAR 32.66. 47 Huxley, 1865, p. 35. Huxley’s authoritative redescription of the osteology of Glyptodon includes a useful history of this and related fossil genera. 48 Quoted in Huxley, 1865, p. 35. 378 PAUL D. BRINKMAN

Figure 2. Richard Owen’s ‘‘Gigantic Extinct Armadillo.’’ Sections of armor from these animals were initially referred to Megatherium. However, many ﬁeld naturalists, Darwin included, recognized their obvious resemblance to armadillos. Owen’s title acknowledges this resemblance also (courtesy of the Library of Congress via Sandra Herbert).

Darwin went on to describe three more fossil vertebrate specimens in his November letter to Henslow: 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.49 These examples, together with the Cavia mentioned previously, serve to show that Darwin did make an eﬀort to compare his fossil vertebrate specimens with living species in South America and that he did not keep his geological enterprise entirely separate from the zoological. Indeed, it would not make sense for a naturalist of Darwin’s period to treat geology and zoology as entirely distinct categories. They also demonstrate that Darwin was competent enough at comparative anatomy to recognize and appreciate certain similarities between fossil and living species. In his geology notes, Darwin elaborated on some of the other fossil remains he found in the Bahia Blanca area:

49 Burkhardt and Smith, 1985, pp. 279–281. CHARLES DARWIN’S BEAGLE VOYAGE 379

I could perceive traces of 4 or 5 distinct animals: two of which certainly belonged to the Rodentia. One must have been allied to the Agouti; the tarsi & Metatarsi belong to an animal less than the present. common inhabitant, Cavia patagonica. The Agoutis are all proper to S. America; & none have hitherto been found in a fossil state.50 Note that Darwin knew, even during the voyage, that agoutis are endemic to South America, and that his was the ﬁrst and only known fossil representative of this unusual group of rodents. Eighteenth- and nineteenth-century naturalists, including Darwin’s hero Alexander von Humboldt, were particularly interested in delineating patterns in the distribution of plants and animals. According to Michael Paul Kinch, a primary reason for this interest was because these patterns of distribution were expected to elucidate the question of species’ origins.51 One year later, in late August, 1833, Lt. Bartholomew J. Sulivan assumed charge of a shore party, including Darwin, encamped near Punta Alta for surveying. He patiently indulged Darwin’s compulsion to collect delaying the entire party while his shipmate waited for low tide to comb the beach for fossils. Several discoveries took place here, including one specimen embedded in solid rock just at the low-water mark. The lieu- tenant provided men to dig the specimen out while the rest of the party, Darwin included, repaired to Bahia Blanca.52 The commander of the nearby fort told Darwin that he had ‘‘often seen and heard of large Paludo Scales on cliﬀs,’’ by which he likely meant more examples of the giant osseous polygonal plates.53 The specimens collected on this visit were ‘‘more perfect’’ than those found previously, including ‘‘nearly an entire skeleton.’’54 Later in September, near the coastal town of Guardia del Monte, Darwin found ‘‘a perfect piece of the case of the Megatherium.’’55 In October 1833, Darwin searched for fossils north of Buenos Aires, in the locality described by Falkner, near the town of Santa Fe, and along the Rio Parana. On his way north, he spent a night just outside the town of Lujan. The original Megatherium specimen described by

50 CUL-DAR 32.70-71. 51 See Kinch, 1980. For a more comprehensive history of biogeography, see Browne, 1983. For more on the global mapping and measuring which was characteristic of ‘‘Humboldtian’’ science, see Dettelbach, 1996. 52 Sulivan, 1896, pp. 36–43. 53 See Barlow, 1946, p. 196. 54 Letter, C. Darwin to Caroline Darwin, 20 September (1833). See Burkhardt and Smith, 1985, p. 331. 55 Keynes, 2001[1988], p. 190. 380 PAUL D. BRINKMAN

Cuvier had been found in a bank of the Rio Lujan which Darwin crossed the following morning.56 He spent a day searching the cliffs of the Rio Carcarvana for fossils, finding and collecting a single, curious tooth. On the Rio Parana, he hired a canoe in an effort to collect some bones jutting from the bank just above the water. Unfortunately, these large bones, which he attributed to Mastodon, were very fragile and difficult to collect – Darwin escaped with only a few skeletal fragments and some teeth.57 During this journey Darwin recognized that there were at least two different types of bony carapace: ‘‘clearly two sorts of Megatherium—contemporaneous with Mastodon,’’ he wrote, ‘‘case of taller, two or three inches thick.’’ He also took the trouble to investigate a large ‘‘Paludas case in a Barranca [cliff] of red Tosca—The shell formed a well between 4 and 5 feet across, entire, but soft, no bones except a lump: the bones were said to be less than a full grown cow.’’58 It is a singular misfortune that Darwin was never able to locate significant skeletal material associated with one of these fossil shells in order to have something to compare directly with some of the other ‘‘Meg- atherium’’ bones he collected elsewhere. Absent this evidence, the anecdotal account he recorded here established – albeit tentatively – that the missing ‘‘Paludas’’ bones were allegedly less than a ‘‘full grown cow,’’ and therefore considerably smaller than a Megatherium (which Pidgeon likened to the size of a rhinoceros59). Lastly, Darwin collected a fossil horse’s tooth ‘‘well buried’’ in the ‘‘red compact Tosca.’’60 Stricken with fever during this trip, he returned to Buenos Aires by boat to rendezvous with the Beagle. A delay in sailing afforded Darwin the opportunity to make another overland collecting trip from Montevideo north to the Rio Negro in November. One of his acquisitions there was a large skull purchased from a local landowner.61 In his diary he identified this specimen as ‘‘a part, very perfect, of the head of a Megatherium.’’62 On this trip he also observed, but apparently did not collect, a number of interesting fossils, including: ‘‘Paludas with a tail very heavy and solid, fragment 17 inches

56 Keynes, 2001[1988], p. 191. Darwin neglected to mention the river’s paleontological significance, something about which he seems to have been unaware. He also seems to have been unaware of Lujan’s most prominent citizen, Francisco Javier Muniz, Argentina’s first naturalist and first fossil vertebrate collector. 57 Keynes, 2001[1988], p. 193. 58 Barlow, 1946, pp. 209–210. 59 Pidgeon, 1830, p. 35. 60 Barlow, 1946, p. 210. 61 Darwin, 1839, p. 181. The purchase price for the skull was eighteen pence. 62 Keynes, 2001[1988], p. 204. CHARLES DARWIN’S BEAGLE VOYAGE 381 long, circumference (longest) 11’’ at end before Vertebra with attach- ments to the case – extraordinary weapon.’’63 Given the specimen’s size, and its unusual description, this could only be another example of a giant bony carapace, this one with a distinctive tail.64 From Montevideo, the Beagle sailed south along the Patagonian coast, where Darwin fell on some hard luck. He spent Christmas, 1833, at Puerto Deseado, but he found no fossil vertebrates. Early in January, 1834, the Beagle reached Puerto San Julian, where he noted a distinct stratum filled with gigantic fossil oysters. On a terrace above the cliff face, in a relatively young deposit of earthy matter, he collected some incomplete post-cranial fossil mammal material, which he tentatively identified as another Mastodon. Thereafter, the Beagle departed, and Darwin never collected another fossil mammal.65 What can we make of Darwin’s work with vertebrate fossils during the voyage? Too much has already been made of his so-called mistakes and misidentifications. First of all, field identifications are necessarily provisional under almost any circumstances. Darwin’s fossils were fragmentary and at least partially obscured by matrix. His working conditions on the Beagle were difficult, at best. He had few comparative materials to work with. Worst of all, almost everything he collected was completely new to science. It is not reasonable to fault Darwin for failing to recognize fossil taxa that previously had never been described and named. By the time the Beagle sailed, only two fossil genera known to occur in South America, the celebrated Megatherium and the nearly ubiquitous Mastodon, had been described in the scientific literature.66 It should therefore be no surprise that Darwin attributed a significant number of his fossils to these two taxa.67 Darwin acknowledged his own limits as a vertebrate zoologist referring once in a letter to Henslow to his ‘‘entire ignorance of comparative Anatomy.’’68 He treated his field identifications of fossil ver-

63 Barlow, 1946, p. 216. 64 I am grateful to vertebrate paleontologist Darin Croft, who helped with this conclusion. 65 Simpson, 1984, pp. 30–31. Darwin did, however, play a role in promoting further fossil vertebrate exploration in South America (see Brinkman, 2003). 66 Owen, 1840, pp. 13–14 cites the Megatherium as well as three species of Mastodon. See also Simpson, 1984 for the state of vertebrate paleontology in South America before Darwin. 67 Interestingly, Sulloway (1969, pp. 91–92) made the same point while arguing that Darwin ‘‘knew little about paleontology.’’ 68 Letter, C. Darwin to J. S. Henslow, March 1834. See Burkhardt and Smith, 1985,p. 368. 382 PAUL D. BRINKMAN tebrates as tentative and routinely expressed doubts about them in his notes and letters. For want of comparative material, he was often re- duced to mere guesswork. One specimen, for example, he identified on the basis of its size and solidity, writing: ‘‘At Port St Julian I found some very perfect bones of some large animal, I fancy a Mastodon.—the bones of one hind extremity are very perfect & solid.’’69 In some notes written later in the voyage, however, he recorded his uncertainty about this specimen, largely on what we would call ecological grounds: Is the animal of St Julian a Mastodon? Even if a change of climate could be granted it is scarcely possible to believe the plains of gravel [in Patagonia] ever could have supported a much more luxuriant vegetation[.] We must suppose like the Camel of Eastern climes or the Guanaco which now lives here that it was fitted for a stunted Vegetation.70 Darwin found this particular specimen in some very recent deposits and confessed that he had ‘‘no idea at the time, to what kind of animal these remains belonged.’’71 It is interesting that in speculating about the fossil animal’s adaption to its environment he should compare it to the guanaco, one of the very few large terrestrial vertebrates living in Pata- gonia. He had an abundance of guanaco bones at his disposal during the voyage. Perhaps he compared his fossil to these remains and found no morphological resemblance. Still, the fact that he compared them in any respect might help explain why he was so profoundly impressed by Richard Owen’s later identification of this fossil as an ‘‘extinct Gua- naco’’ – another example of fossil vertebrate succession (which was later shown to be false).72 Had Darwin anticipated this result? Despite his shortcomings as a paleontologist, Darwin’s record of fossil vertebrate identifications is remarkably good. According to Owen, Darwin’s collection from South America comprises ten great quadrupeds and at least two rodents, mostly new to science which will be discussed in the order in which Owen described them. First, Darwin collected the complete skull and lower jaws of Toxodon platensis (Figure 3); and some miscellaneous post-cranial material of Macrauchenia patachonica (Figure 4). Darwin tentatively identified these as Megatherium and Mastodon, respectively. These are arguably his two most notorious

69 Darwin to Henslow, March 1834 in Burkhardt and Smith, 1985, p. 369. 70 CUL-DAR 42.99. 71 Darwin, 1839, p. 208. 72 See Rachootin, 1985; Herbert, 2005, pp. 320–324. The phrase ‘‘extinct Guanaco’’ is Darwin’s. CHARLES DARWIN’S BEAGLE VOYAGE 383

Figure 3. The skull and palate of Toxodon platensis. (From Owen, 1840, Plate I, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).).

‘‘misidentifications.’’ But his error makes sense when one considers that these were the first specimens ever found of two very unusual orders of extinct mammals unique to South America (which would later be called Notoungulata and Litopterna). Darwin identified the rest of his fossil vertebrates more-or-less correctly. He referred a number of specimens 384 PAUL D. BRINKMAN

Figure 4. Foot bones from Macrauchenia patachonica. (From Owen, 1840, Plate XI, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). to Megatherium, Megalonyx, or to an ‘‘edentate,’’ including: a skull fragment of Glossotherium sp. (Figure 5); a lower jaw and teeth of Mylodon darwinii (Figure 6); a partial skeleton of Scelidotherium CHARLES DARWIN’S BEAGLE VOYAGE 385

Figure 5. Skull fragments of Glossotherium sp. (From Owen, 1840, Plate XVI, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). 386 PAUL D. BRINKMAN

Figure 6. Lower jaw of Mylodon darwinii. (From Owen, 1840, Plate XVIII, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). CHARLES DARWIN’S BEAGLE VOYAGE 387

Figure 7. Skull and teeth of Scelidotherium leptocephalum. (From Owen, 1840, Plate XXI, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). leptocephalum (Figure 7); a lower jaw of Megalonyx jeffersonii (Figure 8); and a skull of Megatherium cuvieri (Figure 9). All five of the aforemen- tioned genera are similar in appearance. All are giant ground sloths and members of the order Edentata (Xenarthra). The first three were new to science, the fourth was very poorly known, and only the last was relatively well known. Small wonder that Darwin could not distinguish them. He also collected some dermal armor and two small bones, which Owen identified as ‘‘Large Edentata’’ (Figure 10) (probably referable to Hoplophorus). Darwin often followed European scientific opinion by referring these remains to Megatherium, but, as noted above, he recognized the resemblance of the dermal armor to armadillos, and he often privately referred to them as such. He also correctly identified the molar of an extinct horse, Equus sp. (see Figure 10) and the teeth and skeletal elements of a Mastodon, which Owen referred to Mastodon angustidens. Other fossil remains he recovered at Monte Hermoso and Punta Alta 388 PAUL D. BRINKMAN

Figure 8. Lower jaw of Megalonyx jeﬀersonii. (From Owen, 1840, Plate XXIX, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). CHARLES DARWIN’S BEAGLE VOYAGE 389

Figure 9. Skull fragments of Megatherium cuvieri. (From Owen, 1840, Plate XXX, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). 390 PAUL D. BRINKMAN

Figure 10. Sections of dermal armor and toe bones of Hoplophorus (top); Equus sp. tooth (lower left); and teeth, jaw, and foot bones of Ctenomys. (From Owen, 1840, Plate XXXII, reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://Darwin-online.org.uk/).). included a jaw and hind foot of an extinct member of Ctenomys (see Figure 10), an extant genus of rodent endemic to South America; and a molar and bony fragments (see Figure 10) of an unnamed animal CHARLES DARWIN’S BEAGLE VOYAGE 391 resembling the capybara, another extant rodent of unusual size. Darwin identiﬁed these remains as agoutis and unspeciﬁed rodents.73

The Lyellian Context for Darwin’s Work

Darwin’s interest in fossil vertebrate succession, as well as the rest of his geological research program, was profoundly influenced by a careful reading of Lyell’s Principles of Geology during the course of the voyage. Henslow had recommended bringing Lyell’s first volume – recently published – but ‘‘on no account’’ to accept all of its views. Darwin devoured the book at sea, and then read the second and third volumes as soon as they were sent to him in South America. He found the geologist’s uniform approach – soon dubbed ‘‘uniformitarianism’’ – so helpful in disentangling the geological problems he encountered on the voyage that by the time he had reached Peru he considered himself a ‘‘zealous disciple’’ of Mr. Lyell.74 Lyell’s second volume, which Darwin received in 1832 in Montevi- deo, is a book-length treatise on earth’s organic processes, and an especially critical appraisal of Jean Baptiste de Lamarck’s transmutation theory. Lyell was bound by his own actualistic approach to reject transmutation on the grounds that it has never been seen to occur – not even in the mummified remains of animals collected in Egypt. Yet he conceded that one advantage of Lamarck’s theory is that it precludes the repeated appeal to a ‘‘First Cause’’ to explain the appearance of new species. Lyell argued that the successive extinction of species is a constant and regular process of nature, much like erosion or deposition. In order to maintain a stable system, he had to account for the regular appearance of replacement species. But because he could not appeal to any known ‘‘cause now in operation,’’ he was forced to argue that the appearance of new species is sudden, and so uniformly distributed in time and space, that it would be extremely unlikely for a naturalist to observe it. Lyell’s readers are left to infer that there is, as yet, an unknown secondary cause for the origin of species.75

73 For the original descriptions on which this paragraph is based, see Owen, 1840. 74 Desmond and Moore, 1994[1991], pp. 108, 168. On Darwin’s relationship to Lyell, see also Browne, 1995, pp. 186–187, 189–190; Herbert, 2005, pp. 63–70; Secord, 1991, pp. 150–153. 75 Lyell, 1830–1833, Vol. II, pp. 169–179. See also Cannon 1961;Rudwick,1970,pp.18– 21; Rudwick, 1976[1972], pp. 181–183; Ruse, 1999[1979], pp. 77–78. Ruse writes (on p. 78): ‘‘This is how Lyell’s friends and critics read him, and he agreed with this interpretation.’’ 392 PAUL D. BRINKMAN

Lyell concluded Chapter XI of the second volume with a discussion of the potential role of fossils in ﬁnding this unknown secondary cause. He wrote: Naturalists may, in the course of future centuries, accumulate po- sitive data, from which an insight into the laws which govern [the appearance of new species] may be derived…. [G]eological monu- ments [i.e., fossils] alone are capable of leading us on to the discovery of ulterior truths. To these, therefore, we must now appeal, carefully examining the strata of recent formation wherein the remains of living species, both animal and vegetable, are known to occur. We must study these strata in strict reference to their chronological order as deduced from their superposition, and other relations. From these sources we may learn which of the species, now our contemporaries, have survived the greatest revolutions of the earth’s surface; which of them have co-existed with the greatest number of animals and plants now extinct, and which have made their appearance only when the animate world had nearly attained its present condition.

From such data we may be enabled to infer whether species have been called into existence in succession or all at one period; whether singly or by groups simultaneously.76 This is precisely the kind of investigation that Darwin was undertaking with his vertebrate fossils. In a March, 1834 letter to Henslow, he described the geological context for his fossil vertebrate work and noted a curious example of succession involving the bones of an agouti: I am quite astonished that such miserable fragments of the Meg- atherium should have been worth all the trouble Mr Clift has be- stowed on them. …[A] part were found in a gravel with recent shells, but others in a very diﬀerent bed.—Now with these latter there were bones of an Agouti, a genus of animals I believe now peculiar to America & it would be curious to prove some one of the same genus coexisted with the Megatherium.77 Why was Darwin so ‘‘curious’’ about his agouti fossil? In notes that date to the latter part of the voyage, Darwin wrote speculatively about the gradual birth and death of species, making speciﬁc reference to Lyell:

76 Lyell, 1830–1833, Vol. II, p. 183. 77 Darwin to Henslow, March 1834 in Burkhardt and Smith, 1985, p. 368. CHARLES DARWIN’S BEAGLE VOYAGE 393

With respect then to the death of species of Terrestrial Mammalia in the S. part of S. America. I am strongly inclined to reject the action of any sudden debacle.— Indeed the very numbers of the remains render it to me more probable that they are owing to a succession of deaths, after the ordinary course of nature.— As Mr Lyell supposes Species may perish as well as individuals; to the arguments he adduces, I hope the Cavia [agouti] of B. Blanca will be one more small instance, of at least a relation of certain genera with certain districts of the earth. This co-relation to my mind renders the gradual birth & death of species more probable.78 Historian Dov Ospovat argued that Darwin here invoked a kind of Lyellian environmental determinism to explain the appearance of new species. Lyell supposed that the tight relationship between animals and their environments could make sense of the regular extinction and replacement of species: as environmental conditions gradually change, animals adapted to those conditions disappear, while new ones arise, better adapted to the changed conditions, to take their places.79 Of course, this is, at best, an incomplete solution to the problem of the origin of new species. It begs the question: where do new species come from? Are they formed from organic material? From the dust? Michael Paul Kinch, who calls Lyell’s hypothesis ‘‘natural creation,’’ argued that those nineteenth century naturalists seeking a naturalistic explanation for global patterns of distribution of species (past and present) found Lyell’s idea to be no more credible than special creation. Why? Because his theory called for the sudden appearance of new species yet it did not provide a mechanism.80 Nor was Darwin satisﬁed with such a theory. Though Darwin’s remarks about his agouti fossil are not openly transmutationist, they nevertheless show him contemplating his own example of fossil vertebrate succession in the context of the gradual appearance of new species. Given what he claimed about the important role of succession in inspiring his speculations on the origin of species, it seems certain that Darwin was privately contemplating transmutation by this point of the Beagle voyage. Because Darwin left no reliable record, no ‘smoking gun,’ it is impossible to date precisely the moment of his switch to transmutation- ism. Consequently, it is no longer tenable to pit the early conversion

78 CUL-DAR 42.98. 79 Ospovat, 1995[1981], pp. 24–25. See also Rupke, 2005 for a discussion of ‘‘autochthonous generation.’’ 80 Kinch, 1980, pp. 113–117. 394 PAUL D. BRINKMAN hypothesis against the late conversion hypothesis as though one or the other is correct. Instead, it seems clear that Darwin’s adoption of transmuta- tionism, like the elevation of the plains of Patagonia, happened gradually, by degrees. The record shows that he was ‘‘greatly struck’’ in March, 1837, and that he opened his first transmutation notebook in July, after consultations with expert naturalists on the nature of his Beagle specimens. But does this represent the moment of his conversion, or his conviction? Is it the moment he became personally convinced of the fact of transmutation, or the moment he realized he had the means to convince his fellow naturalists of the fact of transmutation, or something else? Darwin filled hundreds of pages with notes on his observations of nature during the Beagle voyage and spent his idle shipboard hours contemplating their possible explanations. When he returned to Eng- land, expert naturalists verified many of his observations and leant additional credibility to his speculations on the mechanisms of nature. Owen’s conclusions about the fossil vertebrate evidence, for example, did not strike Darwin like a bolt from the blue. Rather, Owen’s work confirmed an idea that Darwin had already conceived during long months – even years – of private speculations about the curious nature of his specimens. Fossil vertebrate succession is a phenomenon that Darwin recognized on his own and the idea ‘‘haunted’’ him.81 It led him to speculate about the gradual appearance of new species during the voyage. It led him to ask ‘‘why is present and past life on any one spot so closely related?’’82 Lyell’s notion of ‘‘natural creation’’ was one possible answer to this question, but one that raised many new problems. Transmutation was the only other naturalistic alternative. Darwin was already familiar with the idea of transmutation from several sources, including his grandfather Erasmus Darwin’s Zoonomia. He had read Lyell’s book-length refutation of Lamarck’s transmutation theory during the voyage. But he was not so awed by Lyell that he refused to call the geologist’s conclusions into question on occasion.83 Moreover, Lyell’s book showed that the idea of transmutation deserved serious consideration in light of the geological evidence and Darwin had new evidence to consider. Transmutation, therefore, must have been one of the possible secondary causes that Darwin weighed to explain the pat- tern of fossil succession. That he never recorded this heretical notion directly in his notebooks, diary or letters only shows that he was already keeping it fairly close to his chest – his records were, after all, subject to

81 Darwin, 1959[1887], Vol. 1, p. 67. 82 Desmond and Moore, 1994[1991], p. 210. 83 Darwin’s theory of coral reef development is a perfect example. See Stoddart, 1976. CHARLES DARWIN’S BEAGLE VOYAGE 395 the scrutiny of Capt. FitzRoy.84 Nevertheless, it was the contemplation of fossil vertebrate succession, during the voyage, which set Darwin on the path to transmutation.

Conclusion

Because Darwin emphasized the role of fossil vertebrate succession in inspiring him to contemplate the origin of species, the question of when and how he ﬁrst recognized the phenomenon is crucially important. A shipboard familiarity with the concept of succession, for example, raises the possibility (again) that Darwin was seriously contemplating transmutation during the voyage. If so, then historians need to reconsider the role of London’s expert naturalists in enabling his adoption of the transmutationist hypothesis. Most historians argue that Darwin’s switch could have taken place only with the aid of experts in metro- politan London, and that he was incapable of understanding the transmutationist implications of his specimens on his own. On the contrary, it could be that travel on the periphery and distance away from the scrutiny of expert naturalists gave Darwin the autonomy he needed to speculate freely about species’ origins. In that case, the true role of the experts – after the voyage had ended – was in helping Darwin to fashion the argument and to marshal the evidence necessary to convince the wider scientiﬁc community of the fact of transmutation. A re-evaluation of the role of the fossil vertebrate evidence paints a new, unorthodox picture of Darwin’s work on the species question. Historians and scientists alike should recognize that the actual relationships between the fossils he collected and the extant fauna of South America are irrelevant. The only relevant question is: what did he think about possible relationships? The historical record shows that the fossil vertebrate evidence played an early and important role in convincing Darwin personally of the impermanence of species.

84 Sandra Herbert has argued that ‘‘an unrecorded presence may still be a presence (Herbert, 2005, p. 297).’’ There are many reasons why Darwin might have chosen to leave transmutation out of his notebooks. It is worth noting that Alfred Russel Wallace, who, like Darwin, was also convinced of the fact of transmutation by the spatial and stratigraphical succession of species, also neglected to record his ‘‘great moment of illumination’’ about his species theory in his ﬁeld journals (see Raby, 2001, p. 132). 396 PAUL D. BRINKMAN

Acknowledgements

This paper originated as a project in John Beatty’s ‘‘Darwinian Revo- lution’’ course at the University of Minnesota. I am most grateful to John, David Sepkoski, Michel Janssen, other Minnesota friends and colleagues, and especially Richard Bellon for many critical discussions of this topic, as well as some very helpful feedback. Three anonymous reviewers also made many valuable suggestions. I am likewise grateful to the Columbia History of Science Group/Friday Harbor meeting participants of 2003 for their lively and (mostly) hostile reaction to an earlier version of this paper. Rick Madden and David Quammen motivated me to take-up this project again after a long hiatus. There- after, I benefited from an exciting and enlightening discussion on this subject with Niles Eldredge. Two web resources, ‘‘The Complete Work of Charles Darwin Online’’ (http://darwin-online.org.uk/) and ‘‘Darwin Correspondence Project’’ (http://www.darwinproject.ac.uk/) have been immeasurably helpful in providing easy access to essential Darwiniana. Vince Schneider gave me the time to work on this paper. Lori Belk helped me with the figures. Nancy Loquet translated a difficult French text for me. Darin Croft advised me on the fossils. Janet Edgerton, Librarian at the North Carolina Museum of Natural Sciences, filled my unusual book and article requests. Christine Giannoni, Associate Librarian at The Field Museum, provided many helpful services while I worked on this project in Chicago. Finally, thanks to the attentive staff of the Player’s Retreat in Raleigh, where the better part of this paper was written.

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