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Darwin Correspondence Project

Darwin in letters, 1847-1850: Microscopes and barnacles


Joseph Dalton Hooker
Joseph Dalton Hooker
CUL DAR 257: 114
Cambridge University Library

Species theory

In November 1845, Charles Darwin wrote to his friend and confidant Joseph Dalton Hooker: ‘I hope this next summer to finish my S. American geology; then to get out a little zoology & hurrah for my species-work’ (Correspondence vol. 3, letter to J. D. Hooker, [5 or 12 November 1845]). In the event, the ‘little zoology’ turned out to be a study of barnacles that spanned eight years and resulted in two monographs, each of two volumes, describing all the known forms of that neglected and hitherto confusing sub-class of Crustacea, Living Cirripedia (1851, 1854) and Fossil Cirripedia (1851, 1854).

What led Darwin to engage in this work when he was clearly so eager to get on with his species theory? The correspondence reveals how his initial interest in a singular species found during the Beagle voyage developed into an investigation of the comparative anatomy of other cirripedes and then into what became an authoritative and comprehensive taxonomical study of the entire group. Light is shed on the close relationship between Darwin’s systematic descriptive work and the species theory that provided the background to this work and that guided his observations through a difficult, often frustrating taxonomical maze. Throughout these years, Darwin was also struggling with a recurrent debilitating illness that eventually drove him to Malvern in Worcestershire to try the fashionable water-cure. After several months therapy and rigorous regimen, explained in detail in letters to friends and relatives, Darwin felt sufficiently restored in health to work for two or three hours a day at his ‘beloved cirripedia’.

International collaboration

Nevertheless, it is evident from his correspondence that Darwin’s two hours at the microscope did not preclude a prodigious amount of other scientific activity. There are, for example, twenty lengthy letters to Charles Lyell from these years and a series of interesting exchanges on geology with David Milne, Robert Chambers, John Phillips, and Daniel Sharpe, demonstrating the extent of Darwin’s continued involvement in contemporary geological issues. Further correspondence with American and European geologists and naturalists, most notably James Dwight Dana, Henry Darwin Rogers, and Bernhard Studer, and the attention that Darwin and his colleagues gave to their views reveals a recognition on the part of British naturalists that science was fast becoming an international collaborative enterprise.

Of special interest are the nine letters from Joseph Dalton Hooker written during his expedition in the Himalayas. They describe his dramatic and frequently dangerous travels through the mountains. Hooker writes of the complicated geology of the area, of the community of scholars and officials in British India, of rajahs and princes, elephants and mountain-sickness, as well as of his own practical and personal achievements in gaining access to Tibet and previously closed areas of the Sikkim Himalaya. In the midst of all this activity, Hooker responds to Darwin’s particular queries and sends information he believes will be of interest to Darwin’s species work or geological theories. They write about their long-standing debate over the origin of coal deposits, about barnacles and species, and by their easy familiarity show the affection with which they had come to regard each other.

Geology, and geological controversy

Hooker’s letters illuminate the role of the British government in scientific research during the period. Darwin also contributed to these official endeavours through his participation in the publication of an Admiralty Manual of scientific enquiry (1849) designed to guide the scientific work of naval officers and travellers in general. Darwin was asked by the editor, Sir John Frederick William Herschel, to write the chapter on geology (letter to J. F. W. Herschel, 4 February [1848]). Letters between Darwin and Richard Owen, author of the zoological chapter, show how they divided the topics of geology and zoology between them. Owen included in his chapter notes by Darwin on the use of microscopes on board ship (see letter to Richard Owen, [27 March 1848]). Darwin’s chapter plainly calls on his Beagle experiences; his instructions are clear and simply stated, with attention being given to the areas in which observations would contribute most to the leading questions of contemporary geology. Perhaps with himself in mind, he explained ‘These and many other such wide views in the history of the world are open to any one, who, applying thought and labour to his subject, has the good fortune to geologise in little frequented countries.’ (Collected papers 1: 228).

Not surprisingly, the leading questions and wide views spelt out by Darwin in the Admiralty Manual are also those on which he spent much time during the early years of this volume, particularly as important evidence relating to his views on the widespread and recent alterations in the level of land came under renewed scrutiny. In 1847 David Milne, the Scottish geologist, published an important paper on Glen Roy and other associated glens around Inverness, in which he maintained that the terraces, which Darwin believed to be of marine origin, were the shores of an ancient lake formed when detritus, since eroded, blocked the valley. Darwin was much shaken by Milne’s evidence, especially as he realised that it made plausible the theory of Louis Agassiz, who had proposed that a glacier blocking the foot of Glen Roy had produced a lake and the consequent beaches. Darwin carefully re-examined his own thesis in letters to Milne, Lyell, and Robert Chambers, and, in addition to writing to Milne directly, he sent a long rejoinder in the form of a letter for publication in the Scotsman. Yet when the editor, Charles Maclaren, maintained that it would be too technical for his readers and forwarded it to Robert Jameson, editor of the Edinburgh New Philosophical Journal, Darwin asked for it to be destroyed. Only the draft of Darwin’s letter remains (letter to the Scotsman, [after 20 September 1847]). Other letters to colleagues at this time indicate that Darwin was beginning to feel that the Glen Roy question was not fully answered by his marine-beach theory, and he went so far as to complain that his original fieldwork was ‘time thrown away’ (letter to Charles Lyell, 8 [September 1847]).

The second geological theory Darwin felt the need to defend had to do with the explanation of boulders found far from their native rock formations. Darwin’s explanation, originally suggested by Lyell, was that the boulders were transported by floating ice, a notion which was roundly criticised by William Hopkins in 1848. Hopkins maintained that transport by icebergs could not account for the location of some boulders at heights above those of their parent formations and proposed that only a great rush of water could carry them up hills. Darwin’s response was to explain such cases as a consequence of coastal ice transport occurring in combination with subsidence, a view which—with his Glen Roy argument—was later superseded by Agassiz’s glaciation theory.

A third theme in the geological correspondence is the distinction between stratification, on the one hand, and the phenomena of cleavage in slaty rocks and foliation in metamorphic rocks, on the other. Darwin maintained that cleavage was the result of differences in mechanical tension that affected the crystallising and concretionary processes in rocks and that foliation was a further stage of the same process. Both were thought to be independent of the process of sedimentation by which the original beds had been formed. This hypothesis had failed to convince other prominent geologists, among them Lyell, so Darwin was keenly interested in what appeared to be corroborative evidence for his theory in the work of the Swiss geologist Bernhard Studer and of Daniel Sharpe, and he eagerly entered into correspondence with them. He encouraged Studer to visit him at Down and recommended Studer’s papers to others interested in the subject.

The letters also reveal that Lyell sought Darwin’s advice in the preparation of new editions of his Principles of geology and Manual of elementary geology. In addition, Lyell asked for Darwin’s view of his major new theory of ‘craters of denudation’, with which he hoped finally to lay to rest the controversy over the origin of volcanic craters. The issue centred on whether great volcanic craters or calderas were what Christian Leopold von Buch and Jean Baptiste Armand Louis Léonce Élie de Beaumont called ‘craters of elevation’, formed all at once by an upheaval of strata which had once been horizontal or nearly so, or whether they had grown gradually, as Lyell maintained, from one envelope of lava being superimposed over another, over and over again. The critical point in the controversy, and the point on which Lyell at the time felt it necessary to concede, was the assertion that lava could not solidify on slopes with dips of more than three or four degrees. Even Darwin, Lyellian though he was, had contended from observations on Mauritius and other volcanic islands that some craters could not be explained by Lyell’s view. Apparently convinced by Lyell’s ‘denudation’ theory, Darwin wrote that it would be a ‘thorn in the side of É de B.’ (letter to Charles Lyell, 3 January 1850).


Over a hundred letters in these years were written in connection with Darwin’s work on Cirripedia. Having sent off the proofs of Geological observations on South America, the final volume of the geological results of the Beagle voyage, on 1 October 1846, Darwin at last found time to follow up his interest in these organisms.

this illformed little monster

Most of the zoological collections from the Beagle voyage had already been described by the taxonomists Darwin had enlisted in 1837 and published in the five volumes of the Zoology or elsewhere. Darwin had originally planned to describe some of the marine invertebrates himself (see Correspondence vol. 2, letter to Leonard Jenyns, 10 April [1837]) and it was intended that invertebrates be included in Zoology, but by 1844 it had become clear that the Treasury grant of £1000 would be exhausted before the work was completed. Darwin therefore published separately a paper on his observations of the ‘curious’ genus Sagitta ( Collected papers 1: 177–82) and another on marine and terrestial Planariae ( Collected papers 1: 182–93).

Darwin also undertook a detailed study of an unusual cirripede he had found on the Chonos Archipelago off the coast of Chile. His Beagle diary of zoological observations (DAR 31.1: 305–8) contains a description, dated 8 January 1835, of a minute animal embedded in the shell of the mollusc Concholepas peruviana. Darwin was able to identify it as a new, extraordinarily aberrant, kind of Balanus, drawings of which are preserved in DAR 29.3 (Plate 20, figs 1–13) and reproduced in Correspondence vol. 3, facing p. 320. At the end of his detailed description he asks: ‘Who would recognize a young Balanus in this illformed little monster? Fig 12.— . . . It is manifest this curious little animal forms new Genus.—’

By 1 October 1846 Darwin had written a short paper on his new balanus and called it ‘Arthrobalanus’, a name later given up in favour of Cryptophialus minutus. He sent the manuscript to Richard Owen, presumably for his comments, but it was not published—perhaps because Darwin had become intrigued with the structure and metamorphoses of barnacles and wished to make additional observations to supplement his original work. The sequence of entries in his ‘Journal’ (Appendix I) suggests that he came to believe the existing description of ‘Arthrobalanus’ needed further work in order to establish its uniqueness fully.

Sometime before the end of December 1847, John Edward Gray, keeper of the zoological department of the British Museum, urged Darwin to undertake a monograph devoted to the taxonomy of all the Cirripedia, both living and fossil. This proposal, no doubt strengthened by his own widening interest and by Louis Agassiz’s opinion that such a monograph was a ‘desideratum’ (letter to J. L. R. Agassiz, 22 October 1848), was accepted by Darwin, and he immediately took steps to borrow as many collections of both fossil and recent specimens as possible. Gray relinquished his own plans to classify the group, turned over some notes he had made, and, early in 1848, obtained permission for Darwin to borrow the collection of the British Museum. Samuel Stutchbury, curator of the Bristol Institution, had earlier offered ‘his truly beautiful collection’ of fossil cirripedes, which Darwin had declined before deciding to undertake a monograph, but then accepted, and Hugh Cuming and others made their own specimens available. Thus Darwin methodically began to spin a web of friends and associates which, over the next five years, was gradually enlarged to encompass Irish, French, German, Belgian, Scandinavian, and American, as well as British, naturalists, all of whom participated with remarkable generosity in the sharing of information and the loan of materials. Even further afield, Syms Covington, Darwin’s servant during the Beagle voyage, then residing in New South Wales, was enlisted to collect Australian barnacles.

you will perhaps wish my Barnacles & Species theory al Diabolo together

During 1848, Darwin examined the genera Ibla and Scalpellum and was surprised to find that some species were like Arthrobalanus in having two distinct sexes, and that the males were minute, rudimentary organisms, living virtually as parasites within the carapace of the females. Moreover, Darwin found that Ibla and Scalpellum also included hermaphrodite species which possessed tiny, ‘extra’ males that seemed to aid or complement the ordinary reproduction of the hermaphrodites. His astonishment at these sexual arrangements and his excitement with the discovery are clearly expressed in his letters; he also pointed out to Hooker that it was his species theory that had enabled him to understand the meaning of the observations. Darwin believed he had found a series of forms illustrating the gradual emergence of separate sexes in barnacles: Ibla quadrivalvis and Scalpellum vulgare represented a stage where the male organs were still present in the hermaphrodite but were superfluous and reduced owing to the presence of the minute ‘complemental’ males, as Darwin called them. Other barnacles which possessed two separate sexes represented a later stage in which the male was similarly dwarfed in relation to the female and lived, as did the complemental males, as ‘mere bags of spermatozoa’ attached to the female (Living Cirripedia (1854): 23). Darwin had previously worked out this theory of the gradual divergence of the sexes from an ancestral hermaphrodite in his Notebook D (Notebooks) and had subsequently pursued the idea by examining the structure of flowers, particularly those with abortive stamens or pistils (Correspondence vol. 2, letter from J. S. Henslow, 21 November 1840). The sexual relations of barnacles seemed to him to mirror the probable emergence of dioecious plants from monoecious forms (Living Cirripedia (1851): 214; (1854): 29, 528 n.) and, at another level, to explain the existence of structures that properly belonged in only one of the sexes appearing as vestigial or rudimentary organs in the other. The impact of following through his hitherto purely theoretical views on these subjects must have been of great significance to Darwin, not only in confirming his theory of the emergence of separate sexes, but also in strengthening his commitment to wider theories about descent and modification. Something of this importance comes through in his happy protestations to Hooker: ‘But I can hardly explain what I mean, & you will perhaps wish my Barnacles & Species theory al Diabolo together. But I don’t care what you say, my species theory is all gospel.—’ (letter to J. D. Hooker, 10 May 1848).

Once Darwin had decided to undertake a full monograph of the cirripedes, both living and fossil, he brought his species theory into play in other areas of comparative anatomy and morphology that seemed likely to establish the systematic relations of these animals. He attempted to show how the five valves or plates of the cirripede’s external covering were fundamentally equivalent, although often fused and altered in different ways throughout the group, and he examined the metamorphoses of the larvae in detail, intending to demonstrate how the free-swimming larva is modified into a sessile, adult organism (Correspondence vol. 4, Appendix II). One particular aspect of his work discussed at length in his correspondence is the supposed modification of parts of the ovarian tubes into glands that exude the ‘cement’ that attaches the adult barnacle to a substrate—an interpretation of observations that Darwin announced at a meeting of the British Association for the Advancement of Science in Birmingham in September 1849.

At Birmingham, Darwin made the acquaintance of the invertebrate taxonomist Albany Hancock, and subsequently wrote at great length to him about Hancock’s study of Alcippe, another new and aberrant barnacle with similarities to Arthrobalanus, and about Lithotrya, a barnacle that burrows into rocks. Darwin and Hancock discussed the boring mechanisms that might be involved, ultimately disagreeing about their nature.

Taxonomy and scientific naming conventions

Working on Cirripedia, he soon became caught up in the practical problems of systematics, and this led him to an exchange of letters with Hugh Edwin Strickland, an old friend, on the difficulties of naming his species in accordance with the ‘rules of nomenclature’ that had been drawn up in 1842 by a British Association committee on which both had served (Correspondence vol. 2, letters to H. E. Strickland). Darwin’s task was complicated by the abundance of wholly inadequate species descriptions and the confusion surrounding the precise affinities of the cirripedes (Correspondence vol. 4, Appendix II). In some cases he felt he might be compelled to sacrifice the rule of priority for the sake of expedience (letter to H. E. Strickland, [4 February 1849]), but in the end he adhered to the British Association guidelines except in one instance (the genus Pollicipes), which he justified in a lengthy footnote (Living Cirripedia (1851): 293 n.). The problem that bothered Darwin most, however, was the practice—again recommended by the British Association—of appending the name of the first describer to the species description. He saw this as putting a premium on naming rather than on careful descriptions and condemned it as ‘the greatest curse to natural History’ (letter to H. E. Strickland, 29 January [1849]). He also wrote a paper, which he sent to Strickland, setting out his case, and although he did not publish it nor, so far as is known, keep it in his files, he was apparently by no means convinced that his position was wrong. Strickland’s abstract of this paper, preserved with other papers on zoological nomenclature at the Cambridge Museum of Zoology, has been transcribed with Darwin’s letter to H. E. Strickland, 29 January [1849]. As Darwin wrote to J. D. Hooker, who had warned him against keeping up the battle, he gave up only from fatigue and ill health (letter to J. D. Hooker, 9 April 1849).

Health and wealth

Fatigue and ill health were indeed a severe problem for Darwin during this period, especially in 1847 and during the last half of 1848 and the beginning of 1849. When his father Robert Waring died on 13 November 1848, he was too ill to set out on the journey to Shrewsbury until the 17th. He arrived just after the funeral had begun and did not attend the burial service. In March of the following year, in search of a cure for the recurrent vomiting and other disorders from which he suffered, he and his family took up residence at James Manby Gully’s hydropathic establishment in Malvern, Worcestershire. The water-cure that he embarked on there, and that he continued to follow when he returned to Down in June, is the subject of several letters in 1849. Darwin was convinced that it was a success, and there is no doubt that his health improved, at least temporarily. This is borne out by his health diary (Down House MS) that he kept for the next five years. In December 1849, for example, he had 25 days that he rated ‘Well, very’ with a double underline under the ‘very’. But the old symptoms of boils, flatulence, and wakeful nights eventually returned, as did the complaints about his stomach and nausea. Even when these symptoms were absent he still found ‘excitement’—by which he meant stimulating conversation with friends or discussions at learned societies—unsettling and tiring. It was at this time that Darwin established a rigid routine at home that was evidently intended to reduce the likelihood of unsettling occasions and was, in outline, similar to the routine undertaken at Malvern.

Robert Waring Darwin’s estate was a large one. In his will he distributed his farmlands in Lincolnshire between his two sons and his daughter Susan. His unmarried daughters, Susan and Catherine, were guaranteed the right to occupy the family home in Shrewsbury and associated land and buildings for as long as both or either remained unmarried. Each daughter was bequeathed £10,000, Charles was bequeathed £15,500, and his older brother Erasmus £20,000; the remainder of the estate was to be divided so that a quarter went to each son and an eighth to each daughter. It is probable that Darwin inherited at least £45,000 (Keith, p. 225), making his personal wealth considerable. In the year between September 1849 and September 1850, Darwin’s Account Books (Down House MSS) record that his income was £2981 12s. 8d., a sum that must represent investment capital of something in excess of £80,000. Further income was generated by the substantial sum that had been placed in trust for Emma Darwin when she married. The accounts also reveal that the total expenditure of the Darwin household over the same period was £1584 7s. 2d. Darwin was easily able to support a large family (three more children, Elizabeth, Francis, and Leonard, were born during this period) and free to undergo expensive medical therapy.

Village life

As a man of wealth and standing, his responsibilities did not end with his family but extended to the community of Down village. In these years Darwin took an active part in the running of local ‘self-help’ insurance schemes, becoming treasurer of the Down Coal Club and helping to establish the Down Friendly Society for which he also acted as treasurer. He kept the accounts of both organisations with the same thoroughness that he showed in managing his own financial affairs. Darwin’s careful records of his life and work reveal a methodical mind keen to establish order: he chronicled his health, his daily and household accounts, his scientific and personal activities (Appendix I), the behaviour of his children (Appendix III), the books and journals he read (Appendix IV), and his investments and financial transactions. Likewise he gathered and recorded everything he could find pertaining to species and varieties—information that was carefully classified and filed in numbered portfolios against the day when he would turn to writing up his theory in full. The success of his taxonomic work also illustrates Darwin’s skill in ordering information. In this respect, his response to a questionnaire intended to elicit the characteristics of men of science, circulated many years later by his cousin Francis Galton, is particularly revealing. Under the section headed ‘Special talents’, Darwin wrote: ‘None, except for business as evinced by keeping accounts, replies to correspondence, and investing money very well. Very methodical in all my habits.’ (LL 3: 179)