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Apoda Classification Essay

Darwin’s work on barnacles, conducted between 1846 and 1854, has long posed problems for historians. Coming between his transmutation notebooks and the Origin of species, it has frequently been interpreted as a digression from Darwin’s species work. Yet when this study is viewed in the context of Darwin’s earlier interests, in particular his studies of marine invertebrates carried out during his student days in Edinburgh and later on board the Beagle, the monograph on the Cirripedia seems less anomalous. Moreover, as the letters in this volume suggest, Darwin’s study of cirripedes, far from being merely a dry, taxonomic exercise, was a highly theoretical work which addressed several problems at the forefront of contemporary natural history. Treating a group of organisms of considerable interest to mid-nineteenth century naturalists and approaching their classification using the most recent methods available, Darwin was able to provide a thorough taxonomic study that has remained a standard work in cirripede morphology and systematics. For Darwin personally, the barnacle work can be viewed as having perfected his understanding of scientific nomenclature, comprising both theoretical principles and technical facility with the methods of comparative anatomy. It also provided him with an empirical means of testing his views on the species question (Crisp 1983).   

Darwin’s interest in invertebrate zoology stemmed from his years as a student in Edinburgh and, in particular, his contact with Robert Edmond Grant. In his Autobiography (pp. 49–50), Darwin recalled: ‘Drs. Grant and Coldstream attended much to marine zoology, and I often accompanied the former to collect animals in the tidal pools, which I dissected as well as I could.’ In his Edinburgh notebook kept during this period (Notebooks; Collected papers, 2: 285–91), for example, there are numerous references to the ova of various invertebrates, and Darwin’s first scientific paper, presented before the Plinian Society in 1827, related his discovery that the so-called ova of the bryozoan Flustra foliacea were in fact larvae of the leech Pontobdella (Autobiography, p. 50; Hodge 1985; Sloan 1985).   

This interest in marine organisms was exercised during the Beagle voyage. Darwin expressed his current enthusiasm in a letter to William Darwin Fox, 23 May 1833 (Correspondence vol. 1):   

The invertebrate marine animals, are however my delight; amongst them I have examined some, almost disagreeably new; for I can find no analogy between them & any described families.— Amongst the Crustaceæ I have taken many new & curious genera: The pleasure of working with the Microscope ranks second to geology.— I strongly advise you instantly to buy . . . a simple microscope . . . & then make out insects scientifically by which I mean separate, examine & describe the trophi: it is very easy & exceedingly interesting; I speak from experience, not in insects, but in most minute Crustaceæ.   

Given this background and, in particular, his earlier researches in Edinburgh on the ova of invertebrates, Darwin was particularly well prepared to appreciate the unusual nature of a cirripede he collected during the Beagle voyage.   

In 1835, in the Chonos Archipelago off the coast of Chile, Darwin found ‘most curious’ minute cirripedes buried within the shell of a gastropod mollusc. In the zoological notes made during the Beagle voyage, Darwin recorded: ‘The thick shell of some of the individuals of the Concholepas Peruviana is completely drilled by the cavities formed by this animal.—’ (DAR 31.2: 305). He gave a detailed description and tentatively identified this burrowing barnacle as a member of the Balanidae, or sessile acorn barnacles, most frequently found attached to rocks. Yet from the absence of a shell and its unusual parasitic nature, Darwin recognised that it differed greatly from common barnacles.   

It was perhaps Darwin’s further discovery of developing eggs within the base of the barnacle that most captured his interest. He recorded seeing four different stages in the larval development of this ‘Balanus’ and remarked on the resemblance of one stage to that observed in the metamorphosis of Crustacea (DAR 31.2: 307). This observation was notable, for in 1835 the presence of larval stages of cirripedes was still a matter of dispute among naturalists.   

Prior to the publication in 1830 of John Vaughan Thompson’s account of the developmental history of cirripedes, which pointed out the similarity of barnacle larvae to those of Crustacea, most naturalists had followed Linnaeus and Cuvier in classifying the cirripedes as molluscs because of their external shelly covering and because their mantle cavity contained sea-water (Winsor 1969). Thompson’s sequential observations of the metamorphosis of nauplius and cypris larvae into adult barnacles, reinforced (and reinterpreted) a few years later by Karl Hermann Konrad Burmeister (Burmeister 1834), first revealed the developmental stages of these organisms. The consequent sudden shift of the Cirripedia from one branch of the animal kingdom to another—from the Mollusca to the Articulata—indicated to mid-nineteenth century naturalists that a revaluation of the group, based on a systematic and anatomical comparison intra se and with other Crustacea, was needed.   

Such a revaluation had not been undertaken when, in 1846, Darwin began to examine several invertebrates that remained undescribed from his Beagle collections. After writing two papers, one on the marine invertebrate Sagitta and another on the Planariae, he began to work on his curious South American cirripede, which he initially called Arthrobalanus and later renamed Cryptophialus minutus. ‘I had originally intended,’ he explained in the preface to Living Cirripedia (1851): vii, ‘to have described only a single abnormal Cirripede, from the shores of South America, and was led, for the sake of comparison, to examine the internal parts of as many genera as I could procure.’ For fourteen months Darwin pursued an anatomical study of pedunculated and sessile cirripedes, during which time he realised the profound state of disarray in the taxonomy of the group. Late in 1847, John Edward Gray, keeper of the zoological collections at the British Museum and himself a cirripede expert, suggested to Darwin that he prepare a monograph of the entire sub-class. Gray provided him with his own collection, arranged access to the museum’s specimens, and advised him on procuring other collections. At the time Darwin committed himself to this study, he could not have foreseen that it would occupy his attention for the next seven years. To appreciate why Darwin would have undertaken such a study, it is helpful to review the science of systematics and some of the concerns of natural history in the mid-nineteenth century.   

Classification was a primary concern of eighteenth and nineteenth-century naturalists (Knight 1981). Many of Darwin’s contemporaries—Edward Forbes, Richard Owen, Louis Agassiz, William Sharp Macleay, James Dwight Dana, Henri Milne-Edwards, and Christian Gottfried Ehrenberg—undertook a taxonomic study of one major group or another. But it was perhaps the invertebrates, selectively treated by Linnaeus and Cuvier, that presented the greatest challenge to systematists. Only recently have historians begun to penetrate the taxonomic systems of these naturalists to reveal, among other things, how an individual’s conception of the order of nature shaped the particular classificatory scheme they developed (Desmond 1982; Richards 1987; Winsor 1969).   

Darwin’s views on classification were tempered by his attempts to classify specimens during the Beagle voyage, his subsequent discussions with the various experts describing his collections, and his consideration of the subject in the light of his views on the transmutation of species. Several historians have noted his interest in the late 1830s in the quinarian system of Macleay, with its emphasis upon analogy and affinity in arranging groups (S. Smith 1965; Ospovat 1981, p. 108). Darwin’s frequent discussions with Owen no doubt helped to sharpen his thinking about classifying. Influenced by the philosophical anatomy of Étienne Geoffroy Saint-Hilaire, Owen was at this time formulating new guidelines for taxonomy, including a precise definition of homology (a term that he introduced to replace the vaguer notion of ‘affinity’), a particular application of the data of comparative anatomy, and an archetype to represent the common design perceived among organisms. Within Darwin’s maturing evolutionary perspective, the principles of natural classification began to assume a new meaning (Ospovat 1981). As Ghiselin has noted, they ‘ceased to be merely descriptive and became explanatory.’ (Ghiselin 1969, p. 83).   

By the early 1840s, then, Darwin’s ideas on classification were well developed, as is shown by his correspondence with George Robert Waterhouse. On [26 July 1843] (Correspondence vol. 2), for example, Darwin confidently proclaimed his particular understanding of the meaning of a natural classification: 

It has long appeared to me, that the root of the difficulty in settling such questions as yours,—whether number of species &c &c should enter as an element in settling the value or existence of a group—lies in our ignorance of what we are searching after in our natural classifications.— Linnæus confesses profound ignorance.— Most authors say it is an endeavour to discover the laws according to which the Creator has willed to produce organised beings— But what empty high-sounding sentences these are— it does not mean order in time of creation, nor propinquity to any one type, as man.— in fact it means just nothing— According to my opinion, (which I give every one leave to hoot at, like I should have, six years since, hooted at them, for holding like views) classification consists in grouping beings according to their actual relationship, ie their consanguinity, or descent from common stocks— In this view all relations of analogy &c &c &, consist of those resemblances between two forms, which they do not owe to having inherited it, from a common stock.— To me, of course, the difficulty of ascertaining true relationship ie a natural classification remain just the same, though I know what I am looking for.— This being the case viz ignorance of a distinct object I think, we ought to look at classification as a simple logical process, i.e. a means of conveying much information through single words—   

Receptive to Geoffroy Saint-Hilaire’s philosophical anatomy, Darwin incorporated the concepts of analogy and homology within his theory of classification. But within the context of his new interpretation of classification, homological relations became more than simply tools for description. Whereas for many naturalists homology represented structural resemblances among forms arising from a similarity in their basic plan of organisation, for Darwin homology revealed actual phylogenetic relationship. It is no accident, then, that homology played such a prominent role in Darwin’s classification of the cirripedes. Homology provided the key to making out the evolutionary relationships linking members of a group and the possible line of descent of one species from another previously existing form.   

Darwin’s evolutionary interpretation of the meaning of classification also explains why he readily adopted embryology as a methodological tool for revealing homologies. Through the work of naturalists such as Robert Brown, Martin Barry, and Owen in England and Henri Milne-Edwards in France, the substance of Karl Ernst von Baer’s work on embryological development began to enter discussions of classification in the late 1830s and early 1840s (Ospovat 1976; Richards 1987; Appel 1987). Darwin first came into contact with these ideas through his association in Edinburgh with Grant (A. Desmond 1984; Sloan 1985). In 1846, at the start of his exploratory studies of the Beagle invertebrates, Darwin was reintroduced to embryological considerations through his reading of Milne-Edwards’s influential essay on classification (Milne-Edwards 1844). Like von Baer, Milne-Edwards recognised that comparative embryogenesis could be used to yield information about systematic relationships. Within members of the same branch of the animal kingdom, the progress of development appeared to illustrate an increasing divergence from an early resemblence shared by all members of a class to those later features special to the particular order, family, genus, and, finally, to the individual species.   

Milne-Edwards drew from this generalisation several principles that proved to be of importance for classification in the mid-nineteenth century: (1) the most general structures of a class appear earliest in development, and these establish higher taxonomic affinities, (2) characters shared by organisms reflect the degree of zoological parentage, (3) some organisms, in contrast to the general phenomenon of ‘progressive’ development, exhibit arrested or retrograde development, (4) increasing specialisation in embryogenesis illustrates the tendency in higher organisms toward a ‘division of physiological labour’ and this principle could be used to determine ‘lowness’ and ‘highness’ in particular groups, and, finally, (5) that embryology, by revealing homological relationships in development, was the best means for classification.^1^   

Despite the fact that these ideas were not new to Darwin, he was particularly struck by Milne-Edwards’s formulation. In the lengthy abstract that he made of this essay in December 1846—prefaced by the statement, ‘—This is the most profound paper I have ever seen on Affinities’—Darwin noted all the major topics discussed by Milne-Edwards (DAR 72: 117). In his commentary, Darwin frequently relied upon his current concern—the cirripedes—to help illustrate particular points:   

p 77. remarks on the difficulty caused by development in some cases making organisms less complicated, as in Lernæa, (which I sh^d^ think was the strongest case known.^2^ Barnacles in some sense, eyes & locomotion, are lower, but then so much more complicated, that they may be considered as higher. (a) If we consider the number of changes as highness, then Lernæa a mere reproductive sack wd be higher; but this is too counter to common sense. What a fertile source of series are the Entomostraca, good to illustrate my theory; see his Treatise.^3^ It is evidently most difficult to make out old in higher & so no wonder little accordance with Geolog. History. (B) p 77. considers Ornithorhyncus as obtaining its beak by retrograde ‘recurrent’ development.^4^ (When we consider that Bird & Mammal have come from same original cell, it ceases to be less wonderful that under nearly similar circumstances, (compare Plancental & Marsupial animals) w^d^ be similarly or parallely developed. The case of female apterous moths which never leave case & glowworm— a curious case.— We then see that highness does not depend on perfection & number of organs, but on development. B. All that we ought to expect is that if our fossils were perfect, that embryonic forms were the oldest; & hence in this discussion, leave out the term higher & lower.— (DAR 72: 117–19v.)   

When analysing the monograph on the Cirripedia, begun the year after these notes were made, it is interesting to see how many of the points discussed by Milne-Edwards re-emerged within the specific context of Darwin’s treatment of the natural history and systematics of the cirripedes.   

In a later volume of the Annales des Sciences Naturelles (Zoologie), read shortly after reading Milne-Edwards’s essay on classification, Darwin encountered another important paper on classification by Gaspard Auguste Brullé (Brullé 1844). In this work, Brullé argued that the most complex and characteristic organs in a group were the first to develop, thereby adding the dimension of time and functional importance to Milne-Edwards’s more general principle concerning embryological development (Rachootin 1984). In his notes on this paper, Darwin commented:   

This shows something which I do not understand: it seems that to mature an organ, a certain time is required, & that the earlier changes can alone be hurried.  This at once nearly explains the gradual loss of embryonic characters; & with shortening time of development the loss of embryonic or larva characters in certain tribe in classes remarkable for embryonic metamorphoses, as we shall see presently in Hippoboscus &c  states that in Crust, antennæ & parts of mouth appear before legs— the mouth-parts appearing before antennæ— The former have assumed their forms, when the legs begin to appear. (DAR 72: 123 and v.)   

Darwin’s own understanding of embryological development, as outlined in his essay of 1844 (Foundations, pp. 57–255), accorded well with the views put forward by Brullé.^5^ In Living Cirripedia, Darwin appears to have used Brullé‘s law, to some extent implicitly, in establishing some of the embryological homologies.^6^   

Thus, it is clear that long before Darwin commenced his study of barnacles, he was intrigued by the processes of generation and embryonic development of marine invertebrates. Moreover, he was well informed about the current views of leading systematists concerning the theory of classification. By the time he took up the study of barnacles, he was familiar with the embryological criterion of homology offered by respected naturalists such as Milne-Edwards and Brullé, and he studied their works with a high degree of interest and attention, drawing out of their writings statements that directly bore on the Cirripedia and which needed to be followed through in order to satisfy his own particular understanding of a methodology that could yield a classification based on phylogenetic relationships. In addition, after his year-long study of cirripede anatomy, he had gained confidence in his skill in anatomical dissection, an important prerequisite for the preparation of a taxonomic monograph.   

Darwin, then, clearly believed that a classification based on homologies established through embryology as well as anatomy would best reveal possible genetic relationships. Accordingly, homology became a key element in Darwin’s taxonomic evaluation of the cirripedes. It justified, for example, his decision to rank the Cirripedia as a separate sub-class of the Crustacea rather than subsuming the group within another sub-class (Milne-Edwards 1852) or elevating it to a separate class altogether (R. Owen 1855). Milne-Edwards and Owen also relied heavily upon the use of homology, but, in the case of the Cirripedia, with a different interpretation of the information homology provided. Milne-Edwards placed greater emphasis on retrograde development for assigning taxonomic rank, and since the cirripedes most nearly resembled degraded Podophthalmia, he ranked them as a separate order within this sub-class of Crustacea. Owen, on the other hand, recognised the Crustacea-like appearance of the imperfect or larval stage, but his particular reliance upon the homology of comparative anatomy—that is, the comparison of adult forms—led him to regard the characters of the mature cirripede as more important for classification than those of its developmental history. Since two of the characters commonly used to define the class of Crustacea were their power of locomotion and the presence of separate sexes, according to Owen’s classificatory principles the Cirripedia could not be included within this class. He therefore ranked this group as a distinct class between the Crustacea and the Annelida (R. Owen 1855).^7^ Darwin, however, with his particular embryological criterion of homology, believed that the resemblances in the metamorphosis of Crustacea and the Cirripedia indicated their community of descent. Since cirripedes exhibited characters common to two different crustacean sub-classes, he decided, in contrast to Milne-Edwards’s classification, to rank the Cirripedia as a separate sub-class of Crustacea.^8^   

An understanding of the theoretical principles upon which Darwin drew thus provides the context for assessing the monograph on the Cirripedia. Yet it is Darwin’s particular application of these classificatory concepts that is the key to interpreting the barnacle study, and this can perhaps best be illustrated by citing a few of the leading problems treated in this work. In his Autobiography (pp. 117–18), Darwin himself described what he believed to be some of the more notable aspects of the cirripede monograph:   

My work on the Cirripedia possesses, I think, considerable value, as besides describing several new and remarkable forms, I made out the homologies of the various parts—I discovered the cementing apparatus, though I blundered dreadfully about the cement glands—and lastly I proved the existence in certain genera of minute males complemental to and parasitic on the hermaphrodites. . . The Cirripedes form a highly varying and difficult group of species to class; and my work was of considerable use to me, when I had to discuss in the Origin of species the principles of a natural classification.   

In both volumes of Living Cirripedia (1851 and 1854), Darwin devoted an introductory section to a description of the metamorphosis of cirripedes, explaining that this was necessary ‘on account of the great importance of arriving at a correct homological interpretation of the different parts of the mature animal.’ (Living Cirripedia (1851): 25). As a basis for his homologies, Darwin relied upon Milne-Edwards’s model of an archetypal crustacean consisting of twenty-one segments, variously divided in different organisms between cephalic, thoracic, and abdominal somites (Milne-Edwards 1834–40; Appel 1987, pp. 218–19). Darwin identified seventeen of these twenty-one segments in the archetypal cirripede and assumed that the four terminal crustacean segments were missing in barnacles.^9^ This archetypal cirripede was essential to Darwin’s treatment of the group; it formed the basis for making out the anatomical organisation of specimens and guided his assessment of taxonomic rank. Furthermore, studying the developmental history of cirripedes and thereby analysing the homologies of the adult, Darwin found some of the basic tenets of his species theory well demonstrated: the homology of parts in related organisms, the loss of useless organs (e.g., the abdominal segments and the swimmerets), and the transformation in function of homologous organs (e.g., thoracic limbs for walking into cirri for feeding).   

The use of homology as a means to assess relationship was particularly central to Darwin’s discussions of difficult or anomalous forms. For example, together with Cryptophialus and Proteolepas (an extremely aberrant barnacle found in the West Indies),^1^0^ the genus Alcippe presented the greatest challenge to Darwin’s method of classification. For these unusual forms, classification became a critical test of Darwin’s embryological methodology. Relying upon the superficially distinctive characters of Alcippe—its boring powers and the absence of a shell—Darwin initially assumed that it, like Cryptophialus, would form a distinct family. For this reason he deferred examining it for four years, until he completed the systematic description of the common barnacles (the Lepadidae and Balanidae) in 1853. Upon dissecting Alcippe and identifying its homologies, however, he was surprised by what he discovered. The female differed from all other cirripedes (and, indeed, from all Crustacea) in having no rectum or anus. Its pupa had fewer abdominal and caudal segments than any other cirripede, and it had no appendages on its three thoracic segments, where normally there were three pairs of cirri. But it also exhibited some structural similarities to certain genera in the Lepadidae and, along with the pedunculated cirripedes Ibla and Scalpellum (and in contrast to most cirripedes), Alcippe was not hermaphrodite but possessed separate sexes. Given the affinity  between the males of Alcippe and the parasitic males of Ibla and Scalpellum, and his further assumption that ‘the female Alcippe had partially assumed characters confined to the males of the other genera’, Darwin finally decided that the genus was best placed among the Lepadidae (Living Cirripedia (1854): 527–8).^1^1^   

Both Alcippe and Proteolepas, moreover, provided extreme examples of the process of abortion of parts in nature. Darwin’s discussion of this point again illustrates how much his methodology relied upon embryology and the assumption that larvae reveal the most general characters of a group; it also indicates how an archetype represented for him the hypothetical ancestral form:   

The archetype crustacean consists of twenty-one segments; of these the seventeen anterior segments can be clearly made out in the archetype Cirripede: now, in the male Alcippe, the first three segments are largely developed, forming all that is externally visible, but the remaining fourteen segments are absolutely aborted. . . To show the wonderful diversity in nature, even in the same sub-class, I may be permitted to remark, that whilst in Alcippe only the three anterior segments are developed, the fourteen succeeding segments being rudimentary, in Proteolepas . . . these fourteen segments are all largely developed, whilst the three anterior segments are quite aborted . . . (Living Cirripedia (1854): 562–3)   

Indeed, Proteolepas presented such an extreme example of the abortion of parts that it was difficult, he maintained, even to establish its relation to the common cirripedes. Yet again homology guided his particular taxonomic decision:   

Proteolepas has no particular affinity to any other cirripede; it resembles, indeed, Cryptophialus in one important point, but only in one point, namely, in the number of the segments of its body. It is really beautiful to see how the homologies of the archetype cirripede, as deduced from the metamorphoses of other cirripedes, are plainly illustrated during the maturity of this degraded creature, and are demonstrated to be identical with those of the archetype Crustacean. I was at first inclined to rank Proteolepas in one division, and all other cirripedes in another division of equal value; but as it may be inferred from the characters of the prehensile antennæ, that the pupa did not differ much, if at all in any important character, from the pupæ of other cirripedes, I have thought the three orders, which I have instituted, would be the most natural arrangement. (Living Cirripedia (1854): 588)   

The fact that the pupal antennae resembled the common cirripede type was the main reason Darwin advanced for including Proteolepas in the sub-class. Relying upon the antennae, which survive in the adult as the means of attachment, and the law of correlation of parts, he felt able to infer the structure of the unseen pupa and included a representation of it with his figure of the mature animal (Living Cirripedia (1854), Plate XXV).   

Throughout his work, Darwin placed great emphasis on the antennae as features of taxonomic importance. They performed a significant role in the larva as organs of locomotion and touch and in the adult as the indispensable means of attachment, and, most importantly, they were embryonic features that survived largely unchanged in the adult. The structure of the antennae in the degraded parasitic males of Ibla and Scalpellum enabled Darwin to be sure of the true affinity of these most unusual forms. The table of measurements of antennae in the various genera of Lepadidae (Living Cirripedia (1851): 286–7), which he later supplemented with measurements for the aberrant species Alcippe, Cryptophialus, and Proteolepas, indicates the importance Darwin ascribed to these features and bears out his assertion that ‘embryonic parts, as is well known, possess the highest classificatory value’ (Living Cirripedia (1851): 285).^12^   

For delineating the higher taxa, Darwin was guided by a comparison of the metamorphosis and the segmentation of particular forms with the segments of the archetypal cirripede. Using the location of appendages as the leading criterion for characterising the orders, he placed Proteolepas in the Apoda (no legs) and Cryptophialus in the Abdominalia (legs on abdomen), with all other cirripedes making up the Thoracica (legs on thorax).^13^   

One further example of the importance of embryological homologies for Darwin’s monograph comes from his description of the organs which served to transform a previously mobile crustacean into a state of permanent attachment. Such a system was of particular interest to him from a theoretical standpoint. In cirripede larvae in the last stage of development, Darwin observed ‘two long, rather thick, gut-formed masses, into the anterior ends of which the cement-ducts running from the prehensile antennæ could be traced’, and he came to believe that these were the incipient ovaria and the cement glands of the organism (Living Cirripedia (1851): 20). This association suggested to him that the cementing apparatus was homologically equivalent to the ovarian tube. The case appeared to be a striking instance of how an organ had been transformed to perform a new function:   

I have entered on this subject at some length, (and I wish I had space for more illustrations,) from its offering, perhaps, the most curious point in the natural history of the Cirripedia. It is the one chief character of the Sub-class. I am well aware how extremely improbable it must appear, that part of an ovarian tube should be converted into a gland, in which cellular matter is modified, so that instead of aiding in the development of new beings, it forms itself into a tissue or substance, which leaves the body in order to fasten it to a foreign support. But on no other view can the structure, clearly seen by me both in the mature Cirripede and in the larva, be explained, and I feel no hesitation in advancing it. (Living Cirripedia (1851): 37–8)   

In Living Cirripedia (1854), Darwin ventured to suggest the possible ‘evolution’ of this organ system from the ancestral crustacean:   

To conclude with an hypothesis,—those naturalists who believe that all gaps in the chain of nature would be filled up, if the structure of every extinct and existing creature were known, will readily admit, that Cirripedes were once separated by scarcely sensible intervals from some other, now unknown, Crustaceans. Should these intervening forms ever be discovered, I imagine they would prove to be Crustaceans, of not very low rank, with their oviducts opening at or near their second pair of antennæ, and that their ova escaped, at a period of exuviation, invested with an adhesive substance or tissue, which served to cement them, together, probably, with the exuviæ of the parent, to a supporting surface. In Cirripedes, we may suppose the cementing apparatus to have been retained; the parent herself, instead of the exuviæ, being cemented down, whereas the ova have come to escape by a new and anomalous course. (Living Cirripedia (1854): 151–2)   

Crisp (1983) has pointed to Darwin’s interpretation of the cement glands as an example in which evolutionary views influenced, in this case erroneously, his understanding of the phenomena. Certainly Darwin’s view did accord well with Milne-Edwards’s principle of the division of physiological labour as a means for the development of structural specialisation and with Darwin’s understanding of how pre-existing organs could become modified to fulfill a new function in an organism.^14^ It is also true that Darwin found it difficult to abandon this homology when it was challenged in 1859 by August Krohn. As he admitted in a letter to Charles Lyell, 28 September 1860 (Life and Letters, 2: 345–6), ‘It is chiefly the interpretation which I put on parts that is so wrong; & not the part which I describe.’ But this ‘blunder’ (as Darwin called it in his Autobiography and in his letter to Lyell), was more than a matter of interpretation. It challenged his best case for descent with modification and ultimately his picture of how the archetypal cirripede had evolved from the ancestral crustacean.   

Perhaps the clearest example of how Darwin’s transformist views may have influenced his taxonomic decisions comes from his discussion of the sexual relations of cirripedes. The hermaphroditism of cirripedes is one of the major characters distinguishing the majority of the group from other crustaceans. Soon after commencing the monograph, however, Darwin discovered in the genus Ibla a species in which small, rudimentary males were found parasitic on the female. Subsequently Darwin encountered a further sexual peculiarity which he regarded as even more significant. In both Ibla and Scalpellum, he also found minute or, as he called them, ‘complemental’ males attached, not to a female, but to a hermaphrodite. Apart from the ‘marvellous’ fact that these complemental males were so ‘utterly different in appearance and structure’ from the hermaph%rodite—the two representing such ‘diverse beings, with scarcely anything in common, and yet all belonging to the same species!’ (Living Cirripedia (1851): 293)—this discovery was unique in the animal kingdom, and it touched upon, Darwin realised, the question of sexuality and its evolution.^1^5^   

Darwin viewed his discovery of males and complemental males in these two genera as a significant confirmation of his transformist views. Here seemed to be a prominent illustration of a gradual change in nature within a few members of a group from one condition to another—in this case, from hermaphroditism to separate sexes:   

In the series of facts now given, we have one curious illustration more to the many already known, how gradually nature changes from one condition to the other,—in this case from bisexuality to unisexuality. (Living Cirripedia (1854): 29)^16^   

Darwin’s elation over this discovery was frequently expressed in his correspondence. Immediately after finding little males attached to hermaphrodite cirripedes, for example,  Darwin informed Hooker of this interesting discovery and specifically addressed how it related to his species theory. On 10 May 1848, Darwin wrote:   

I have been getting on well with my beloved cirripedia, & got more skilful in dissection: I have worked out the nervous system pretty well in several genera, & made out their ears & nostrils, which were quite unknown. I have lately got a bisexual cirripede, the male being microscopically small & parasitic within the sack of the female; I tell you this to boast of my species theory, for the nearest & closely allied genus to it is, as usual, hermaphrodite, but I had observed some minute parasites adhering to it, & these parasites, I now can show, are supplemental males, the male organs in the hermaphrodite being unusually small, though perfect & containing zoosperms: so we have almost a polygamous animal, simple females alone being wanting. I never sh^d^. have made this out, had not my species theory convinced me, that an hermaphrodite species must pass into a bisexual species by insensibly small stages, & here we have it, for the male organs in the hermaphrodite are beginning to fail, & independent males ready formed. 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.—   

It was certainly his species theory that prompted him to look for similar relations in closely allied genera, and it contributed to his decision to include Alcippe, with its separation of sexes,  within the same family as Ibla and Scalpellum.   

Another notable aspect of Darwin’s study of the Cirripedia was the great amount of variation shown by members of the group, particularly among the Balanidae. Darwin was surprised by the amount of variation presented in this family. Interesting though this was from a theoretical point of view, it caused him considerable difficulty in deciding whether such variations indicated different species or merely varieties (Southward 1983). In Living Cirripedia (1854), Darwin clearly stated the consequences such variation had for taxonomic evaluation, concluding with a generalisation that appeared again in chapter five of the Origin:   

After having given up several years to the study of this class, I must express my deliberate conviction that it is hopeless to find in any species, which has a wide range, and of which numerous specimens from different districts are presented for examination, any one part or organ,—which from differing in the different species is fitted for offering specific characters,—absolutely invariable in form or structure. I may in one respect even go further, and affirm, that, if in a species, any part or organ differs remarkably from the same part in its congeners, then if many specimens are examined, especially when collected from different districts, such part or organ will be found eminently variable. (Living Cirripedia (1854): 155)   

One of the first systematists to break from the type-concept of species, Darwin realised that within a given species variation must necessarily occur. But he had difficulty deciding, as do modern systematists, where species variations stopped and distinct species began. It was, perhaps, because Darwin had an overall concept of evolutionary change that, in Balanus amphitite and B. tintinnabulum (now called Megabalanus), he ‘confounded some taxa and unjustifiably separated others’ (Henry and McLaughlin 1975, p. 8).   

Darwin specifically addressed how his views on species had influenced his taxonomic decisions on several occasions in his correspondence with Hooker. On 13 June [1850], for example, Darwin wrote:   

You ask what effect studying species has had on my variation theories; I do not think much; I have felt some difficulties more; on the other hand I have been struck (& probably unfairly from the class) with the variability of every part in some slight degree of every species: when the same organ is rigorously compared in many individuals I always find some slight variability, & consequently that the diagnosis of species from minute differences is always dangerous. I had thought the same parts, of the same species more resembled than they do anyhow in Cirripedia, objects cast in the same mould. Systematic work w^d^ be easy were it not for this confounded variation, which, however, is pleasant to me as a speculatist though odious to me as a systematist.—   

Toward the end of his study of Balanus, in a letter to Hooker on 25 September [1853] (Correspondence vol. 5), Darwin again discussed this issue, stating:   

In my own cirripedial work . . . I have not felt conscious that disbelieving in the permanence of species has made much difference one way or the other; in some few cases (if publishing avowedly on doctrine of non-permanence) I sh^d^. not have affixed names, & in some few cases sh^d^. have affixed names to remarkable varieties. Certainly I have felt it humiliating, discussing & doubting & examining over & over again, when in my own mind, the only doubt has been, whether the form varied today or yesterday (to put a fine point on it, as Snagsby would say). After describing a set of forms, as distinct species, tearing up my M.S., & making them one species; tearing that up & making them separate, & then making them one again (which has happened to me) I have gnashed my teeth, cursed species, & asked what sin I had committed to be so punished: But I must confess, that perhaps nearly the same thing w^d^. have happened to me on any scheme of work.—   

It was, in other words, Darwin’s views on the impermanence of species that led him to lump what we today recognise as many separate species together as varieties of one species.   

One final aspect of Darwin’s study of the Cirripedia that is not often noted is how well it illustrates his practical skill in the techniques of anatomical dissection. Huxley paid Darwin a high compliment when he touched upon this matter in his 1857 lecture on cirripedes. In his praise of the monograph, Huxley commented: ‘It is one of the most beautiful and complete anatomical and zoological monographs which has appeared in our time, and is the more remarkable as proceeding from a philosopher highly distinguished in quite different branches of science, and not an anatomist ex professo.’ (T. H. Huxley 1857, p. 238 n.).   

While Darwin’s dissecting abilities were not inconsiderable on board the Beagle, they were perfected by his work on the cirripedes. At the outset, Darwin realised the need for a careful examination of the internal parts of each specimen. Indeed, because naturalists had previously regarded the group as molluscs and had relied primarily upon external characters alone in their descriptions, earlier systematic treatments of the group were generally poor. This helps to explain why Darwin devoted the first sixty-five pages of Living Cirripedia (1851), and a lengthy section in Fossil Cirripedia (1851), to a detailed description, among other things, of the various features on the underside of the valves to which the living parts attached. In addition, he provided a thorough description of the anatomical characters of the soft parts of cirripedes. His astute recognition of subtle changes in shell structure, opercular valve configuration, cirral counts, etc. continue to be used by cirripede systematists.   

To facilitate the numerous dissections that were required, Darwin purchased a new compound microscope and later a simple dissecting microscope made to his specifications. This latter instrument suited his purposes well; he reported in a letter to Richard Owen, 26 March 1848, that he strongly recommended it to others pursuing microscopic anatomy. The acumen that Darwin developed in dissecting, preserving, and mounting his specimens is well demonstrated by a letter he wrote to Charles Spence Bate, 13 June [1851] (Correspondence vol. 5), in which Darwin advised Bate on the art of microscopical technique:   

I think you will find it useful to preserve small objects, in a way in which I have been accustomed to preserve the results of most of my minute anatomical researches, namely in common water without any spirits, with a Bit of thin glass over the object (without any cell) & gold size all round the rough edge—objects thus prepared will sometimes keep for a long time & generally for some months.— If you are inclined to take the trouble to rear any larvæ to the second stage, you could so send them me or better in a very minute bottle of spirits  Every cirriped that I dissect I preserve the jaws &c. &c. in this manner, which takes no time & often comes in very useful.  This very day I have been using preparations thus made two years since, & they are perfectly clear & with some colour preserved, As I am in the way of suggesting—I would strongly advise you to get one of the glass ruled micrometers to slip in eye piece, the whole does not cost much above half a guinea, you would only have to send medium eyepiece to London & you could measure to the 120,000^th^ or less of an inch, without delaying your work half a minute.—^17^   

By the close of the monograph on the Cirripedia, Darwin was undoubtedly highly skilled in the art of anatomical dissection. He was an experienced systematist, well aware of the practical and theoretical problems this pursuit entailed. He was also thoroughly familiar with the current views of naturalists in such areas as comparative anatomy, embryology, and palaeontology. This knowledge guided his work on the cirripede monograph, within which many of the major tenets of his theory of evolution can be recognised. Indeed, both Hooker and Huxley believed that the cirripede work was crucial in preparing Darwin for his next work on the origin of species.^18^   

Darwin’s long and often tedious labour in producing the monograph on the Cirripedia was soon rewarded, for he received the Royal Medal of the Royal Society of London in 1853, even before completing the second parts of Living Cirripedia and Fossil Cirripedia. Informing Darwin about the award (Correspondence vol. 5, letter from J. D. Hooker, [4 November 1853]), Hooker wrote: ‘The RS. have voted you the Royal Medal for Natural Science— All along of the Barnacles!!! . . . Portlock proposed you for the Coral Islands & Lepadidæ. Bell followed seconding, on the Lepadideæ alone, & then, followed such a shout of pæans for the Barnacles that you would have smile to hear.’   

The announcement of the award, printed in the Proceedings of the Royal Society 6 (1853): 355–6, mentioned both Coral reefs and Living Cirripedia (1851), but it was the latter work that received detailed attention. The text is here reprinted in full to illustrate what Darwin’s scientific peers considered to be the primary merits of the work:   

In your Monograph on the Pedunculated Cirripeds, you have treated generally of the structure, economy, and zoological relations of these animals, and given a systematic arrangement and description of the different species. In the accomplishment of your task, you have not only made use of previously existing materials with sound and enlightened criticism, but, by the discovery of new facts and the promulgation of original views, contributed most materially to advance the department of knowledge to which your researches more immediately belong, and rendered valuable service to physiological science in general.   

In the course of your inquiries you have confirmed and widely extended the observations of your predecessors respecting the larval condition of the Cirripeds, and have shown that all the perfect Lepads and Balanids pass through successive stages of metamorphosis. You have also added largely to our knowledge of the anatomy of the larva, and brought to light the curious fact, that in one of its stages its mouth is altogether rudimentary, and perfectly closed up by the external covering, so that the creature in this stage is in fact a ‘locomotive pupa,’ incapable of feeding. You have further observed that the prehensile antennæ with which the larva fixes itself in its final change, invariably remain permanent in the adult animal at the attached end of its peduncle, and in many cases afford important characters for zoological discrimination.   

The knowledge thus gained from the study of development is most sagaciously and happily applied by you to explain the homological relations between the Cirripeds and Crustaceans; and in this way you have conclu%sively shown that the peduncle of the mature Lepad corresponds with the three anterior segments of the Crustacean. Again, by your discovery of Proteolepas, a new parasitic Cirriped of low organization, you have been able successfully to compare the remaining segments of the body in the two classes; for whilst the chain of evidence is in some measure broken by the absence of two segments near the middle of the series in Cirripeds generally, the missing links are supplied by the newly-discovered animal referred to.   

The existence of an eye with a pair of ophthalmic ganglia in adult Lepads, as had been previously shown in Balanids,—the presence too of organs seemingly intended for hearing and smelling—the chemical nature of the tegumentary coverings—the cement-gland and ducts, yielding a plastic material for attaching the peduncle and for other special purposes in particular instances, and the singular organic connexion between that gland and the ovaries, are all most interesting discoveries in comparative anatomy, first made known in your work.   

Classification essays rank the groups of objects according to a common standard. For example, popular inventions may be classified according to their significance to the humankind.

Classification is a convenient method of arranging data and simplifying complex notions.

When you select a topic, do not forget about the length of your paper. Choose the topic you will be able to cover in your essay, do not write about something global or general.

Consider these examples:

  • Evaluate the best to worst methods of upbringing.
  • Rate the films according to their influence on people.
  • Classify careers according to the opportunities they offer.

You should point out the common classifying principle for the group you are writing about. It will become the thesis of your essay.

It is important for you to use clear method of classification in your essay, especially when you are dealing with subjective categories such as "quality" or "benefit". Make sure you explain what you mean by this term.

To organize a classification essay, the writer should:

  • categorize each group.
  • describe or define each category. List down the general characteristics and discuss them.
  • provide enough illustrative examples. An example should be a typical representative of the group.
  • point out similarities or differences of each category, using comparison-contrast techniques.