Carl Gegenbaur (1826–1903): IntegratingComparative Anatomy and Embryology

MANFRED D. LAUBICHLERn

School of Life Sciences, Arizona State University, Tempe, Arizona

During his lifetime, Carl Gegenbaur (Fig. 1) waswidely respected as ‘‘the most important morphol-ogist in the world’’ and credited to be the one‘‘who first created a scientific comparative anat-omy.’’1 At the time of his death, on June 14, 1903,Gegenbaur had taught and influenced severalgenerations of German and international anato-mists and morphologists through his courses andtextbooks; founded the ‘‘Gegenbaur School’’ incomparative morphology; established and editedone of the premier journals in the field, theMorphologisches Jahrbuch, for 37 years; proposedinfluential theories about two of the main pro-blems in vertebrate evolution, the origin of pairedappendages and the segmentation of the verte-brate skull; and, last but not least, contributedsubstantially to his and his friend Ernst Haeckel’s(1834–1919) program in evolutionary morphology.Given the scope and nature of his contributions itis only fitting that a vignette in the history ofevolutionary developmental biology is devoted toCarl Gegenbaur honoring the centennial of hisdeath.

Carl Gegenbaur and his contributions to evolu-tionary morphology are of more than just histor-ical interest, although no pre-history ofevolutionary developmental biology could be writ-ten without reference to Gegenbaur either. In thewake of Ernst Haeckel’s several formulations ofthe biogenetic law the importance of embryologi-cal data for phylogenetic reconstructions as well asthe role of development as an evolutionarymechanism for morphological transformationwere widely discussed.2 Gegenbaur’s contribu-tions to these debates, especially his views on theprimacy of comparative anatomy over embryologyand his interpretations of such central concepts ofevolutionary morphology as homology stand out as

insightful analyses that have methodological im-plications for today (Laubichler and Maienschein,2003).

Gegenbaur’s early years and the beginningof his academic career

Carl Gegenbaur was born on August 21, 1826 inWurzburg, Bavaria into a family of civil servantsand lawyers.3 Due to his father’s rotating assign-ments, he grew up in the idyllic countryside ofFranconia in an environment that fostered hisinterest in natural history from an early age. Hisyears at the Gymnasium in Wurzburg (1838–1845)were less happy, largely due to the strict Catholicorientation of the school. However, like manyothers who experienced the strict discipline of aCatholic institution, these years left him with anindependent mind and a solid classical education.After passing his exit exams, the Absolutorium, in1845 Gegenbaur enrolled at the University ofWurzburg to study medicine with the intention toconcentrate on the natural sciences.

During Gegenbaur’s time as a student theUniversity of Wurzburg made a series of fortunatehires that placed it in the top league of anatomicaland medical research for years to come. In 1847Albert von Kolliker (1817–1905) was offered thechair in physiology and comparative anatomy.Kolliker’s research and teaching focused oncomparative anatomy and histology, morphology,and embryology (Entwicklungsgeschichte).4 He

nCorrespondence to: Manfred D. Laubichler, School of LifeSciences, Arizona State University, PO Box 874501, Tempe, AZ85287-4501. E-mail: Manfred.Laubichler@asu.edu

Received 7 October 2003; Accepted 21 October 2003Published online in Wiley Interscience (www.interscience.wiley.

com). DOI: 10.1002/jez.b.00043

1Letter of Wilhelm Waldeyer to Max Furbringer, 2 April 1897.Senkenbergische Bibliothek, Nachlass Max Furbringer, A. 1. 2752. Seealso Hobfeld (2003) and Nyhart (2003).

2Two developmental mechanisms that could lead to morphologicaltransformations were heterochrony and terminal addition. Onheterochrony see e.g. Gould (1977) for a recent discussion on thesubject.

3The main accounts of Gegenbaur’s life are his own autobiography,Erlebtes und Erstrebtes (Gegenbaur, ’01), the account of Gegenbaur’slife by his student Max Furbringer (’03), and Friedrich Maurer (’26),and, in English, several studies on Evolutionary Morphology byLynn Nyhart (’95, 2002, 2003).

4Throughout most of the 19th century German morphologistsand embryologists referred to their embryological studies asEntwicklungsgeschichte (developmental history). Entwicklungs-geschichte implied a comparative perspective and adherence to themethod of morphology. The descriptive-morphological perspective of

r 2003 WILEY-LISS, INC.

JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 300B:23–31 (2003)

had made a name for himself as the first torecognize the role of spermatozoa during repro-duction and he observed the importance of thenucleus during embryogenesis while studying theembryology of cephalopoda. Kolliker also madesignificant contributions to the methodology ofmorphology, which he identified with ‘‘wis-senschaftlicher’’ (scientific) anatomy. For Kollikerthe goal of morphology was the establishment oflaws of transformation. His main focus wasdevelopmental, but in pursuing laws oftransformation he was far more cautiousabout the interpretation of these laws thanmany others (Kolliker, 1852, 1861). Kollikerargued that at most the morphologist couldreach an ultimate level of descriptivegeneralization, but not of final causation in theexplanation of morphological form (Kolliker,1852). In his later career and especially in his

interpretation of the biogenetic law, proposed byhis friend Haeckel, Gegenbaur would displaymuch of the same methodological caution as histeacher Kolliker.

The second prominent hire during Gegenbaur’stenure as a student in Wurzburg was RudolfVirchow (1821–1902) who was offered the chair inpathological anatomy in 1849. Virchow, who todayis best known for his contributions to cellularpathology and public health, further reinforcedthe developmental approach to anatomy and theimportance of microscopic studies that Kollikerhad already introduced in Wurzburg. For twoyears Gegenbaur was a loyal student of Virchow,often attending the same lecture several timesthus observing the development of Virchow’sviews on anatomical theory and practice as wellas Virchow’s style of lecturing (Gegenbaur ’01, p.48). Wurzburg also had an active scientificsociety, the physikalisch-medizinische Gesellschaft(a common phenomenon in German Universitytowns during the 19th century), where students,such as Gegenbaur regularly met with theirprofessors and the local academic and medicalcircles.

As a student, Gegenbaur focused mainly onanatomy and embryology and in 1851 he publiclydefended his dissertation with Kolliker as theopponent. In his dissertation Gegenbaur discussedthe instability of species and the importance ofembryology for the understanding of animal andplant forms. Some of the arguments in his thesiswere on the surface quite similar to Darwin’sideas, yet nobody (including Gegenbaur) drew anyfurther conclusions from this formal academicdisputatio. Darwin, of course, had already devel-oped a first draft of his theory in his (still) privatenotebooks and manuscripts. By the time of hisgraduation, Gegenbaur had spent his entirestudent years at the University of Wurzburg, ashe saw no reason to go some other place when hecould study with Kolliker and Virchow. In 19thcentury Germany this was quite unusual. Moststudents studied at different universities andattended courses with several prominent teachers.

So Gegenbaur had some catching up do as heleft Wurzburg for his scientific Wanderjahre. Hevisited Johannes Muller in Berlin and later met upwith his mentor Kolliker in Messina, where hespent more than a year working on marineorganisms. Upon returning to Wurzburg, Gegen-baur soon obtained the venia legendi in anatomyand physiology (in 1854) and started teachingzoology (a field that did not overlap with either

Fig. 1. Carl Gegenbaur lecturing in the anatomy hall atThe University of Heidelberg, 1892 (from Friedrich Maurer,‘‘Carl Gegenbaur,’’ and Lynn Nyhart ‘‘Biology Takes Form’’).

Entwicklungsgeschichte is highlighted by the contrast to the metho-dology of Entwicklungsmechanik (developmental mechanics), whichdeveloped during the late 19th century. For the remainder of thisessay I will use embryology as the translation of the German termEntwicklungsgeschichte, even though it is, strictly speaking, not fullyaccurate.

M.D. LAUBICHLER24

Kolliker or Virchow).5 In 1855 he received an offeras auberordentlicher Professor associate professor)of zoology at the University of Jena’s medicalfaculty. For the next three years Gegenbaurtaught zoology, comparative anatomy, histologyand embryology and in 1858 he was offered thechair in anatomy and physiology at Jena.

Even though it was still customary that a chairwould combine anatomy and physiology, Gegen-baur insisted that the two disciplines had devel-oped to such a degree that they should beseparated and covered by two specialists. He wasable to convince the authorities and from 1858served as the chair in anatomy and zoology at themedical faculty of the University of Jena whileAlbert von Bezold (1836–1868) was offered anassociate professorship in physiology. Correspond-ing with his duties as the new chair in anatomy atJena’s medical faculty, Gegenbaur’s researchfocus gradually shifted from the study of inverte-brates to vertebrate anatomy and embryology andhe began his still famous studies of the skeletaland nervous system of vertebrates (especiallyGegenbaur, 1864, 1872, 1898, ’01).

Gegenbaur, Haeckel, and evolutionarymorphology

In 1861 Ernst Haeckel (Fig. 2) obtained hishabilitation in zoology at Jena. Gegenbaur knewHaeckel (who was seven years his junior) from thelatter’s student days in Wurzburg and encouragedhim, after Haeckel obtained his Ph.D. withJohannes Muller in Berlin, to begin his career inJena. Only a year later, in 1862, Gegenbaur passedhis duties in zoology on to Haeckel, who wassubsequently promoted to the chair in zoology in1865. Gegenbaur and Haeckel complemented eachother in many ways; Gegenbaur was more meth-odical, careful, grounded in observation, and

politically more conservative; Haeckel was crea-tive, speculative, a gifted artist, and politically leftleaning. Despite these differences their scientificcollaboration and personal friendship blossomedand it was further deepened by shared tragedywhen in 1864 both Gegenbaur and Haeckel losttheir beloved wives. They found comfort in theirfriendship and the total immersion in science anduniversity matters allowed them to cope with theirloss. These were exciting years for anatomy andzoology. Darwin’s theory of descent with modifica-tion had revitalized morphology and Gegenbaurand Haeckel both reoriented their research pro-grams to establish what has come to be known asevolutionary morphology. Gegenbaur, who hadexpressed proto-Darwinian ideas during the publicdefense of his dissertation, became the ‘‘foremostpractitioner’’ of this new field, while Haeckel

Fig. 2. Ernst Haeckel in 1870.

5 The German university system of the 19th century was quitedifferent from current practices in many countries. The standardacademic career proceeded through the following steps: (1) disserta-tion, followed by a period of research, usually as an (sometimes paid)assistant to a professor; (2) habilitation, often referred to as a seconddissertation. A habilitation was composed of proof of independentresearch after the dissertation (usually a series of papers or amonograph) and proof of teaching abilities (often performed as partof the duties as an assistant.) Habilitation implied the right to teachcourses (venia legendi) in a specified field and granted the title ofPrivatdozent, but no salary, save of lecture fees from courses; (4)auberordentlicher Professor (Extraordinarius), roughly the equivalentto associate professor. This was a tenured and salaried position, butdid not come with all the benefits of a chair; (5) ordentlicher Professor(Ordinarius). An Ordinarius held the chair in a specific recognizedacademic field at a university. In most cases there was only one chairin a field at the University. In many ways the powers of a chair weresimilar to those of a feudal lord; he had several assistants, control of aninstitute’s budget, and all people of lower rank reported directly tohim.

CARL GEGENBAUR 25

functioned as its ‘‘most efficient spokesperson’’(Nyhart, ’95, p. 153).

The emphasis of evolutionary morphology asperceived by Gegenbaur and Haeckel differed fromDarwin’s original program in crucial ways; notindividual variation and its consequences forgradual change by means of natural selectionwas the main focus of study, rather it was the ideathat common ancestry could provide a newfoundation for comparative anatomy and systema-tics as well as a new perspective for the search ofmorphological laws of transformation.

Gegenbaur’s textbook Grundzuge der verglei-chenden Anatomie reflects this transformation ofmorphology, especially in its General Introduc-tion. In the first edition, published in 1859,Gegenbaur was mainly concerned to establishthe internal logic of comparative anatomy and toclarify its relations to physiology and embryology.In the second edition, published in 1870 (Fig. 3),the focus has broadened to include a discussion ofthe importance of morphology for systematics,clearly asserting the primacy of the former: ‘‘Thezoological system thus represents the currentstate of morphological research. Therefore, theposition of an organism within the (zoological)system establishes the genealogical connections ofthe former (p.4).’’6 By 1870 Gegenbaur alsoreflected on the far-reaching consequences ofDarwin’s theory of descent with modification formorphology: ‘‘Inheritance and adaptation are thusthe two principles that can explain the diversity of(animal) organizations as well as the similaritiesbetween them (p.19).’’7 The ideas of commondescent and of inheritance thus provide ana posteriori explanation for the similaritiesbetween organisms that has previously beencaptured by the idea of type or Bauplan.

Methodologically, evolutionary morphology bothrelies on comparative anatomy (‘‘Wherever weestablish by means of rigorous comparison thesimilarity between organizations, this similarity,as an inherited property, indicates commondescent’’ (p.19)8) and refocuses the practice ofcomparative anatomy (‘‘The method of compara-tive anatomy thus becomes more rigorous and a

whole series of comparisonsFthose that focus onisolated organs without first considering whetherthe organization of the whole organisms allows forthe possibility of close genealogical relation-shipsFwill become superfluous’’ (p. 19)9). InGegenbaur’s opinion Darwin’s theory of descenthas initiated a new period in the historyof comparative anatomy, but far from beingrelegated to the fringes of the biologicalsciences, comparative anatomy was strengthenedas the methodological foundation for all researchinto the principles of morphological and phylo-genetic transformation as well as the basis forsystematics.

Fig. 3. Frontispiece of the second edition of Gegenbaur’stextbook Grundzuge der vergleichenden Anatomie, publishedin 1870.

6 ‘‘das zoologische System reprasentiert somit den jeweiligenStandpunkt der morphologischen Forschung. Die Stellung einesOrganismus im System wird daher die Verwandtschaftsverhaltnissedes ersteren hervortreten lassen’’ (p. 4).

7 ‘‘Vererbung und Anpassung sind somit die zwei wichtigenMomente, aus denen sowohl die Mannichfaltigkeit der Organistationals das Gemeinsame derselben verstandlich wird’’ (p. 19).

8 ‘‘Wo wir durch prazise Vergleichung nachgewiesene Ueberein-stimmung der Organisation treffen, deutet diese, als eine vererbteErscheinung, auf gemeinsame Abstammung hin’’ (p. 19).

9 ‘‘Die vergleichende Anatomie wird dadurch auf eine strengereMethode verwiesen und ganze Reihen von Vergleichungen werdenhinfallig, jene namlich, die willkuurlich nur einzelne Organe im Augehaben, ohne vorher zu prufen, ob die Verhaltnisse des Gesamtorga-nismus verschiederer Formen die Moglichkeit des Bestehens einernaheren Verwandtschaft zulassen’’ (p. 19).

M.D. LAUBICHLER26

For Gegenbaur, comparative anatomy alsoprovided the reference frame within which allresults from embryology needed to be interpreted.By the time he published the second edition of theGrundzuge in 1870, Gegenbaur’s conception of therelationship between embryology and comparativeanatomy already differed from Haeckel’s, whocontinued to emphasize the importance of thebiogenetic law.

In the course of the 19th century embryologybecame an increasingly important part of com-parative anatomy as comparisons were extendedto include embryos and as the analysis of devel-opmental stages contributed to a deeper under-standing of the connections between organs bothwithin an organism and between organisms ofdifferent species (see e.g., Gould, ’77; Hall, ’92;Nyhart, ’95). This trend was further reinforced bythe emerging emphasis on the cellular basis of allorganisms (including their pathologies) and thenew trend (by mid-century) to study marineorganisms in their local settings (see e.g.,Coleman, ’77).

In the context of Darwin’s theory of evolution,ontogeny was also seen as an important source ofevidence for the phylogenetic relationships be-tween organisms. Much has been written aboutHaeckel’s biogenetic law (e.g., Russell, ’16; Gould,’77; Di Gregorio, ’95; Nyhart, ’95). Here I onlywant to discuss it in connection to Gegenbaur’sideas of the relations between comparative anat-omy and embryology. The basic methodologicalpremise of the biogenetic law is that the embryo-nic stages provide a more or less accuraterecapitulation of the phylogenetic stages in anorganism’s ancestry, thus allowing for the recon-struction of phylogeny. The basic assumptions ofthe biogenetic law are that: (1) all life is mono-phyletic, (2) more primitive types of organisms arephylogentically older, (3) all evolutionary changes

have to be realized in the course of development,(4) individual development passes through onto-genetic stages that represent primitive phyloge-netic stages, (5) most progressive evolutionarynovelties arise as terminal additions in the courseof development, and (6) this linear parallelismbetween ontogeny and phylogeny can, however, bemasked due to functional demands on the devel-oping embryo and larvae (caenogenesis).

Ideally, ontogenetic evidence would thus com-plement comparative data in establishing a nat-ural system based on genealogic relationships andtherefore embryologists and anatomists wouldboth contribute to the reconstruction of phylogenyand to the establishment of morphological lawsof transformation, the goal of evolutionary mor-phology. In reality, practitioners of evolutionarymorphology would emphasize one or the othermethodological approach, depending on their ownbackground and convictions (Nyhart, 2002). De-spite his close working relationship with Haeckel,who had dedicated the first volume of theGenerelle Morphologie to Gegenbaur (the secondone was dedicated to the fathers of the theory ofevolution; Goethe, Lamarck and Darwin), Gegen-baur soon emphasized the primacy of comparativeanatomy over embryology (Gegenbaur, 1870,1874, 1875, 1888, 1889). He argued that due tothe complications of caenogenetic patterns ofdevelopment embryological data were intrinsicallyambiguous and needed to be interpreted withinthe more reliable and methodologically moreprecise framework of comparative anatomy. Onlyif weighted against the reference frame of sys-tematic comparison could embryological data beused in establishing phylogenetic relationships orcontribute to the search of morphological laws oftransformation.

In 1873, after 18 productive years in JenaGegenbaur accepted the chair in anatomy at theUniversity of Heidelberg, a move that onlyaccelerated the growing separation betweenthe practice and the advocacy of evolutionarymorphology.

The Gegenbaur School

An important feature of nineteenth centuryscience was the emergence of research schools,defined by Princeton historian Gerald Geison(’81, p. 23) as ‘‘small groups of mature scientistspursuing a reasonably coherent program of re-search side-by-side with advanced students in thesame institutional context and engaging in direct,

Fig. 4. Archipterygium theory of the origin of pairedvertebrate appendages: A schematic representation of a gillarch and rays; B, C, evolution of a featherlike fin, archipter-ygium, from A; D-F, evolution of the elasmobranch fin fromthe archipterygium (from J.S. Kinsely, Comparative anatomyof the Vertebrates, Philadelphia: P. Blakiston’s Son (1912),p.114 and Lynn Nyhart ‘‘Biology Takes Form’’).

CARL GEGENBAUR 27

continuous social and intellectual interaction.’’ Inthe history of comparative anatomy (and, assuggested here, the pre-history of evo-devo) theGegenbaur School, which blossomed duringGegenbaur’s Heidelberg years (1873–1903) wasa prime example of this trend (Nyhart , ’96, 2003).Focusing on the Gegenbaur School is helpful as itallows us to analyze the intellectual and metho-dological content of science embedded within itsinstitutional, social, and cultural context, thusproviding us with a richer description of thehistory of science as well as with some stimulatingperspectives on current developments in thehistory of evo-devo. The following discussiondraws heavily from Nyhart’s analysis of thehistory of German morphology.

The Gegenbaur School defined itself institution-ally through a network of former students ofGegenbaurFespecially Max Furbringer (1846–1920), Georg Ruge (1852–1919), Friedrich Maurer(1859–1936), Hermann Klaatsch (1863–1916), andErnst Goppert (1866–1945), as well as some ofGegenbaur’s ‘‘scientific grandchildren,’’ such asRichard Semon (1859–1918), Hermann Braus(1868–1924), and Hans Bluntschli (1877–1962).

This core group was further connected to acircle of mostly foreign students, who spend sometime in Gegenbaur’s lab before returning to theirhome countries, thus further spreading Gegen-baur’s conception of evolutionary morphology andcomparative anatomy. Members of this grouphelped each other to secure positions, defined acoherent educational mission by spreading Gegen-baur’s approach to teaching (including his text-books), and exerted control over importantpublication outlets, such as the MorphologischesJahrbuch.

Intellectually, these scientists followed a set ofmethodological principles that Gegenbaur devel-oped and refined during the 1860s and 1870s(inspired both by his teacher Kolliker and hisfriend Haeckel) that defined the relationshipsamong anatomy, histology and embryology in thecontext of evolutionary morphology. They alsosupported Gegenbaur’s position with regard totwo major problems of comparative anatomy andvertebrate evolutionary morphology; the origin ofpaired appendages and the segmentation of theskull.

The Archipterygium Theory

Gegenbaur’s best known contribution to evolu-tionary morphology is the Archipterygium theory,

the idea that paired appendages (fins and limbs)evolved by means of a transformation of gill archesto limb girdles and rays to fins. Soon afterGegenbaur first proposed his transformationalhypothesis (Gegenbaur, 1870b, 1873, 1876) it waschallenged by the alternative side-fold hypothesisproposed by James Thatcher, Francis Balfour andSt.George Mivart as well as by the Germanmorphologists Anton Dohrn, Carl Rabl, andRobert Wiedersheim. The side fold hypothesissaw the origin of paired appendages in thereduction of a post-cranial lateral fold used tostabilize the movement of primitive fishes.

In the context of vertebrate evolution the originof paired appendages is one of the major problemsof evolutionary innovation and textbooks to thisday discuss both the side-fold and the Archipter-ygium hypothesis, even though the latter is widelythought to have been discredited. Gegenbaurarrived at his hypothesis through a thoroughcomparative investigation of elasmobranch fishes,which suggested to him a transformational sce-nario from gill arch to girdle and from rays to fins(Fig. 4). Gegenbaur subsequently modified hishypothesis in light of the new discovery of thelungfish Ceratodus forsteri, now arguing that inthe primitive condition the fin rays branched offon both sides of the base (only to be restricted toone side in later phylogenetic transformations).Central to all of Gegenbaur’s arguments is theconviction that the origin of anatomical innova-tions is transformational, i.e. that innovationsderive from the modification of previously existingstructures, rather than arise de novo. These ideascan be traced back to Goethe’s transformationalprinciples of morphology, further expanded byGeoffroy de St. Hilaire, and the correspondingnotion of a fundamental (primitive) vertebrateplan (Appel, ’87; Nyhart, 2002).

The core of Gegenbaur’s program of evolution-ary morphology was to discover these structuraltransformations and to place them into the correctphylogenetic sequence. By the time of the limbcontroversy Gegenbaur has become even moreforceful in his assertion of the methodologicalprimacy of comparative anatomy over embryology,arguing that the frequent caenogenetic patterns ofdevelopment make it very hard to uphold thebiogenetic law as a guiding principle of evolu-tionary morphology. As most of his opponents inthe limb controversy employed embryologicalargumentsFsome, like Anton Dohrn, explicitlywithin the context of the biogenetic lawFthecontroversy also was one about method and the

M.D. LAUBICHLER28

role of different forms of evidence. This disagree-ment over accepted standards of evidence and thereach of different methods contributed to thedecline of the program in evolutionary morphology(Nyhart ’95, 2002). Current proponents of evolu-tionary developmental biology should be inter-ested in the fate of evolutionary morphology, as ithighlights the importance of shared evidentiarystandards for the success of an interdisciplinaryresearch program (Wagner et al., 2000).

The question of the origin of paired appendagesis still not completely resolved and recent studieshave added new twists to the problem (Tabin, ’92;Coates, ’94, 2003). In this context Gegenbaur’stransformational approach has gradually gainednew acceptance, albeit in a radically differentempirical and experimental context (Coates,2003). One of the main tenets of evolutionarydevelopmental biology is the importance of con-served developmental modules and pathways andthe role of regulatory evolution (Carroll et al.,2001). In establishing the homology between thepectoral, pelvic and branchial skeletons Gegen-baur referred to this relationship as homodyna-mousFindicating a similarity between parts thatare expressed sequentially along the anteriorpos-terior axisFand he also used the pectoral fins asan example of incomplete homology, arguing thatthe teleost fin has lost elements still present inthe fins of selachier. If we do not focus on theempirical details of Gegenbaur’s claims about theorigin of paired appendages (which have mostlybeen falsified, especially in light of new phyloge-nies), but rather on the methodological implica-tions of his conceptual framework of comparativeanatomy, we can easily see that our currentconcern about integrating molecular and develop-mental with evolutionary mechanisms facessimilar conceptual problems as evolutionarymorphology. For instance, any assertion of homol-ogy between phenotypic characters due to thepresence of similar molecular and developmentalmodules will have to be carefully evaluated in lightof comparative data, as molecular data, such aspatterns of gene expression, can have the sameproblems as embryological evidence in that cae-nogenetic processes can distort underlying phylo-genetic patterns.

On the other hand, the fact that a comparativelysmall number of regulatory modules control thedevelopment of many phenotypic characters givesnew life to older ideas that most evolutionarychanges are transformational variations of anunderlying basic repertoire. In light of these

findings, it might be worth revisiting Gegenbaur’smethodological reflections and see whether theycan, in modified form, contribute to the theoreticalstructure of current evolutionary developmentalbiology.

A current example in which Gegenbaur-likereflections have led to interesting new hypothesesabout evolutionary transformations is the problemof the homology of the digits in the avian hand(Wagner and Gauthier, ’99). Here anatomical andembryological data each suggest a different homo-logical relationship of avian digits with the digitsof other vertebrates; the anatomical evidenceindicates the avian digits correspond to the digitsI, II, and III whereas embryological data suggestthat the avian digits are derived from condensa-tions II, III, and IV. The proposed solution, namelythat a subsequent frame shift in developmentalidentities turns the embryological anlagen forcondensations II, III, and IV into digits I, II, andIII (Wagner and Gauthier, ’99) resolves thisapparent conflict by arguing that (1) developmen-tal origin and subsequent individualization of acharacter can be dissociated and (2) thereforeembryological data need to be evaluated withinthe context of comparative evidence and cannotper se be used to establish homology. In otherwords, the present-day preformist temptation, touse genetic or developmental evidence to ascertainhomology needs to be resisted and claims abouthomology need to be evaluated within a frame-work of comparative anatomy. Gegenbaur couldnot have agreed more.

CONCLUSION

For evolutionary developmental biologists at thebeginning of the 21st century Carl Gegenbaur,the ‘‘most important morphologist in the world’’at the end of the 19th century, is still an importantfigure to remember. He has earned his place in thepantheon of science for his contributions tomorphology, comparative anatomy, and embryol-ogy; his ideas about the origin of limbs and thesegmentation of the vertebrate skull are stilldiscussed, albeit in the radically different contextof Hox genes and conserved developmental signals(Kuratani, 2003; Coates, 2003; Grandel, 2003);and the role of Gegenbaur School and his friend-ship with Ernst Haeckel are still the subject ofanalyses by historians of biology (e.g., Krause, ’96;Nyhart, 2003).

But, I would argue, the most important aspect ofGegenbaur’s legacy for 21st century evolutionary

CARL GEGENBAUR 29

developmental biology lies in his methodologicalreflections about the relationship between com-parative anatomy and embryology and his empha-sis of the comparative method and of careful anddetailed observation over tempting and far reach-ing speculation. Current research in developmen-tal evolution aims to integrate evolutionary anddevelopmental mechanisms and perspectives inorder to explain the patterns of morphologicalevolution. In doing so, it faces similar challengesas those of 19th century evolutionary morpholo-gists, namely to distinguish between severalpossible phylogenetic scenarios. We all agree thatthis is an enormously difficult task, some of useven voice concerns about whether such a task ispossible at all (Wagner, 2001), but we can all findinspiration for the painstaking and detailed workthat is needed to solve any of these problems inGegenbaur’s statement that: ‘‘The inclusion ofontogeny has completed comparative anatomy, itsrange has been broadened to incorporate numer-ous problems that could only have been solved bymeans of ontogenetic analysis. The foundation ofcomparative anatomy has been deepened, asontogeny allows also to analyze the history oforgans and not just their structure. Through itsrelationship to comparative anatomy, the impor-tance of ontogeny, however, is not diminished,rather it is elevated, as only the integration ofontogeny with comparative anatomy allows us toreach the final goal [of comparative biology], theestablishment of phylogeny’’ (Gegenbaur, 1875).10

ACKNOWLEDGMENTS

The author wishes to thank Gunter Wagner,Nick Hopwood, Jane Maienschein, and LynnNyhart for helpful discussions on the subject.

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