Darwin's Diagram of Divergence of Taxa as a Causal Model ...

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Darwin’s Diagram of Divergence of Taxa as a Causal Model for the Darwin’s Diagram of Divergence of Taxa as a Causal Model for the

Origin of Species Origin of Species

Juan Luis Bouzat Bowling Green State University, [email protected]

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DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA AS A CAUSALMODEL FOR THE ORIGIN OF SPECIES

Juan L. BouzatDepartment of Biological Sciences, Bowling Green State University

Bowling Green, Ohio 43402 USA

e-mail: [email protected]

keywordsCharles Darwin, common descent, evolution, natural selection, phylogeny

abstractOn the basis that Darwin’s theory of evolution encompasses two logically independent processes

(common descent and natural selection), the only figure in On the Origin of Species (the Diagramof Divergence of Taxa) is often interpreted as illustrative of only one of these processes: the branchingpatterns representing common ancestry. Here, I argue that Darwin’s Diagram of Divergence of Taxarepresents a broad conceptual model of Darwin’s theory, illustrating the causal efficacy of naturalselection in producing well-defined varieties and ultimately species. The Tree Diagram encompasses theidea that natural selection explains common descent and the origin of organic diversity, thusrepresenting a comprehensive model of Darwin’s theory on the origin of species. I describe Darwin’sTree Diagram in relation to his argumentative strategy under the vera causa principle, and suggestthat the testing of his theory based on the evidence from the geological record, the geographicaldistribution of organisms, and the mutual affinities of organic beings can be framed under thehypothetico-deductive method. Darwin’s Diagram of Divergence of Taxa therefore represents a broadconceptual model that helps understanding the causal construction of Darwin’s theory of evolution,the structure of his argumentative strategy, and the nature of his scientific methodology.

Now let us see how this principle of great benefit being derived from divergence of character,combined with the principles of natural selection and extinction will tend to act. The accompa-nying diagram will aid us in understanding this rather perplexing subject (Darwin 1859:116).

Introduction

SINCE the publication of the first editionof On the Origin of Species by Means of Nat-

ural Selection by Charles Darwin in 1859,there have been numerous debates not onlyabout the nature of Darwin’s theory (or theo-ries) and its significance (Kohn 1985; Hodgeand Radick 2003; Ruse 2009), but also on the

actual structure of what Darwin himself de-scribed as “one long argument” (Darwin1859:459; Hodge 1977; Mayr 1991; Waters2003). Ernst Mayr (1991) argued that “Dar-win’s theory” involved at least five indepen-dent theories, which included Evolution assuch (i.e., descent with modification in con-trast to Special Creation), Common Descent

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(i.e., the relationship among species by an-cestry), the Multiplication of Species (to ex-plain the origin of diversity), Gradualism(regarding the mode and tempo of evolution-ary change), and Natural Selection (as amechanism to explain adaptations). Argu-ably, in a historical context, these supposedlyindependent theories were considered byDarwin as part of a single, unified theory ofevolution. That is, Darwin viewed these as a“logically inseparable package” (Mayr 1991:37), which would justify the use of a commonargumentative strategy (i.e., one long argu-ment) throughout the Origin.

Today, most biology textbooks describecommon descent and natural selection asthe two major components of Darwin’s the-ory of evolution (e.g., Reece et al. 2011; Free-man et al. 2014; Urry et al. 2014). Theseelements are considered conceptually as wellas logically independent hypotheses (e.g.,Mayr 1982:505–510; Sober and Orzack 2003:426), given that the truth or falsity of thesehypotheses is independent from each other.This interpretation is consistent with thethinking of some of Darwin’s contempo-raries as well as modern biologists. For exam-ple, Ernst Haeckel (1868) accepted naturalselection, but believed major groups of con-temporary organisms originated indepen-dently. Motto Kimura (1968) accepted thecommon ancestry of genes and species, butbelieved evolutionary changes at the molec-ular level were mostly neutral (i.e., driven bygenetic drift rather than natural selection;Sober and Orzack 2003). Throughout thispaper “Common Descent” refers to thebranching patterns of divergent speciesoriginating from common ancestralforms. Other authors (e.g., Sober, personalcommunication) interpret common ancestryas a claim that all life forms can be tracedback to one or a few ancestral lineages. Com-mon ancestry is also often assumed to be partof Darwin’s idea of “descent with modifica-tion.” However, the concept of “descent withmodification” may be applied narrowly tothe idea of evolutionary change within a sin-gle lineage. One thus can argue that theconcept of common descent comprises theidea of descent with modification; however,the opposite is not necessarily true.

Here, I will argue that Darwin’s one longargument assumes that there is indeed acausal relationship between the two pro-cesses; and that Darwin’s Diagram of Diver-gence of Taxa (Darwin’s Tree Diagram), theonly figure illustrated in the Origin of Species,represents a causal model of his theory onthe origin of species by means of naturalselection (Figure 1). I will argue that thestructure of Darwin’s argument, as repre-sented by the Tree Diagram, suggests thatDarwin did not consider common descentand natural selection to be logically indepen-dent processes. Under Darwin’s perspective,natural selection explained the branching pat-terns of species; i.e., natural selection pro-vided the mechanism to understand how thepatterns of common descent originated. Thecausal relationship between natural selectionand common descent is clearly described byM. J. S. Hodge’s (1977) interpretation ofDarwin’s argumentative strategy following thevera causa principle (Kavaloski 1974; Hodge1977). Hodge specified the three necessaryelements underlying the principle of veracausa: i.e., the existence of a cause, its compe-tence to produce an effect, and its putativeresponsibility for the phenomenon to be ex-plained. Hodge argues that, in the Origin,Darwin structured his argument followingJohn Herschel’s (1830) argumentative idealsfor the testing of theories, which requiredindependent evidence for causal existence,competence, and responsibility. The argumenta-tive structure of the Origin thus makes thecase for the existence of natural selection(Chapters I–III), its competence to producenew species (Chapter IV), and its responsibil-ity for the production of extant and extinctspecies (Chapters X–XIII; Hodge 1977). TheTree Diagram provides therefore a concep-tual model of Darwin’s thesis, centered onthe mechanism of natural selection as thevera causa for the origin of species.

The ideas presented here are consistentwith Sober’s (2009) characterization of Dar-win’s argumentative strategy in writing theOrigin in the specified order in which it waswritten, with an initial description of artificialselection, followed by natural selection, andfinally common ancestry. According to So-ber, Darwin assigns “causal priority” to natu-

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ral selection, since it is selection that leads to(i.e., explains) branching. In fact, in his “BigSpecies Book” (the manuscript from whichthe Origin represented only an abstract of theoriginal book Darwin intended to complete)Darwin stated that “[n]o theory of the deri-vation of groups of species from a commonparent can be thought satisfactory until itcan be shown how these wondrous correla-tions of structure can arise” (Stauffer 1975:175). The theory of natural selection thussolved this question, providing a mechanisticexplanation for the correlations of structure,both between traits in an organism andamong distinct species.

In this paper, I first describe the majorcomponents of Darwin’s theory, establishingthe causal efficacy of natural selection as theexplanatory mechanism driving divergenceof character and common ancestry. I willthen argue that Darwin’s Diagram of Diver-gence of Taxa represents a causal model forhis theory on the origin of species by meansof natural selection, establishing the case for

competence under the principle of vera causa.Finally, I suggest that Darwin’s Tree Diagrammay be interpreted as a working hypothesis(the case for competence), as one can infer aseries of predictions that can be testedthrough experimentation/observation in ahypothetico-deductive framework (providingthe case for responsibility). Darwin’s evidentialobservations (presented in Chapters X–XIII ofthe Origin) can, thus, be directly related to spe-cific predictions that emerged from the TreeDiagram as a conceptual model of evolution.

Darwin’s TheoryAlthough Darwin refers to “my theory” 57

times throughout the Origin, he mostly usesthis term in relation to expectations, difficul-ties, and objections of the theory, or to de-scribe evidence that would be consistentwith, or which would allow rejection of, histheory. Darwin never formally defined whathe calls “my theory.” However, at the begin-ning of Chapter XIV of the Origin (Recapit-

Figure 1. Darwin’s Diagram of Divergence of Taxa Presented in the ORIGIN

The diagram integrates the Principle of Divergence and the Principle of Natural Selection together with theprocess of extinction into a conceptual model of evolution.

March 2014 23DARWIN’S DIAGRAM OF DIVERGENCE OF TAXA



ulation and Conclusion) he refers to the“theory of descent with modification throughnatural selection” (p. 459), which impliesthat his was a unified theory that combinedcommon descent and natural selection. Thisidea is consistent with one of the passages inwhich Darwin describes the two great laws ofnature, Unity of Type and the Conditions ofExistence, in relation to what he calls “mytheory” (p. 206). It is there where he ex-plicitly provides an argument for a uni-fied theory of common descent and naturalselection, as he emphasizes that the Law ofConditions of Existence (which embracesnatural selection) is a higher law that encom-passes that of Unity of Type (which is ex-plained by unity of descent; see below). I willargue that a concise description of Darwin’stheory is summarized in the title of his bookOn the Origin of Species by Means of NaturalSelection, or the Preservation of Favoured Races inthe Struggle For Life. Although the title doesnot make any explicit allusion to commondescent, Darwin’s theory was intended toprovide an evolutionary mechanism (naturalselection) as a causal explanation for the or-igin of species and thus the observed pat-terns of common ancestry, in contrast to theprevalent idea that species were created in-dependently.

The overall structure of Darwin’s “onelong argument” is shown in Figure 2. Darwinstarts with the description of established factson the variation of domestic and wild speciesas an essential feature of populations (Chap-ters I and II). These chapters provided thefoundations (arguing from analogy; Hodge1977) to infer an evolutionary mechanismfor adaptive change, i.e., the process of naturalselection. In Chapter III (entitled Struggle forExistence), Darwin provides a detailed expla-nation of the major elements involved in theprocess of natural selection (i.e., variation,differential fitness, inheritance, and the Mal-thusian principle), which characterize the so-called “struggle for existence.” Chapter III ofthe Origin provides a precise description ofthe Principle of Natural Selection and thefirst explicit definition (since the term wasmentioned only briefly on page 5 of the In-troduction): “I have called this principle, bywhich each slight variation, if useful, is pre-

served, by the term of Natural Selection” (p.61). However, this chapter does not develop,as many would argue, what Darwin calls “mytheory.”

It is not until Chapter IV (entitled NaturalSelection) that Darwin puts forward the ideaof natural selection in the context of theorigin of species. That is, Chapter IV formal-izes what Darwin calls “my theory,” the causallink between natural selection as a creativeforce leading to adaptive divergence into theorigin of new species, thus generating pat-terns of descent, ancestry, and extinction. Inthe first 35 pages of this chapter, Darwinprovides multiple examples of natural selec-tion operating in nature, describes the pro-cess of sexual selection, and emphasizes theimportance of interbreeding, reproductiveisolation, and gradual change. Darwin’s goalin this chapter was, however, to present aunified theory for the origin of speciesthrough the process of natural selection.Darwin uses the only figure in the Origin, theDiagram of Divergence of Taxa (Figure 1),to illustrate the logical connection betweennatural selection, adaptive divergence, andextinction in explaining descent with modi-fication and common ancestry. As first de-scribed on page 116 of the Origin, the TreeDiagram illustrates the principles of diver-gence and natural selection (conditions ofexistence), together with the process of ex-tinction, into a single conceptual model ofevolution.

The Tree Diagram is therefore not a sim-ple schematic of the pattern of common an-cestry, as presented by many (Ragan 2009;Tassy 2011). The Tree Diagram represents aconceptual model of Darwin’s theory, bywhich he assigns causal efficacy to the pro-cess of natural selection as an explanatorymechanism for the origin of species. That is,in this model, natural selection is the mech-anism that explains adaptive divergence andthe branching tree. In this sense, I wouldargue that the “adaptive end” of natural se-lection, which explains biological design(Chapter III), is not the most important as-pect of Darwin’s theory as presented in theOrigin. It is natural selection, when presentedas a creative force leading to adaptive diver-gence, which provides causal efficacy in Dar-




win’s theory on the origin of species. In away, the Origin was written as one long argu-ment to provide a mechanistic explanation(natural selection) as the vera causa for theorigin of species.

It is unquestionable that in the Origin Dar-win described both Unity of Type (due to com-mon descent) and Conditions of Existence(natural selection) as the “two great laws” thathave formed all natural beings. It is also indis-

putable that Darwin assigned causal priority tonatural selection over common ancestry, as hestated that “the law of the Conditions of Exis-tence is the higher law; as it includes, throughthe inheritance of former adaptations, that ofUnity of Type” (Chapter VI; p. 206). In a way,one could argue that Darwin’s thesis in theOrigin may be better interpreted not as a directresponse to William Paley’s Natural Theology(Paley 1802) arguments on Divine Design (the

Figure 2. The Structure of Darwin’s “One Long Argument” in the ORIGIN OF SPECIES

The schematic highlights the three major elements of the Principle of vera causa in Darwin’s argument: thecase for existence, competence, and responsibility of natural selection on the origin of species.




“adaptive end” of natural selection), but toGeorges Cuvier’s (1817) ideas on the indepen-dent creation of species (i.e., natural selectionas a mechanism for adaptive diversification).

The idea that Darwin’s theory should not beinterpreted as common ancestry plus naturalselection, but rather natural selection as thecausal mechanism to explain common ances-try and the origin of organic diversity, is alsosupported by the chronological developmentof some historical events leading to the pub-lication of the Origin. The idea that livingorganisms may have descended from com-mon ancestral forms predated Charles Dar-win. Early naturalists, including Buffon(1749), Erasmus Darwin (1794), and tosome extent Lamarck (1809), had previouslysuggested that species may change into dif-ferent organisms and that some groups mayhave descended from a single ancestralform. However, the actual formalization ofthe idea of common descent into a tree-likestructure of species ancestral/descent rela-tionships was first published by Alfred RusselWallace in his 1855 paper, “On the law whichhas regulated the introduction of new spe-cies.” Here, Wallace introduced for the firsttime “the analogy of a branching tree, as thebest mode of representing the natural ar-rangement of species and their successivecreation” (Wallace 1855:191). In fact, althoughno formal figure was represented in this ar-ticle, Darwin’s own personal copy of Wal-lace’s (1855) paper revealed a hand-drawnsketch of a tree on its margins and the an-notation “[u]ses my simile of tree” (Brooks1984:245; Kottler 1985:382).

What was still missing, however, was a the-ory that could provide a mechanistic expla-nation for the tree-like patterns of ancestraldescent, i.e., the vera causa for the origin ofspecies. And Darwin thought that natural se-lection was the key element to solve “themystery of mysteries.” After all, Darwin didnot panic when he read Wallace’s (1855)paper describing precisely the idea of com-mon ancestry and a tree-like structure for theorganization of diversity, even though hehad independently had this exact same ideaabout 18 years earlier, when he described a“coral-like tree” and the “I think” tree in hisNotebook B (pp. 26 and 36) on the Trans-

mutation of Species (1837; Figure 3). In fact,Darwin thought that Wallace’s (1855) paperdid not add anything new to the question ofhow species originated (Kottler 1985). Al-though in his paper Wallace described thephenomenon of branching divergence, hedid not actually explain it (Kotler 1985:381);i.e., he did not provide an evolutionarymechanism to account for it. However, Dar-win felt indeed forced to demonstrate prece-dence when he received Wallace’s (1858)letter describing the independent formula-tion of the principle of natural selection. Itwas the independent development of theprinciple of natural selection by Alfred Rus-sel Wallace, and not the idea of commondescent, which led Charles Lyell and John D.Hooker to rush a joint presentation ofDarwin and Wallace’s ideas to the Lin-nean Society, less than two weeks afterreceiving Wallace’s letter on June 18,1858 (Darwin and Wallace 1858).

Darwin’s Diagram of Divergenceof Taxa

The first description of Darwin’s Diagramof Divergence of Taxa appears in Chapter IVof the Origin. The diagram is a schematicrepresentation of three major processes (p.116): the principle of divergence; the princi-ple of natural selection; and the process ofextinction. The Tree Diagram is not just adescriptive model of the patterns of com-mon descent. As indicated above, evidentialpriority of common ancestry had beenshown by Darwin himself in his Notebook Bon the transmutation of species many yearsbefore the publication of the Origin (Barretet al. 1987).

I argue that, through the Diagram of Di-vergence of Taxa, Darwin’s intention was todraw a logical connection between naturalselection and the divergence of species intovarieties and newly formed species. Naturalselection was presented as a creative forceleading to the adaptive divergence of speciesinto varieties and subsequently into new spe-cies, thus generating the observed patternsof descent and common ancestry. The struc-ture of the argument is formalized under theconceptual frame of two competing theories:evolution or the transmutation of species




and independent creation. It was this “causaltheory” that Darwin thought “banish[es] thebelief of the continued creation of new or-ganic beings” (pp. 95–96). Interestingly, inhis “Big Species Book” Darwin framed histheory as an alternative to fortuitous chancerather than creation, as he stated that “noone I should think could extend this doc-trine of chance to the whole structure of ananimal, in which there is the clearest relationof part to part, & at the same time to otherwholly distinct beings” (Chapter V of Dar-win’s Natural Selection; Stauffer 1975:174).

Darwin’s Diagram of Divergence of Taxa(Figure 1) is a theoretical tree that representsa truncated section of a group of species (A–L)with varying degrees of morphological sim-ilarities (based on their relative proximity

on the horizontal axes), and their patternsof ancestral descent. The schematic of the“little fan of diverging dotted lines” (p. 117)is perhaps the key element in the Tree Dia-gram since this represents the causal effi-cacy of natural selection in leading toadaptive divergence and ultimately the ori-gin of new species. In Darwin’s own words,the “little fan of diverging dotted lines ofunequal lengths proceeding from (A), mayrepresent its varying offspring” (p.117). It isnot clear if he is referring specifically to theindividuals’ offspring or to species/varieties’offspring (i.e., each dotted line representinga group of individuals). Most would inter-pret these dotted lines as incipient varietiesanalogous to those selected by breeders,which Darwin described in Chapter I. In any

Figure 3. Darwin’s “Coral-Like” Tree and the “I Think” Tree Described in 1837These trees were sketched on pages 26 and 36, respectively, of his Notebook B on the Transmutation of

Species (Barret et al. 1987). (Reproduced with permission of Cambridge University Library.)




case, his statements emphasizing that the“variations are supposed to be extremelyslight” and that “[o]nly those variations whichare in some way profitable will be preserved ornaturally selected” (p. 117), clearly refer to thepopulation-level process of natural selectionbased on individual variations described inChapters I through III of the Origin.

Perhaps Darwin’s Tree Diagram may bebetter interpreted as a “zoom-in/zoom-out”picture, representing different processes atmultiple scales. That is, at some point, wecan expand the scope of the diagram to rep-resent a larger temporal scale, denoted bythe numbered horizontal lines (I–XIV).According to the diagram, each horizontalline represents 1000 generations (or even10,000). At that scale, the dotted lines repre-sent varieties and their modified descen-dants. “When a dotted line reaches one ofthe horizontal lines, and is there marked bya small numbered letter, a sufficient amountof variation is supposed to have been accu-mulated to have formed a fairly well-markedvariety” (p. 117).

The causal efficacy of natural selection inthe formation of well-marked varieties isthen extended into the origin of new speciesby depicting two general processes: the ten-dency of variability to be in itself hereditaryand the principle of divergence in character,both represented in the Tree Diagram. In-heritance is broadly represented by the con-nection of the small numbered letters (a1–a2,a2–a3, etc.) and the continued variation (rep-resented by the fan of dotted lines) that isretained across generations. That is, varieties“will tend to vary, and generally to vary innearly the same manner as their parents var-ied” (p. 118). The principle of divergence, bywhich extreme varieties or varieties with ex-treme characters will have the best chance ofsurviving, is represented by the increased dif-ferentiation of modified descendants in eachof the lineages (e.g., from a1–a10 or m1–m10),as indicated by the fact that, e.g., variety a2

differs more from A than does variety a1 (asa1 is more closely located to A in thehorizontal axis of morphological varia-tion). These two processes (inheritanceand character divergence) lead to twomajor inferences: that modified varieties

originate from a common parent; andthat both the number and divergence ofvarieties will generally increase over time.

Through the Diagram of Divergence ofTaxa, Darwin assigns therefore “causal prior-ity” to natural selection as an explanatoryprocess for the origin of varieties and, ulti-mately, the origin of new species. As describedabove, after many generations populationscan be transformed through natural selec-tion into well-marked varieties, subsequentlyinto the “doubtful category of sub-species”(p. 120) and, ultimately, into well-definedspecies. In Darwin’s own words, “the diagramillustrates the steps by which the small differ-ences distinguishing varieties are increasedinto the larger differences distinguishingspecies” (p. 120). The causal effect of naturalselection in generating descent with mod-ification and common ancestry is also ev-idenced in the description of Darwin’s originaldiagrams represented in Chapter VI of his“Big Species Book” manuscript (Stauffer1975). The Tree Diagram in the Origin wasderived as a composite figure of Diagrams I,III, and IV in the “Big Species Book,” repre-senting the expected patterns generated bythe principle of natural selection, and theprinciple of divergence and the process ofextinction as bearing on natural selection.In contrast, Diagram II reveals the evolu-tionary patterns of the same ancestral spe-cies but expected under the alternativeexplanation of chance, where the preserva-tion of varieties is not driven by the processof natural selection, but is left to randomsurvivorship (Figure 4). Darwin’s descriptionsof the theoretical patterns derived from Dia-grams I and II emphasize the role of naturalselection as a causal effector of directionalchange, increased divergence, and increaseddiversification, as these changes are not pre-dicted under the doctrine of chance, i.e., inabsentia of natural selection (Figure 4).

The Tree Diagram illustrated in the Originalso embodies the gradual nature of naturalselection and evolutionary change, as naturalselection acts slowly over long periods oftime, producing slightly different varieties,which will become well-defined varieties and,eventually, species. As described in ChapterIV, at one level, variation of diverging dotted




lines in the Tree Diagram is supposed to beextremely slight (p. 117). Gradual change ischaracterized by increased divergence incharacter, as the taxa represented by the dot-ted lines become well-marked varieties whenreaching successive generation intervals, andvarieties are intermediate between ancestraland descendant forms (i.e., a3 � a2 � a1).

Thus, in the Tree Diagram, the concept of“natura non facit saltum” emerges as a conse-quence of the principle of divergence throughthe process of natural selection.

Darwin’s Diagram of Divergence of Taxaalso represents causal efficacy for the hierar-chical organization of emerging lineages andthe origin of higher taxonomic groups. This

Figure 4. Diagrams I and II from Darwin’s “Big Species Book”These diagrams were part of Darwin’s Natural Selection manuscript being the second part of his “Big Species

Book” written from 1856 to 1858 (Stauffer 1975). The diagrams illustrate the effects of natural selection onspecies divergence and extinction (Diagram I) and the effects of chance (Diagram II). Under the doctrine ofchance, Darwin predicts no change within a lineage (M–m10), no increased divergence (M–L distance similarto m10–L), and no increased diversification (a1–l1 do not differ much more than a10–l10).




is represented by the condensed version ofthe Tree Diagram from generation intervalsXI to XIV (Figure 1). The origin of subgen-era or even distinct genera is represented bythe relatively recent divergence of closely re-lated species (e.g., a14, q14, and p14) from spe-cies that have diverged at an earlier period(e.g., b14 and f14). The uneven distribution ofthe species in the horizontal axes also showsthe morphological similarities of closely re-lated species. The eight descendant speciesfrom A can thus be grouped into three sub-genera or genera of closely related species.The complete diagram reflects therefore thehierarchical organization of nature, with indi-viduals being grouped into varieties, varietiesgrouped into species, and species groupedinto higher taxa (genera, families, and or-ders); this resulting from the combined effectsof natural selection and common ancestry.The evolutionary foundation of this hierar-chical organization is what provided thebases for modern taxonomy, which definesnatural taxa as monophyletic groups (Mayr1942).

A comprehensive look at the Diagram ofDivergence of Taxa shows that Darwin alsoincorporated unaltered lineages and theprocess of extinction within his conceptualmodel of evolution. In the case of unalteredlineages (e.g., F–F14), causal efficacy can alsobe attributed to natural selection, in this casenot as a diversifying process, but as a stabiliz-ing force weeding out the unfit. This processof negative selection was originally describedby Edward Blyth in 1835 and has been de-scribed by Dawkins as the first conceptualbridge to the evolutionary understanding ofnatural selection (Dawkins 2010:219).

The process of extinction results, however,from the replacement of predecessors andthe original parent by improved descen-dants. The intermediate forms (e.g., a6, d5,k8) and their parent species (A) will generallybecome extinct. Darwin’s explanation con-sistently refers to inherited adaptive advan-tages of diversified varieties in “the naturaleconomy of their country” (p. 122). That is,less improved states of a species will tend tobecome extinct, as they are replaced by moreimproved states (p. 121). The application ofthe principle of natural selection to the de-

scription of the process of modification ofhigher taxonomic groups (p. 125) has ledsome researchers to suggest that Darwin mayhave also provided group selection argu-ments for the origin of diversity (Ruse 1980).As Darwin stated, “the struggle for the pro-duction of new and modified descendants,will mainly lie between the larger groups,which are all trying to increase in number.One large group will slowly conquer anotherlarge group, reduce its numbers, and thuslessen its chance of further variation andimprovement” (p. 125). This statement sug-gests that we may be able to predict thesuccess of highly diversified groups as theyhave higher “fitness” (i.e., they producemore varieties/species) compared to thosethat produced a reduced number of newgroups. I believe, however, that this is just aconsequence of Darwin’s emphasizing onthe causal priority of natural selection forthe origin of diversity, rather than a groupselection argument for the success of diver-sifying taxa, as for predicting the future suc-cess of specific groups based on their pasthistory (reflected in the Tree Diagram),Darwin stated that “which groups will ulti-mately prevail, no man can predict” (p.126).

Darwin’s Tree Diagram as a model of evo-lution has two major consequences for thetemporal dynamics of diversity, whichprovide causal efficacy for explaining thegenealogical relationship by which all liv-ing beings are united, as well as the hierar-chical organization and natural classificationof organic diversity. The process of descentwith modification through natural selectionresults in an increase in the number of spe-cies through time and a tendency of thesespecies to increasingly diverge in characters.This is represented in the tree by the subor-dination of groups; e.g., of closely relatedspecies into genera, closely related generainto families, and families into higher taxo-nomic groups. The Tree Diagram is there-fore used by Darwin as a model to representevolutionary patterns at multiple scales since,at different points throughout the Origin,capital letters (A–L) and numbered let-ters (an–zn) in the diagram refer some-times to well-defined varieties and species




(e.g., Chapter IV), and other times to sub-genera and genera (e.g., Chapters X andXIII). The subordination of groups is thenrepresented by the hierarchical organizationof genera (a14–z14) grouped into subfamilies(e.g., a14–q14–p14), subfamilies grouped intofamilies (a14–q14–p14–b14–f14), and familiesinto two orders (defined by all extant descen-dants from A and I, respectively). At the largestscale, all numbered letters at the generationinterval XIV (a14–z14; representing currenttime) of the Tree Diagram are united intoone class by an ancient but unseen commonancestor of genera A–L (pp. 420–421).

Evidence in Support of the TheoryThe last section of Darwin’s “one long ar-

gument” consists of evidential support forthe theory from his observations on the dis-tribution of species in the geological record,on the geographical distribution of varieties,species and higher taxonomic groups, andon the hierarchical organization of nature(Table 1).

Hodge (1977) has argued that, in doingso, Darwin followed the argumentative strat-egies of Herschel and Whewell, by present-ing evidence favoring responsibility under thevera causa principle, i.e., showing that theproposed theory on the origin of species bynatural selection is indeed responsible forthe observed patterns in nature. I would ar-gue, however, that Darwin’s theory, as repre-sented in the Diagram of Divergence ofTaxa, may be framed under a hypothetico-deductive framework in which the theory canbe tested either experimentally or through ob-servation. That is, the Tree Diagram doesnot represent a simple descriptive model(of what has happened) but a hypo-thetico-deductive model that generates test-able predictions regarding the evolutionaryprocess. Perhaps the best evidence for thisidea comes from Darwin’s “Big SpeciesBook” (Stauffer 1975), in which he contraststhe evolutionary patterns predicted by twoalternative hypotheses: natural selection (Di-agram I) versus chance (Diagram II; see Fig-ure 4). Through the last few chapters of theOrigin (Chapters IX, X, and XII), Darwinconsistently refers back to the Tree Diagramas he presents multiple evidential facts

regarding the temporal and spatial distri-bution of species and the hierarchical or-ganization of nature. The explanatory powerof the theory results, therefore, from the factthat the evidence from apparently indepen-dent phenomena is consistent with the pre-dictions deduced from Darwin’s Diagram ofDivergence of Taxa. Other authors (e.g.,Harman 1965; Thagard 1978), however,have argued that this represents an inductivestrategy by inference to the best explanation,in the sense that a variety of different phe-nomena can be explained by the same causalprocess.

A summary of Darwin’s evidence in sup-port of the theory is presented in Table 1.Following Darwin’s structure on the presen-tation of evidence, one can assess the specificpredictions of the model, as represented in

TABLE 1Structure of the evidence described in the Origin(with corresponding chapter references) in support

of Darwin’s theory of evolution by means ofnatural selection

Evidence in support of the theory

Evidence from the Geological Record (Chapters IX–X)1. The process of modification is gradual (Gradualism)2. The amount of change does not correspond with the

succession of geological formations (Variable Rates ofChange)

3. Ubiquitous nature of Extinction4. Parallelism in successional forms5. Affinities between successional forms

Evidence from the Geographical Distributions of Organisms(Chapters XI–XII)

General Patterns (driven by migration/dispersal andcommon ancestry)

1. Regional affinities of organic beings2. Affinities of groups from different continents (single

centers of creation)Patterns on Oceanic Islands (driven by migration/isolation

and natural selection)1. Depauperate diversity of islands compared to

continents2. Increased number of endemic species3. Deficiency of certain classes4. Affinity of island species with those of the nearest

mainlandEvidence from the Affinities of Organic Beings (Chapter XIII)

1. Hierarchical organization of Nature2. The natural classification system3. Single ancestral group




the Diagram of Divergence of Taxa, in rela-tion to the temporal patterns of successionalforms (evidence from the geological record),the spatial patterns of organisms (evidencefrom the geographical distribution of organ-isms), and the hierarchical organization ofnature (based on evidence regarding the af-finities of organic beings).

evidence from the geological recordThe temporal scale of Darwin’s Tree Dia-

gram, represented by ancestral/descent taxa(small numbered letters) in successive gen-eration intervals (intervals I–XIV; Figure 1)enables one to make predictions regardingthe rate of change and the affinities of spe-cies that existed in the past (Chapters IX andX). Two major predictions become apparentwhen looking at the Tree Diagram. First, theprocess of modification through time mustbe gradual (Gradualism). This is inferredfrom the Principle of Divergence, repre-sented in the diagram as a steady and grad-ual divergence in character while followingthe genealogical sequence of varieties/spe-cies throughout a lineage (e.g., a1, a2, a3, an).We must keep in mind that the horizontalaxes in the diagram are not meaningless;they represent the dimension of morpholog-ical variation such that species in close prox-imity (e.g., A, B, C, or i3, m3) are more similarto each other than to other species in thesame axis (e.g., A, G or i3, x3). The change ina lineage is therefore gradual since, as pre-dicted by the Principle of Divergence, at anytime a taxa would have intermediate mor-phology between ancestral and descendentforms (e.g., a3 � a2 � a1).

The second prediction relates to the rateof change between different groups. Darwinnoted that “The amount of organic change . . .does not strictly correspond with the succes-sion of our geological formations” (p. 313).That is, many times the same geological for-mations showed evidence of massive changes(e.g., adaptive radiations of some groups) atsome locations and limited change at otherlocations. Thus, different groups must havechanged at different rates. This predictioncan be inferred by comparing the two ma-jor clades represented throughout theTree Diagram (i.e., descendant species from

common ancestors A and I). It is apparentthat, over the same time period (i.e., the fulltemporal scale of the diagram), clade Achanged at a higher rate than clade I, sincethe former resulted in eight descendant spe-cies (a14–q14) whereas the latter only led to sixspecies (n14–z14). Ultimately, one can arguethat an extreme example of variability in therate of change is represented by lineage F,which shows no divergence in character at all(i.e., no change). Darwin deliberately incor-porated variation in the rate of change in hisconceptual model of evolution, as he wasfamiliar with many of the contingencies(e.g., variability in fecundity, variation in thedegree of intercrossing, rate of breeding,etc.) that different species exhibit in nature,as well as the existence of ancient “livingfossils.”

A series of predictions from Darwin’s TreeDiagram were assessed by comparing speciespatterns in the fossil record. These includepredictions regarding the ubiquitous natureof the extinction process, affinities of extinctspecies, and the relationships between ex-tinct and extant species in the same geo-graphical region (Table 1). The process ofextinction played a central role in Darwin’stheory. As mentioned above, the Tree Dia-gram illustrates the process of extinction asbearing on natural selection. The diagramshows multiple examples of extinctions, rep-resented by lineages that were not able toreach generation interval XIV (Figure 1).That is, only few species will have descen-dants; in Darwin’s Tree Diagram only speciesA, F, and I from the 11 ancestral species(A–L) have left contemporary descendants.Disregarding the condensed diagram fromgeneration intervals XI to XIV, which doesnot illustrate extinction events, a minimumof 14 extinctions can be identified betweengeneration intervals I and X. Scaling downinto the representation of different varietiesby all dotted lines drawn, the diagram showsa total of 133 extinction events. It is particu-larly at this level in which extinction becomesan essential part of Darwin’s explanatorymodel since, under his theory, extinction is adirect consequence of the replacement ofless successful varieties by more successfulones through the process of natural selec-




tion. One could argue that under the TreeDiagram one may also predict a greater rateof extinctions in those lineages that do nottend to change (i.e., lineages that may not beable to become adapted to potential changesin the environment). This is shown by thefact that only one of the nine nondivergentlineages (lineage F) was able to persist untilgeneration XIV, while those lineages thatshowed divergence (i.e., from ancestralspecies A and I) are represented by 14descendant species (a14–z14). The affini-ties of species in the geological record weretherefore consistent with three major predic-tions associated with Darwin’s Diagram ofDivergence of Taxa on the gradual nature ofchange, the variable rates of change, and theubiquitous nature of extinction (Table 1).

Darwin also emphasized a parallelism inthe successive forms of life throughout theworld, with the same families, genera, andsections of genera being represented in dif-ferent regions, but not necessarily by thesame species (p. 323). However, this predic-tion does not follow logically from Darwin’sTree Diagram, as this pattern is explained onthe basis of new species having formed fromdominant species that spread widely and var-ied in relation to the different environmentsthey encounter.

Predictions on the affinities of extinct andextant species can be clearly inferred fromDarwin’s Tree Diagram, as Darwin refersback to the figure (in Chapter X) to assessthe evidence of extinct taxa being generallyintermediate in character with their modi-fied descendants (p. 333), on the fauna ofeach geological period being intermediatein character between preceding and suc-ceeding faunas (p. 334), and on the fossilsfrom consecutive formations being more re-lated than those from remote formations (p.335). These patterns are also consistent withspecific predictions that can be inferred fromthe Diagram of Divergence of Taxa (Figure1). For example, ancestral taxa A tend to beintermediate in morphology with their de-scendants (e.g., a2 and m2, or a10 and m10),fossils from past formations (e.g., a1, m1)tend to be more closely related than theirdescendants from later formations (e.g., a10,m10), and fossils from successive formations

(e.g., a1–a2) are more closely related thanthose from remote formations (a1–a7).These predictions led Darwin to formu-late a generalized law, the Law of Succes-sional Types, which determined the“relationship in the same continent be-tween the dead and the living” (p. 339).That is, the expected association by com-mon descent between the extant faunaand their ancestral forms.

evidence from the geographicaldistributions of organisms

The Diagram of Divergence of Taxa canalso be interpreted in relation to Darwin’sobservations on the geographical distribu-tion of species (Chapters XI and XII). How-ever, in this case, predictions from the modelrequire linking the Tree Diagram to the pro-cess of migration/dispersal as one of the keyelements driving the geographical distribu-tion of organisms. Darwin emphasized thatthe major causes driving patterns on the af-finities of groups from local areas includephylogeny (as depicted by the Tree Diagram)and barriers to migration. As describedabove, the Tree Diagram has both a tempo-ral dimension represented by the generationintervals and a morphological dimension rep-resenting character divergence on the hor-izontal axes. However, a “spatial scale” ofgeographic distribution is not apparent inthe diagram. One could argue that predic-tions regarding the geographical distribu-tion of organisms can easily be inferredfrom the Tree Diagram, as the temporalcontinuity of lineages through inheritanceand common descent would lead to speciesinhabiting local regions being more closelyrelated to one another than they are to spe-cies in other local regions. However, migra-tion, or lack of thereof as a consequence ofgeographical barriers, must be accountedfor to provide a causal explanation for theobserved patterns.

For Darwin, two general patterns of spe-cies relationships are presented as evidencefrom the geographical distribution of organ-isms (Table 1). The first one refers to theregional affinities of organic forms. As shownpreviously (see the section Evidence fromthe Geological Record), the Tree Diagram re-




flects the grouping of species by commonancestry (e.g., a14–q14–p14 versus o14–e14–m14).Although no explicit geographic dimensionis included in the diagram, one can inferthat geographically isolated groups with re-stricted migration would tend to be moreclosely related to each other than to speciesfrom other locations. For example, if thetwo major diverging lineages from the dia-gram (i.e., lineages originating from ances-tors A and I) would represent diversifyingtaxa from different continents, then onewould predict that organisms on each of thecontinents would be more closely related byancestry, as reflected by the fact that a14

through m14 are all descendants from A, andn14 through z14 are all descendants from I.Similar patterns can be identified at differ-ent scales, as long as a scale of geographicdistribution is overlaid on top of the TreeDiagram.

The second general pattern regarding thegeographic distribution of organisms refersto the affinities of groups from different con-tinents as evidence for the presence of asingle center of creation (as a contrastinghypothesis to the predominant view of mul-tiple centers of creation). This is also exem-plified in the Tree Diagram by patterns ofdivergence followed from dispersal and dif-ferentiation from common ancestral forms.Darwin is puzzled by the simplicity of thisview, in which each species has originatedwithin a single region, as he stated: “He whorejects it [this view], rejects the vera causa ofordinary generation with subsequent migra-tion, and calls in the agency of a miracle” (p.352).

The causal agency of phylogeny (i.e., com-mon ancestry and diversification by naturalselection) in combination with migration/isolation is then used by Darwin to explainthe geographic patterns of species distribu-tions on oceanic islands (Chapter XII). De-creased levels of diversity on islands,increased number of endemic species,deficiency of certain taxonomic groups,and the affinity of island species to those ofnearest species in the mainland are consid-ered evidential support for his theory, as theyare consistent with the conceptual modelrepresented by the Diagram of Divergence

of Taxa in combination with the dispersal/migratory tendencies of species.

In summary, the general patterns of thegeographic distribution of organisms areexplained by Darwin through the causalefficacy of natural selection leading to the di-versification of species into new forms (re-flected by the patterns of ancestral/descentand multiplication of new forms in the TreeDiagram) in combination with a theory ofmigration/isolation as a creative force.

evidence from the affinitiesof organic beings

The conceptual model of Darwin’s theoryon the origin of species, as underscored inthe Diagram of Divergence of Taxa, also pro-vides a causal explanation for the hierarchicalorganization of nature, a natural classificationsystem, and the relationships by which allliving forms are united (Chapters IX and X;Table 1). These three features are explainedby Darwin’s theory, since they “all naturallyfollow on the view of the common parentageof those forms which are considered by nat-uralists as allied, together with their modifi-cation through natural selection, with itscontingencies of extinction and divergenceof character” (p. 456).

The hierarchical organization of groups isthe natural outcome of three major featuresdepicted in the Diagram of Divergence ofTaxa: common ancestry, in combination witha general tendency of species to increase innumber (i.e., multiplication of species), anddivergence in character (as a result of natu-ral selection). “[T]he inevitable result is thatthe modified descendants proceeding fromone progenitor become broken up intogroups subordinate to groups” (p. 412). Thisis exemplified by Darwin’s referring to thenumbered letters in the last generation in-terval of the Tree Diagram (a14–z14; Figure 1).At this level, each letter may represent a ge-nus composed of multiple species. At thesame time, these genera may be groupedinto subfamilies (e.g., a14–p14) and subse-quently into families (a14–f14) based on theirdescent from ancestral species a10 and a5,respectively. At the largest scale, all descen-dants from A and I may represent distinctorders of a large class (encompassing a14–z14)




that have descended from a single commonprogenitor not represented in the tree.

It was the recognition of this subordina-tion of groups resulting from the mutualaffinities of organic beings that provided ajustification for a natural system of clas-sification and the foundations for mod-ern evolutionary taxonomy (Mayr 1942).After Darwin, taxonomic groups couldonly be considered “natural” as long astheir grouping was consistent with this pat-tern of common ancestry and subordination(monophyletic groups). The natural systemof classification, therefore, emphasized thatthe arrangement of groups must be genea-logical but the degree of modification mustbe expressed by ranking groups into highertaxonomic categories. This provides justifica-tion for considering higher taxa as arbitrarygroups (pp. 421–422); that is, as conceptualconstructs based on subjective interpretationsof the “degree of modification” required todefine a particular taxonomic category.

Finally, the Diagram of Divergence ofTaxa provides the basis to explain the natureof the relationship by which all living beingsare united. Going backward in time, a logicalinference from the Tree Diagram suggeststhat all living forms coalesce into a singlecommon ancestral form, i.e., the unseen pro-genitor of species A–L depicted in the dia-gram. The idea of a single origin of life isonly lightly addressed in the Origin (p. 484).Most naturalists that believed in the transmu-tation of species suggested that most organ-isms originated from multiple independentprogenitors. Darwin himself expressed in theOrigin that animals and plants may each havedescended from four or five ancestral proto-types. However, on the basis of his proposedtheory and intuitive speculations on com-mon features of the chemical composition,cellular structure, and laws of growth andreproduction, he conceded to the ideaof a single primordial form, as he con-cludes: “Therefore I should infer from anal-ogy that probably all the organic beingswhich have ever lived on this earth have de-scended from some one primordial form,into which life was first breathed” (p. 484).

ConclusionsThe characterization of nature’s organiza-

tion in tree-like schemas preceded Darwinand the Origin (Ragan 2009). Before Darwin,most tree-like structures aimed at describingrelationships among organisms representedclassifications and keys organized hierarchi-cally, but with no phylogenetic meaning(Ragan 2009; Tassy 2011). That is, theseschemas were conceived to show affinitiesbetween organisms, not evolutionary trans-formations. Lamarck (1809) published thefirst phylogenetic tree to illustrate the rela-tionships between major groups of animals onthe basis of origin. Lamarck’s tree thinkingwas, however, framed under his theory ofevolution, which focused on phyletic trans-formation. Thus, his trees tended to bemore linear (rather than bifurcating pat-terns) with extant groups depicted as ances-tral species. Lamarck’s trees may thereforebe considered prephylogenetic, since theywere conceived using “classified taxa basedon a non-transformational concept of life toillustrate transformation” (Tassy 2011:91). Fur-thermore, Lamarck’s trees were intended asfactual representations of evolutionary trans-formations of specific taxonomic groups (e.g.,for the origin of animals and the origin ofhumans) rather than as theoretical models forthe evolutionary process as a whole.

It was not until Darwin that true phyloge-netic trees illustrating evolutionary transfor-mations and genealogical relationships bycommon descent were conceived. Darwin’sTree Diagram in the Origin was indeed thefirst published theoretical tree that illus-trated both a schema of pattern and a schemaof process (Tassy 2011:93). The pattern re-ferred to the relationships between varietiesand species on the basis of common descentand the process to the actual principle of di-versification driven by natural selection.Throughout this paper, I argue that Dar-win’s Diagram of Divergence of Taxa pre-sented in the Origin goes beyond the de-scriptive aspects of the principle of diver-gence and common descent. In fact, Ishowed that Darwin used the Tree Dia-gram as a conceptual model for his theory ofevolution, to establish a causal relationship




between natural selection leading to diver-sification and common descent.

Darwin’s Diagram of Divergence of Taxa isnot just a theoretical representation of therelationships among living organisms andthe way organic diversity evolved (Darwin’stheory). I believe the Tree Diagram also il-lustrates Darwin’s argumentative strategy forthe origin of species by means of naturalselection under the vera causa principle. Aspreviously shown, the characterization ofnatural selection as the evolutionary mecha-nism that explains divergence and commondescent enabled Darwin to make the case forthe competence of natural selection to producenew species, one of the necessary conditionsof the principle of vera causa (Hodge 1977).In addition, the characterization of Darwin’sTree Diagram as a conceptual model of evo-lution, i.e., as a working hypothesis, allowedDarwin confirmation of his theory throughpredicted patterns observed in the geologi-cal record, the geographical distributions oforganisms, and the affinities of organic be-ings. This diverse number of observations byDarwin made the case for the responsibility ofnatural selection in the production of extantand extinct species, the final condition toascribe vera causa to the phenomenon of theorigin of species.

Darwin’s argumentative strategy also illus-trates that his research program followedboth inductive and deductive principles (Ayala2009). Although Darwin himself claimed thathis research followed “true Baconian princi-ples,” he also emphasized the importance ofmaking observations in relation to support-ing or rejecting a particular view (i.e., a hy-pothesis), what we now consider as the coreelement of the hypothetico-deductive method.In Darwin’s view, the best explanation for awide range of biological and geological datawas the hypothesis that species originated asa result of decent by modification throughthe working of natural selection, as exempli-fied in the Diagram of Divergence of Taxa.This view was presented by Darwin in thecontext of the competing theory of SpecialCreation, the dominant pre-Darwinian par-adigm. This is evidenced at the end of Dar-win’s introduction to the Origin, in which hespecifically indicated that “the view which

most naturalists entertain, and which I for-merly entertained–namely, that each specieshas been independently created–is errone-ous” (p. 6).

Arguably, one could consider that underthe vera causa principle Darwin followed in-ductive reasoning in establishing the case forthe existence of natural selection, as he arrivedto the generalization of the struggle for ex-istence (Chapter III) through the accumula-tion of empirical observations on artificialselection of domestic species (Chapter I),and on variation, reproductive growth, andcompetition under nature (Chapters II andIII). In contrast, on the case for competenceand responsibility, I believe Darwin’s argu-ment represents a classical example of thehypothetico-deductive method. That is, Dar-win’s theory, as depicted in the Diagram ofDivergence of Taxa (Chapter IV), emergesas a classical hypothesis with multiple test-able predictions. This is clearly evidenced inDarwin’s original notes from his “Big SpeciesBook” manuscript (Stauffer 1975), in whichhe presents natural selection as an alterna-tive hypothesis to the doctrine of chance(Figure 4). The actual testing of these hypoth-eses was accomplished through Darwin’s evi-dential observations on the geological record,the geographic distribution of organisms, andthe affinities of organic beings (Chapters IX–XIII), making the case for responsibility (see Fig-ure 2).

My intention throughout the paper was tocapture Darwin’s reasoning in developingthe Diagram of Divergence of Taxa as a con-ceptual representation of his theory of evo-lution. It is true that the idea of commondescent is logically independent from Dar-win’s idea of natural selection as the mainmechanism of species transformation. This isevidenced by the fact that many of Darwin’scontemporary readers accepted the idea ofcommon descent but rejected natural selec-tion as the dominant evolutionary mechanism(Mayr 1991). The argument in the Origin washowever not about natural selection indepen-dently of the splitting of one species into two;the argument was about natural selection driv-ing the actual splitting. That is, under Darwin’s




view, without natural selection there was in-deed no vera causa for transmutation and com-mon descent.

Waters (2003) suggests that Darwin’s argu-ment was flexible enough to provide compel-ling arguments for evolution independently ofnatural selection. Under his perspective, theOrigin offers a strong case for transmutationand common descent, regardless of naturalselection being the dominant evolutionarymechanism. It would be difficult, however, toargue that transmutation and common de-scent were the central elements of Darwin’sone long argument, since both ideas werepublished prior to the publication of the Or-igin, by Lamarck (1809) on the transmuta-tion of species and by Wallace (1855) oncommon descent. It is true, though, thatprior to the Origin there had not been muchevidence presented formally on the issue ofcausal responsibility for these two processes.

Darwin’s one long argument has also beenframed under William Whewell’s idea thatcausality can only be established on the basisof consilience (Waters 2003), the process bywhich a wide variety of apparently indepen-dent phenomena can be explained as a re-sult of the same cause (Whewell 1840; Hull2003). This is consistent with the ideaproposed by Thagard (1978) that Dar-

win’s argument follows the inference tothe best explanation; that is, the criteria bywhich a hypothesis can be judged to providea better explanation than a competing hy-pothesis. In the case of Darwin, the alterna-tive hypothesis to which he consistentlyreferred throughout the Origin was SpecialCreation.

Regardless of anyone’s views about the na-ture of Darwin’s one long argument, onething seems clear: that the only figure in theOrigin of Species, the Diagram of Divergenceof Taxa, is not just a simple tree illustratinggenealogical relationships between organ-isms, but a conceptual model that providessignificant insights into the causal construc-tion of Darwin’s theory of evolution, thestructure of his argumentative strategy, andthe nature of his scientific methodology.

acknowledgments

I would like to specially thank Michael Bradie (Depart-ment of Philosophy, Bowling Green State University)as well as all of the students who have participated inmy seminars on Darwin and Wallace for their construc-tive comments and fruitful discussions on the ideas pre-sented in this paper. I would also like to express mygratitude to Elliot Sober, Francisco Ayala, RichardBurkhardt, and Ronny Woodruff, who provided con-structive comments on earlier versions of the manu-script.

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