Journal of the History of Biology (2010) 43:363–399 Ó 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 oste- ology 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 col- lection 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 ques- tion 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 con- version hypothesis has emerged as the new consensus view among his- torians 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 nat- uralists. 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 zoo- logical 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 spe- cies 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 zoo- logical.’’ 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.