Charles Darwin’s Beagle Voyage, Fossil Vertebrate Succession, …

[Pages:37]Journal of the History of Biology (2010) 43:363?399 DOI 10.1007/s10739-009-9189-9

? Springer 2009

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: Paul.Brinkman@

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.

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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.

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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. Consequently, 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 periodically 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.

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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 insufficient 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 professionalization 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).

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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 Furthermore, 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.

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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.

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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 persuaded 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 lectures, 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.

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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 human.... 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 Darwin's Rio notebook (EH1.10; English Heritage 88202330), p. 47b, establishes this date.

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