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Darwin Correspondence Project

Living and fossil cirripedia

Nomenclature of the valves.jpg

Nomenclature of the valves
Darwin's nomenclature of the valves, Lepadidae, fig. 1

Darwin published four volumes on the crustacean sub-class Cirripedia between 1851 and 1854, two on living species and two on fossil species. These were systematic monographs, written for a specialist audience, and as such are probably among the most challenging and least read of Darwin’s works. Often dismissed as a necessary detour on the road to the development of the theory of evolution through natural selection, these volumes reveal, in fact, how observation, experiment, and classification both informed and were informed by Darwin’s species theory. Every aspect, from the choice of subject to the production of the volumes, reveals something about Darwin’s approach to scientific investigation. While appearing to conform to long-established practice, Darwin introduced a new approach to systematics that challenged the purely morphological methodology of his predecessors.


In 1846, Darwin switched focus from geology to invertebrate zoology, a subject which had interested him since his student days in Edinburgh. While on the Beagle, he had recorded numerous observations on invertebrates, but only those on the octopus and on planarian worms are included in his Journal of researches, his account of the voyage. Moreover, although he had passed on his collections of birds, mammals, and reptiles from the Beagle voyage to experts for systematic description, he had not done so with invertebrates.  By the time Darwin turned his attention to these specimens, he had also completed two outlines of his ‘species theory’ (1842 Pencil sketch and 1844 Essay).

In the course of discussions about species in the autumn of 1845, his close friend Joseph Dalton Hooker had been critical of an essay on this topic by Frédéric Gérard in the Dictionnaire universel d’histoire naturelle, noting, ‘I am not inclined to take much for granted from any one treats the subject in his way & who does not know what it is to be a specific Naturalist himself’. Hooker was probably taken aback by how hard this hit Darwin. When Darwin wrote ‘How painfully (to me) true is your remark that no one has hardly a right to examine the question of species who has not minutely described many’, Hooker was quick to reassure him that ‘your having collected with judgement is working out species’. What part, if any, this exchange played in Darwin’s eventual decision to produce a systematic work is uncertain. In any case, the decision had certainly not been taken even a year later; instead, Darwin told Hooker, that he was ‘going to begin some papers on the lower marine animals,’ which, he thought, would last him ‘some months, perhaps a year’. The first animal to be investigated was an odd little barnacle Darwin had collected from the Chonos Archipelago in the southern tip of South America in January 1835. Darwin told Robert Fitzroy, the former captain of the Beagle, that he had been ‘for the last half month daily hard at work in dissecting a little animal about the size of a pin’s head from the Chonos Arch. & I could spend another month on it, & daily see some more beautiful structure!’. Darwin was more than a little intrigued by his anomalous barnacle, affectionately dubbed ‘Mr Arthrobalanus’ in letters to Hooker, since it seemed to have many segments, almost no shell, and bored into the shell of Concholepas, a Chilean abalone. By February, 1847, Darwin had expanded his study and informed a German correspondent, ‘I have for the present given up Geology, & am hard at work at pure Zoology & am dissecting various genera of Cirripedia, & am extremely interested in the subject. I always, however, keep on reading & observing on my favourite work on Variation or on Species, & shall in a year’s time or so, commence & get my notes in order’.

What began as a plan to write ‘some papers on the lower marine animals’ was slowly moving towards a much larger work of comparative anatomy, but it was only at the end of 1847 that Darwin decided to undertake a systematic study of what he called ‘this difficult order’. He wrote to John Edward Gray to request permission to describe the collection in the British Museum, a request, which under normal circumstances, Gray could have acceded to without formality, but the circumstances were anything but normal. Not only did Darwin want to study the specimens at home, he also required that he be allowed to soak, clean, and disarticulate one specimen of each species. He told Gray, ‘I have resolved not to describe any species, without I can do it thoroughily’. Shortly after his first request, Darwin added that if he was sent the pedunculate (stalked) and sessile (stalkless) barnacles separately, this would be ‘all I could wish’. In February 1848, Darwin received ‘the good tidings of the great liberality of the Trustees’ and began writing in earnest to collectors and acquaintances about to set out on expeditions asking for specimens.  He also offered his work to the recently established Ray Society (minutes of council meeting, 4 February 1848), founded to publish by subscription highly specialised works on natural history with limited appeal. Darwin’s proposed barnacle work was accepted on 18 February 1848.

‘An instinct for truth’

Having already spent well over a year on cirripede anatomy, Darwin wrote a rather reflective letter to his former professor and friend, John Stevens Henslow, musing about the value of his work. ‘I feel within me, an instinct for truth, or knowledge or discovery ... some of my friends laugh at me, & I fear the study of the cirripedia will ever remain “wholly unapplied”’.  He also announced his discovery of what he later called ‘complemental males’, that is, a much reduced male ‘parasitic within the sack of the female’. This is a doubly anomalous feature, for, while most crustaceans had separate sexes, most barnacles were hermaphrodites. Even more amazing was his next find: in a related genus, he discovered small supplemental males attached to hermaphrodites. He excitedly told Hooker, ‘my species theory convinced me, that an hermaphrodite species must pass into a bisexual species by insensibly small stages, & here we have it’.

Although more comfortable with his skill in dissecting and convinced of the importance of his work for his theory, Darwin now had to deal with two bugbears of taxonomy: naming and synonymy. The issue with respect to barnacles was complex, he confided to Henslow, ‘for instance under Balanus punctatus (which must be made a distinct genus) three or four varieties have been called distinct species; whereas one form, which has not been called even a variety, is not only a distinct genus, but a distinct sub-family.’ For Darwin, the practice of appending the name of the first describer to the species name led to ‘hasty work,’ and to the evil of ‘naming instead of describing’. In a long desideratum on the subject, he complained, ‘I find every genus of cirripedia has half a dozen names & not one careful description of any one species in any one genus.— I do not believe that this wd have been the case, if each man knew that the memory of his own name depended on his doing his work well, & not upon merely appending a name with a few wretched lines indicating only a few prominent external characters.’ This ‘tirade’ was written to Hugh Edward Strickland, who, like Darwin, had served on a British Association committee on zoological nomenclature. Strickland’s thoughtful reply defending the principle of priority, and Hooker’s advice to ‘drop the battle about perpetuity of names’ began to wear Darwin down, but he  could not ignore the problem. ‘Literally not one species is properly defined: not one naturalist has ever taken the trouble to open the shell of any species to describe it scientifically, & yet all the genera have ½ a dozen synonyms. ... The subject is heart-breaking.—

While Darwin reluctantly conceded to Strickland on naming, his final act of rebellion was ‘never putting mihi or Darwin after my own species & in Anatomical text giving no author’s names at all, as the systematic Part will serve for those who want to know History of species as far as I can imperfectly work it out.’ Indeed, the cirripede volumes are notable for their sparing synonymy. On the heels of their discussion on naming, Darwin and Strickland again differed on the topic of type species. Strickland explained, ‘Of course you will understand that by type-species I only mean a conventional distinction, referring only to words, not to things; and like human titles, only used as a matter of convenience.’ Darwin countered, ‘I feel some difficulty about your type species: I always arrange genera in as natural order as I can, & then one puts the species nearest allied to former genus first.’ Implicit in Darwin’s method is the idea of a real genealogical connection; this basis also led him to group more species into fewer genera. He happily told an American conchologist who had supplied him with several specimens, ‘I am glad to say that I have had to run far more genera together, than to separate & make new ones’.

Darwin’s most radical move, however, was to recognise the significance of including fossil species in his work, while being aware of the difficulties: ‘I shall do what I can in fossil Cirripedia  … but I do not believe that species (& hardly genera) can be defined by single valves; as in every recent species yet examined their forms vary greatly: to describe a species by valves alone is the same as to describe a crab from small portions of its carapace alone, these portions being highly variable & not as in Crustacea modelled over viscera’. Because the shell plates or ‘valves’, to use Darwin’s terminology, of a barnacle shell are connected by a chitinous membrane which deteriorates after death, most fossils consist of single valves. Darwin’s decision to include fossil species was unusual, but made sense within the framework of his species theory. From early 1849, Darwin worked on both fossil and extant forms and extended his requests for specimens. He seized an opportunity at the Birmingham meeting of the British Association for the Advancement of Science in September 1849, where he met the Danish geologist Johan Georg Forchhammer; later that month wrote to request specimens from the Geological Museum of Copenhagen University and to ask whether Johannes Japetus Smith Streenstrup, professor of zoology there, would make his own collection available.

Characters and homologies: seeing what lies beneath

While at the meeting of the British Association in Birmingham, Darwin heard a paper by Albany Hancock describing a new type of boring barnacle he had named Alcippe. Immediately alerted to a possible related species to his own Arthrobalanus, also a boring species, Darwin told Hancock, ‘In S. America I collected an allied form, parasitic in the Concholepas & which probably will be included in the same order with your’s, but which I think must certainly form a very distinct family’. Only a week later, Darwin was less certain: ‘I think it possible that Alcippe & Arthrobalanus may turn out distinct orders’, he told Hancock, ‘The metamorphosis is certainly different— my larva has no thoracic legs, whereas yours has. mine is binocular, yours uniocular &c &c. Yet the 3 pair of cirri, the great lab〈rum〉 & habits are certainly strong points 〈o〉f resemblance’. A few months later, Darwin still puzzled over Hancock’s, ‘description of Alcippe’ even though ‘it is most clear & definite’. ‘I hope to put Arthrobalanus in same order with it,’ he admitted, ‘but it will be stretching a point to do so, & they must form distinct families, more distinct than any Pedunculate & any Sessile cirripede are from each other’.

Not only was it dangerous to separate species based on what Darwin dismissively described as ‘a few prominent external characters’, but also difficult even to establish a character: ‘the cirripedia are moreover very troublesome, from their great variability, & the necessity of examining whole animal & inside & outside of shell’. Darwin’s student days at Cambridge predisposed him to be wary of multiplying species; his botany professor John Stevens Henslow had alerted his students to the great amount of variability that existed in the same species under different conditions, and even produced herbarium sheets with several distinct specimens as an aid to identifying species. While close examination of many individuals was a necessity for both living and fossil species of barnacles, in the latter there were no soft parts to consider. Nevertheless, Darwin painstakingly examined the underside of the valves of fossil shells and tried as far as possible to compare several specimens. He explained, ‘I much dislike giving specific names to each separate valve, & thereby almost certainly making three or four nominal species for each true species’ Furthermore, Darwin was influenced in the naming of fossil genera by his belief in the material relation of fossils to living species. A significant factor in his renaming as Scalpellum some fossil specimens classed as Pollicipes, was the exceptional differences he had noted in living foms.

Although barnacles, formerly classified as molluscs, had only been revealed to be crustaceans based on the study of their free-swimming larvae, no one before Darwin had tried to homologise the parts of larval and adult forms. Darwin and the American researcher James Dwight Dana began a correspondence that focused on homologies, and revealed the difficulties as well as the advantages of this approach. The term ‘homology’ has many interpretations, and Darwin used it in different senses, from serial homology to larval-adult homology to homologies in species, genus, family, and higher categories. Serial homology refers to corresponding parts in the same organism, such as the legs of an insect or the segments of the vertebrate spine; in Crustacea the thoracic segments would be seen as homologous parts. For animals that undergo significant structural remodelling during development, attempting to equate parts in the early stages of development to their adult forms was another type of homology, one that was especially difficult in cirripedes because they had more than a single larval form. This type of homology became especially important from an evolutionary perspective because it revealed hidden genealogical links to members of other groups.

The homology of parts across species, genus, etc. had been used by naturalists such as the French zoologist Étienne Geoffroy Saint-Hilaire to support his view of a unity of organic composition. In contrast, his rival, Georges Cuvier argued that function determined form so that similarities in morphology resulted only from functional similarities. Darwin followed Geoffroy Saint-Hilaire’s idea of unity of composition (which, in the context of evolution became a unity through common descent), but did not dismiss the importance of function, conceptualising it according to his evolutionary theory as adaptation. This added a new dimension to the exercise of homologising parts and determining relevant taxonomic characters. Indeed, when separating allied species, Darwin’s adaptationist approach allowed him to identify a character from more than a purely structural perspective. Homology was also crucial in Darwin’s attempt to demonstrate the evolution of sessile barnacles from stalked or pedunculate forms, which appear much earlier in the fossil record. In Origin pp. 191-2, he cited the homology of ovigerous frena (folds of skin which hold eggs) in stalked barnacles to the branchiae (respiratory organs) in sessile forms as an example of the adaptation of organs to perform new functions.

Over the course of his barnacle research, Darwin kept returning to species already described in an attempt to homologise parts with those of new species. Nowhere was this more true than in his description of the first barnacle he observed closely—‘Mr Arthrobalanus’, who turned out to be female and accompanied by complemental males as well (Cryptophialus minutus). This was not always a consensual activity: for example, Darwin and Dana disagreed about the homologies of the antennae in early (naupliar) and late (cyprid) larval foms, and continued to discuss these differences for years. Darwin only reported his conclusions about larval-adult homologies in a letter to Dana in December 1853.

Preparing for publication

Until 1850, Darwin had probably expected the Ray Society to publish his work on both living and fossil barnacles. He was working on fossil and living species in concert, dividing his research by subgroup, tackling the stalked or pedunculate species before the sessile ones. This approach would have made the sections of his work more even, since the ratio of pedunculate to sessile species was the opposite in living and fossil forms. On the other hand, it would have been highly unusual to publish a work that included both forms, and the separation of fossils from living species made more sense from this perspective. Darwin was evidently approached by James Scott Bowerbank, who had founded the Palaeontographical Society in 1847. ‘With respect to publication of the fossils, I have not yet thought’, Darwin told Bowerbank in January 1850, ‘your mentioning the Palæont. Soc. makes me think whether my work wd suit them.’ But there was a problem: the work would have to be restricted to British species since the society’s brief was to publish work on undescribed British fossils. Darwin immediately began to think of ways to circumvent this limitation, telling Bowerbank that he would soon ‘receive a lot of fossils from Copenhagen-chalk, named by Beck & Steenstrup; which will be very valuable for comparison with the British specimens’. The fossil cirripedes volume was accepted by the Palaeontographical Society by February 1850, and in the end, Darwin was permitted to include descriptions of non-British species; these were given in a smaller typeface as very long footnotes, but the descriptions themselves were not abbreviated Darwin was equally particular about the illustrations, telling Bowerbank that ‘in a few weeks, when I have got all my drawings & M.S. ready I will communicate again with you for your advice on some points— The drawings must be on copper for all depends on lines of growth:— James de C. Sowerby is making my drawings, & wd. undoubtedly engrave them best.’

The decision to employ James de Carle Sowerby was one Darwin certainly came to regret, but for the moment he focused on his own task of describing fossil species. He was helped by numerous collectors, but especially appreciated access to the unparalleled collection of Norwich collector Robert Fitch. This  enabled Darwin to compare several specimens and he told Fitch that a particular specimen was ‘far more valuable to me than a new species, convincing me that the conclusion at which I arrived viz that P. maximus & sulcatus of Sowerby are only varieties, is correct.’ He later urged Fitch, ‘if you have lately procured even any fragments & do not grudge the trouble of sending me so very many parcels I have no doubt they wd aid me’. By April 1850, he reported to Steenstrup that he had ‘fully described 33 species taking a typical valve in each genus.’ Darwin may have imagined his work almost complete, but the publication of the first barnacle volume was delayed several months largely because of problems in the production of plates. He explained what he wanted to Sowerby: ‘The outlines appear to me very accurate; but yet a few alterations are wanted in most of them; but these are trifling & refer chiefly to lines of growth.— The scale is not at all too large. What I now write for, is to beg you to do them a little harder & with the lines of growth more distinct. Some of the drawings have the muzziness of Lithography,—a style of art, (viz Lithog.) which in my opinion has been highly injurious to Nat. History— I do not care for artistic effect, but only for hard rigid accuracy.

Not all the issues with the plates were due to Sowerby. Darwin had tried to introduce illustrations of some foreign species but now had to tell Sowerby, ‘The Pal. Soc. have just settled not to give me more than I Plate full of foreign species, & therefore, until I see how many will pack in one Plate, I shd be glad if you would not draw Pollicipes Hausmanni, P. carinatus & P. elongatus & perhaps not even P. validus.’ A series of letters from Darwin, cajoling, threatening, and pleading for Sowerby to hurry followed over the next few months, but it did not stop him from making late changes based on newly received specimens, and after requiring late changes by Sowerby in September 1850, told him, ‘I hope to God I have now come to the end of my specimens’. Final corrections were made to the plates, but even close to publication in early 1851, Darwin told Sowerby, ‘I like the looks of the Plates now they are complete; but I see several of my corrections have not been fully attended to; I presume that they required too extensive changes.’ Darwin was also unhappy about the format in which his monograph would appear. ‘What a sad pity it is that you stitch your annual parts in one vol. for the year’, he complained to Bowerbank, ‘What a poor show it makes! & how inconvenient to those who never (as I for one never do) bind their books.’ When the first fossil monograph appeared in June 1851, it was the third part of volume 5 of the Palaeontographical Society’s publications.

As his work on the first fossil volume approached completion in September 1850, Darwin had reported on his progress: ‘I have described 38 fossil Pedunculated Cirripedia; all the recent Pedunculata including the animal’s body, & I have just finished with the 45th. species of the great & difficult genus of Balanus.’ A month later, told Edwin Lankester of the Ray Society that his manuscript on living pedunculated species could be ready in a fortnight, requesting information on the numbers of illustrations he would be allowed, and how many might be coloured. He also explained the limitations of the proposed format: ‘I am not sure whether the Council understands, that by the present division, the Systematic Section will be in two Parts, the first small & the second fully twice as large; and a Third Part not very large on the Anatomy, Habits Range &c.’  With publication of the first fossil volume seemingly within reach, Darwin’s troubles with the volumes on living cirripedes, destined for the Ray Society, were just beginning.

New order

Darwin was clearly not happy to relegate his research on anatomy, habits, and range to a separate section in the volumes of the Ray Society. He also worried about the language of description, the size of the typeface, and the number of plates. In the end, the Ray Society allowed Darwin to integrate material on anatomy, etc. into the specific descriptions, and produce two rather than three parts on living Cirripedia. This was especially important as it allowed Darwin to justify his choice of diagnostic characters with reference to the habits of living species, rather than on the sole basis of museum specimens. With the first fossil volume finally published, Darwin turned to the first part of the living species; having finished writing in July 1851, he corrected proof-sheets from August to November and reported that the 10 plates for the volume were ready for printing in September. Darwin had employed George Brettingham Sowerby Jr, nephew of James de Carle Sowerby, to do the illustrations and translate specific descriptions into Latin. This Sowerby was more reliable in producing work on time, although his Latin evidently did not satisfy Darwin, who hired his old school friend John Price to correct the work. By the time the second volume of Living Cirripedia was published, Darwin had dropped the Latin descriptions entirely. Although the first volume of Living Cirripedia bears the date 1851, it did not appear until January 1852.

By 1852, Darwin was well advanced in his work on sessile species, both living and fossil. The sheer number of recent sessile species would have made the second volume of Living Cirripedia double the length of the first had not Darwin decided to employ a smaller font size for all the specific descriptions. As it was, it still was half again as long as the first volume. Darwin continued to receive new specimens and refine his research so that the volume, at first promised by the end of 1852 then the summer of 1853 was only sent in manuscript form to the Ray Society at the beginning of 1854, where it took longer than the ‘2 or 3 months’ Darwin had hoped for to finally appear. It was no doubt a great relief to tell his friend Thomas Henry Huxley in early September 1854, ‘My second volume on the everlasting Barnacles is at last published’.

A second volume on fossil cirripedes appeared in May 1855 as part of the eighth volume produced by the Palaeontographical Society; the monograph itself was printed in 1854. This volume appears not to have been discussed much in Darwin’s correspondence, but he wrote to the Palaeontographical Society in February 1854 and the society confirmed that he was to complete the part on fossil Balanidae. Darwin had evidently wavered about producing a second volume because the number of fossil species was so small; in fact, the monograph was a scant 44 pages in length.

Taxonomy as history

The barnacle volumes are remarkable works not just in remaining the standard description of barnacles for the next hundred years, but in that they provided a new model for the practice of taxonomy itself. Classification after Darwin changed from an idealistic exercise in imposing order on natural forms to a materialist attempt to create a genealogy of these forms and to explain how changes on Earth had, over time, shaped the production of what Darwin referred to in the closing sentence of Origin as ‘endless forms’.