Iconoclasts of Evolution: Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 1faec(c4 e1 e. kvf( Olneye& tAe oj0 al

KURTM. PICKETT JOHNW. WENZEL STEVENW. RISSING

he modernconcept of biologicalevolution arose relativeimpacts. Indeed, the hypothesis that evolution before the 19th century.Darwin and Wallaceprovided is takingplace at all in any given case is a testable,and the first plausible explanationfor a mechanismof evo- therefore scientific, proposition. Darwin proposed lution by which species change through time and are many tests of evolutionarymechanisms and left us a connected in an ancestor-descendant relationship. legacy of testable hypotheses. The Darwinianview of Indeed, Darwin described many mechanismsof evolu- biological evolution enjoys broad support from studies tion including natural selection, sexual selection, kin including the observed hierarchyof life (systematics), selection, group selection, and inheritanceof acquired fossils (paleontology),developmental patterns (embry- characteristics.Whether any of these mechanisms act ology), and most recently,genetics. However,support on a given population is a testable hypothesis, and provided by embryologyin particularhas been called many fields of evolutionarybiology seek to assess their into question lately. Support from embryology has been challenged KURTM. PICKETTis in the Division of InvertebrateZoology, throughouthistory in various ways by early critics of AmericanMuseum of NaturalHistory, New York,NY 10024; evolutionincluding creationists,who now speak under e-mail: [email protected]. JOHN W WENZELis in the the banner of "IntelligentDesign." Behe (1998) and Departmentof Entomology,The Ohio State University, Columbus, Wells (1999) claimed that embryologicalsupport for OH 43210. STEVENW RISSINGis in the Department of Evolution, Darwinianevolution is based on the drawings of 19th Ecologyand OrganismalBiology, The Ohio State University, century embryologistErnst Haeckel (1834-1919). They Columbus,OH 43210.

ICONOCLASTSOFEVOLUTION 275 reiterateRichardson et al.'s (1997) workindicating what * Richardsonet al.'s (1997) critical claim is that has been known since at least 1894 (Sedgwick): there is no conserved stage during develop- Haeckel's embryologicaldrawings are inaccurateand ment, and it is incidental that Haeckel's draw- perhaps fraudulent.Accordingly, Behe (1998) claims ings are in error that because of this, " ... the problem of development * von Baer's rejection of Darwin's paradigm is within evolutionremains unsolved." Later, Wells (2000, immaterialbecause his empirical data support for Chapter5) claimed that the embryologicalsupport Darwin(as happens commonly in science). Darwinianevolution is based on the work of another 19th century embryologist,Karl von Baer(1792-1876). A companion presentation (available in both Wells says that as von Baerwas not a proponentof evo- Microsoft'PowerPoint and Apple' Keynoteformats) for lution, so Darwin'suse of his embryologicalcontribu- use by teachersthat parallelsthe argumentsdiscussed tions in support of evolutionarytheory is in factmisuse. here is available via free anonymous download at Laterin the same chapter,Wells (2000) claims that von ftp://www.biosci.ohio-state.edu/pub/IBP/. Baer'slaws-which describe continual change and spe- cialization during development-cannot accommodate The Rise of Embryology reality because they do not allow for the Haeckelian notion of a conserved stage midway during develop- Priorto 1830, embryologywas an irregularscience. Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 ment. As we will show, von Baer'sview, and not the sub- Two earlyconcepts were preformationismand epigenesis, sequent view of a mid-embryologicalconserved stage, is both of which seem obviously false today. overwhelmingly supported by available data. Notwithstanding this, Behe (1998) and Wells (2000) Preformationism,formalized by CharlesBonnet in in both claim that the alleged existence of a conserved 1745, argued that all adult structures exist the stage midway through development shows, also, that gametes, and that they merely unfold, or unroll as the Haeckel's biogenetic law, "ontogenyrecapitulates phy- individualdevelops. This, in fact, is the original mean- logeny,"is false. ing of the word evolution.An example of preformation- ism is the concept of the homunculus(Figure 1): a per- Wells and Behe claim that Darwin's fectly formed miniatureperson inside a thesis, as set forth in On the Origin of sperm, who only needs to grow to Species,relied upon embryologicalcon- become adult. Historically,many object- clusions drawn before and after the ed to this view of embryology:In many publication of Origin. Further, they organisms(e.g., frogs)immatures do not imply that current support of resemble adults. Second, if preforma- Darwinism is based on either von Baer tionism were true-if there was no such or Haeckel, and that if embryological thing as development, but simply support for evolution comes from von unfolding-then just as the egg contains Baer,then it is misuse; if it comes from the full-grownmother-to-be, so she must Haeckel, it is fraudulent. While these have in her eggs her own daughters- simplistic claims are easily dismissed by and in their eggs, their daughters-and embryologists and systematists, it can so on, like nested Russiandolls, to infin- seem quite confusing to other biology ity.Thus, the questionablenecessity that teachers. We present the background all generations of humanity were pre- necessary for teachers and students to formed in Eve's body. In addition, pre- evaluate this conflict objectively. formationismdid not account for the Here, we describe the history of mixture of parental traits observed in modern embryology,the roles von Baer Figure1. TheHomunculus offspring. and Haeckel played in that history, and Thismodel of preformationism Epigenesis,literally "outside origin," how that history relates to the formula- wasproposed by microscopists is the opposite of preformation. tion of Darwin's theories. In particular, whoclaimed they had seen a Although today the term generally we show: completeadult in the headof implies nothing more than gradual the validity, or lack thereof, of humansperm. Reprinted by per- developmentfrom formlessness, as orig- Haeckel's drawings or his "bio- missionof the McGraw-Hill inally envisioned (by Aristotle,and later genetic law" has no impact on Companiesfrom Life, Fourth by 19th centuryembryologists), the egg Darwinian evolution because Edition,by Ricki Lewis, Douglas is formlessand outside forcesdetermine its destiny.The egg, like a ball of warm Darwinpublished Originsome 15 Gaffin,Marielle Hoefnagels & clay, has only potential for its fate. years before Haeckel's drawings BruceParker. (2002). were published Epigenesis, in its historical sense, was

276 THEAMERICAN BIOLOGY TEACHER, VOLUME 67,NO. 5, MAY2005 easily rejectedby simple observationsthat, for example, morphology.Here "higher"and "lower"have no evolu- chicken eggs incubatedby a goose still become a chick- tionary meaning,but instead referredto improvement. en. The outside "goose environment"did not make a (Some,even today,might erroneously see "improvement" developing egg become a goose. Most aspects of the as a naturalconsequence of evolution.) egg's identitywere determinedendogenously. von Baeropposed recapitulation.In the second vol- ume of Entwickelungsgeschichteder Thiere,von Baer set von Baer fortha broadargument against recapitulation in the fifth of his six commentarieson the chick embryo.Here von By the early 1830s, the scientific approachtoward Baer presented many objections to recapitulationism; the study of developmenthad takenplace largelydue to two of the most damning were that (1) embryological Karl Ernst von Baer.After meticulous examinations,in featuresof lower animals can appearin adult stages of 1828 von Baerpresented his concept of differentiationin higheranimals (Figure 2), and (2) definingcharacters of detail in Entwickelungsgeschichteder Thiere (Animal higher groups often appearearly in the developmental Development). Von Baer's critical claim was that all sequence. The first point, today understood as paedo- early animal embryos appear similar and tend to look morphosis,is strictlyincongruous with the notion that less so as time passes, and they develop specialized higher animals are improvementsof lower animals. A Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 structures,with modificationusually coming at the end good example of the second point is the amnion: of development. Members of close taxonomic groups Amniotes are defined by their possession of a derived tend to resembleone anotherlonger throughthe devel- trait,the amnioticsac of the egg. But,of course,this trait opmental period. Species that differ accordingto phy- is found at the very beginning of the developmental lum will differearly in embryology.Those that arein the sequence. In order for recapitulationto be correct, all same phylum but differentclasses differlater in embry- lower animalsshould have an amnion as well. von Baer ology. Species that are in the same family are differen- summarizedhis thoughtson developmentin his famous tiablelater still, and so on. The Linnaeanhierarchy cap- laws of development(as cited in Gould, 1977, 56): tures this temporalorder of developmentand remains the strongest,most robustempirical support for the the- 1. Thegeneral features of a largegroup of animals ory of descent from a hierarchyof common ancestors. appearearlier in the embryothan the special features. Duringvon Baer'stime, two overlappingschools of morphologists prevailed:a German Naturphilosophie 2. Lessgeneral characters are developedfrom the school and the French transcendentalmorphologists. mostgeneral, and soforth, untilfinally the most These movements specializedappear. assertedthat organisms are organizedin a chain of higher and lower Figure 2. Myriapoddevelopment2 types, and that this chainappears in embry- 5 ological recapitulation. Formallyintroduced by the Naturphilosophie school (but with ves- tiges in writings of Aristotle [Gould, 1977]), recapitulation holds that organisms if 9 pass through develop- IY. 90 45~~~~~~day 91 mental sequences Newport,A18141). including fully formed, adult stages of lower organisms. According Figure2. Myriapod development to this scheme, human Millipedeshave one pair of legs per segment asadults, but as larvae have only six legs (far left figure embryos, high on the labeled19), like their more derived relatives, theinsects.That insects would, intheir adult stage, take the scale of nature,sequen- embryologicalformof a "lower" animal iscontrary torecapitulationism. Reprinted bypermission ofthe tially look like the publishersfrom Ontogeny andPhylogeny byStephen Jay Gould, Cambridge, MA:The Belknap Press of adultsof fish, frogs,rep- HarvardUniversity Press, Copyright ?1977 by the President and Fellows ofHarvard College (From tiles, and birds, before Newport,1841). they acquire human

ICONOCLASTSOFEVOLUTION 277 3. Eachembryo of a givenspecies, instead of pass- ing throughthe stagesof otheranimals, departs a) moreand morefromthem. of *~Phylogeny A L __ X __ > , X adult stages 4. Fundamentallytherefore, the embryoof a high- er animalis neverlike [the adult ofi a lowerani- mal, but only like its embryo. 0~> von Baer'slaws, including their explicitrejection of recapitulation,formed the basis for Darwin'sdiscussion of embryologyin Origin. -AfL

a) Darwin Relied on von Baer u > Darwinwas heavilyinfluenced by von Baerin forming Ontogeny his theoryof naturalselection (see reviewin Gould,1977). Some choice quotes readilyreveal this. Darwinrefers to Figure3. Terminaladdition of embryological von Baer:"Hardly any point gaveme so much satisfaction Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 innovations when I was at workon the Originas the explanationof the wide differencein many classesbetween embryo and the Species1 (bottom)is the mostancestral, species 6 is the adultanimal, and of the close resemblanceof the embryos mostderived. Rows represent ontogenetic sequence; the within the same class"(cited in Gould, 1977, 1971). He diagonalarrow represents phylogeny. Each symbol repre- wrote in 1860 in a letter:"Embryology is to me by farthe sentsan adultform. Descendants pass through unaltered strongestsingle class of factsin favorof changeof forms" adultstages of theirancestors. According to Haeckel's (citedin Gould, 1977, 1970). scheme,ontogeny will always faithfully reflect phylogeny providedthat developmental additions occur exclusively terminally.Thesimple example of the amnionrefutes this oversimplification.Reprinted by permission of the publish- ers fromOntogeny and Phylogenyby StephenJay Gould, Cambridge,MA:The Belknap Press of HarvardUniversity M IOLWAUKE E Press,Copyright ?1977 bythe Presidentand Fellows of 5tg ~October5-8, 2005 HarvardCollege (From Newport, 1841).

von Baerdid not accept evolution.Like most of his contemporaries,he gathereddata fromhis observations and drew few inferences. It has been claimed (Wells, 2000) that Darwin's reliance on von Baer is invalid becausevon Baerdid not accept evolution.But scientif- ic revolutions necessarily interpret old data in a new light, and scientistswho were leadersbefore the revolu- k I tion are not so much convertedas simply pushed aside (Kuhn, 1986). To say that any scientist preceding Darwinwas not Darwinianis to state the obvious. For example, Einstein'stheory of relativityis not compro- mised by the fact that Newton was not a relativist,even in light of Einstein'sreliance on Newton's data. Indeed, _II the principalmechanism of scientificadvance is simply ijlm'l al,MSl 11i new interpretation,through research,of standing data or concepts, and Darwin'suse of von Baer'sdata is an excellent example. Similarly,Ernst Haeckel reinterpret- ed recapitulationismin light of Darwin'stheory of evo- lution, but the fact that Darwin rejectedrecapitulation- ism does not invalidate Haeckel. Other data were requiredfor that.

278 THEAMERICAN BIOLOGY TEACHER, VOLUME 67, NO. 5, MAY2005 embryologicalinnovations are added to the develop- mentalsequence terminally (Figure 3). More ancient species should have a sim- ple developmentand modi- fications to the develop- ment should occur at the end. Terminal addition causesmore derived species to have more advanced, complicated,and extended development. In order for this sequence of events to i~~~~- ~

remainin the properorder, Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 there could be no shuffling 1~~~~~~~~~~~~~~~~~~~~~~~0 of the sequence. Accord- ingly, Haeckelbelieved the embryo was immune to selection. If this were true, then closely related ani- mals would pass through an identical, identifiable Figure4. Haeckel's1874 drawingsof vertebrateembryos stage of development, the Fromleft to right:"fish; "salamander," "turtle," chicken, pig, cow, "rabbit," and human.The upper- phylotypic stage, which mostrow of illustrationsrepresents a conserved stage across Vertebrata. Many lines of evidence corresponded to the strict recapitulation of their suggestthat no such conserved stage exists."There is no highly conserved embryonic stage in the most recent common vertebrates:implications for current theories of evolutionand development." Reprinted by per- ancestor. Haeckel pointed missionof Springer-Verlagfrom Anatomy and Embryology byMichael K. Richardson, James to the existence of this Hanken,Mayoni L. Gooneratne, Claude Pieau, Albert Raynaud, Lynne Selwood, and Glenda M. stage in his suspect draw- Wright.(1997).Vol. 196, p.93, Figure 2. ings of vertebrateembryos (Figure4). Haeckel & Evolutionary Haeckelwas wrong about his BiogeneticLaw. This was not news, however,as even von Baer himself had Recapitulationism shown 50 years earlierthat recapitulationismis bank- rupt when compared to actual data. The accuracyof In 1874, 15 years after Darwin's publication of Haeckel'sillustrations has been challenged by numer- Origin,Ernst Haeckel published the most comprehen- ous authors across more than a century, including sive treatmentof vertebratemorphology to date, nec- Sedgwick(1894), Lillie (1919), de Beer (1951), Gould essarily considering the theory of natural selection. (1977), and most recentlyRichardson et al. (1997, see Haeckel sought to fuse recapitulationwith the relative- Figure 5). Claims that Haeckel's errors are newly dis- ly new theory of evolution. In doing this, Haeckel covered,or haveunrecognized significance (Wells 1999, thought he found the mechanism of evolution: 2000) are at best poor scholarship. "Phylogenesisis the mechanical cause of ontogenesis" (Haeckel, 1874, 5; cited in Gould, 1977, 1978). Haeckel continued, "Theconnection between them is not of an Enter Richardson et al. (1997) & externalor superficial,but of a profound,intrinsic, and causal nature" (Haeckel, 1874, 6; as cited in Gould, the ID Creationists 1977, 1978). To Haeckel, phylogeny was the engine of It should be clear from chronology that Darwin ontogeny,and thus ontogenyrecapitulated phylogeny, his relied on von Baerbut could not have relied on Haeckel BiogeneticLaw. (Figure5). Nonetheless,Wells (1999, p. 349) states, "Of Haeckelpromoted recapitulationism as intrinsically course, it would be illogical to conclude that Haeckel's tied to evolution.In his 1874 treatisehe proposedthat all distortions invalidate Darwin's theory. Although

ICONOCLASTSOFEVOLUTION 279 Early 1800s: Epigenesis Haeckel publishes Anthropogenie, and Preformationism reign. reasserts recapitulationism, gives German Naturphilosphie birts recB ionism, the and French Transcendental birthto the Biogenetic Law and the Morphologists promulgate phylotypic stage. recapitulationsim

Darwin publishes Origin, interpreting von Baer's laws in 1 r an evolutionary context BEFORE AFTER Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 118.00 1 I

von Baer publishes Sedgwick publishes first l Enterwickelungsgeschichte, account doubting Haeckel's a powerful critique of vertebrate illustrations recapitulationism

Lillie publishes The de Beer publishes Richardson et al. re- Development of the Embryos and Ancestors, assert that Haeckel's Chick, which, in part, , which questions iscuss the depend questions Haeckel's - Haeckel's illustrations. discuss the dependent illustrations. falsity of a conserved _phylotypic" stage.

Gouldpublishes Behe (1998) and Wells (2000) Ontogenyand assert the existence of a conserved Phylogeny,in which stage, despite more than a century he pointsout that of evidence to the contrary,and, recapitulationism,and ironically,point to Richardsonet al. Haeckel'sBiogenetic (1997) as support. are false. Clearly,Darwin c nthvould eido Law, ace'rwnspublishedsoe1er afte Onthe98 Orgndo thelSpcis (contra Beh

[198] cntrWll[199).Nuerpouns authr thavreatelan cosstnl qestiondHenckelilutratieonsrr, thaotrcntdbin Ricarsonetal.(197)Secaif accodinuains,adioically, (97,wa'to Richardsonrn buet al.ace' lutain sta hyso

conservedstagChofnevlopmasqent-oe tadoesno existy(contral Behe [1998];iontar Whellse [2000]).tenthan

280 THEAMERICAN BIOlOGY TEACHER, VOlUME 67,NO. 5, MAY 2005 Darwin consideredthe embryologicalevidence 'second that is highly resistantto evolutionarychange. to none in importance'(Darwin, 1859, p. 346), he did Thisidea is implicitin Haeckel'sdrawings, which not base his theory on that evidence alone"(emphasis have been used to substantiatetwo quite distinct added). Indeed, Darwin could not base his theory on claims.First, that differences between species typi- any of Haeckel'sviews because they would not be pub- callybecome more apparent at latestages. Second, lished for another 15 years. Darwin consideredembry- that vertebrateembryos are virtuallyidentical at ology, not Haeckel'sdrawings, second to none in impor- earlierstages. This first claim is clearlytrue. Our tance. Figure5 clarifiesthis chronology. survey, however,does not support the second claim,and insteadreveals considerable variability Richardsonet al. (1997) found considerablevaria- - and evolutionarylabiality - of the tailbud stage, tion among vertebrateembryos and concluded (p. 105): thepurported phylotypic stage of vertebrates. Thesemodifications of embryonicdevelopment are Haeckel argued that all vertebrateembryos pass difficultto reconcilewith the idea that mostor all througha similarstage of development,sometime mid- vertebrateclades pass throughan embryonicstage way in embryology.This notion is in contrast to the scheme of von Baer, which suggests continual diver- in form of various taxa during development. gence Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 Clearly,Haeckel's conserved stage in development is mutuallyexclusive with von Baer'snotion of continual development.Richardson et al.'s (1997) data (rejecting Haeckel's drawings and his concept of a conserved stage) stronglysupport von Baer'sview of development, upon which Darwinrelied. Behe (1998) used Richardsonet al., (1997) and ref- erences therein to argue " ... that for a century, Darwinismeasily embraceda false descriptionof a fun- damentalprocess and that the problemof development within evolution remains unsolved" and "the earliest stages of developmentare actuallyquite differentacross vertebrate species, and become increasingly similar toward the phylotypic stage. The hourglass pattern of developmentis a conundrum that is not predicted by Darwinism."Indeed, the hourglass pattern referredto (Figure 6) is not predicted by Darwinism, but by Haeckel's false notion of the phylotypic stage (Richardsonet al., 1997). And in any event,as discussed above,the validityof Haeckel'sdrawings, his concept of a phylotypicstage or hourglass model of development does not impactDarwin's hypothesis as these concepts were introduced15 yearsafter the publicationof Origin. Instead, Darwin did rely on von Baer,who embraced neither the phylotypicstage nor the hourglassmodel. Figure6. Thehourglass model of development Wells (1999) argued that Darwin'stheory is partly Thisconcept is a directoutgrowth of Haeckel'snotion of the invalidatedbecause it relies on Haeckel. Interestingly, phylotypicstage. The phylotypic stage (the center of the hour- Wells (2000) subsequently claimed that Darwin's glass)is anoutgrowth of Haeckel'sdrawings of 1874,that have reliance on von Baer was invalid because von Baer's beenshown to beinaccurate.Therefore, skepticism of Haeckel's ideas are inconsistent with the Haeckeliannotion of a drawingslogically necessitates skepticism ofthe hourglass conservedstage. Although Wells (2000, p. 98) acknowl- modelof development."Thereis nohighly conserved embry- edges Richardson's(1999, p. 609) statement that "the onicstage in the vertebrates: implications for current theories phylotypicstage is a misleading concept that needs to of evolutionand development." Reprinted by permission of be reassessed,"he insists that "Nevertheless,no one Springer-VerlagfromAnatomy and Embryology byMichael K. doubts that vertebrateembryos start out looking very Richardson,James Hanken, Mayoni L.Gooneratne, Claude different, converge in appearance midway through (though not at the same time), then Pieau,Albert Raynaud, Lynne Selwood, and Glenda M.Wright. development become increasinglymore different as they continue (Copyright1997).Vol. 196, p. 92, Figure 1. toward adulthood;"this in spite of Richardsonet al.'s

ICONOCLASTSOFEVOLUTION 281 (1997) title: "Thereis no highly conserved embryonic Haeckel, E. (1874). Anthropogenie:Keimes- und Stammes- stage in the vertebrates."The fundamentalfinding of Geschichtedes Menschen.Leipzig: Engelmann. Richardsonet al. (1997) is that the phylotypic stage, Kuhn,T.S. (1986). TheStructure of ScientificRevolutions. New and its corollary hourglass model of development, is York:New AmericanLibrary and Universityof Chicago unsupported. While neither the phylotypic stage nor Press. the hourglassmodel form any basis of Darwin'sdescrip- Lillie, F. R. (1919). The Developmentof the Chick.New York: tion of the theory of naturalselection, these are funda- Holt. mental beliefs of Behe,Wells, and theirstudents in their ongoing Intelligent Design Creationism attack on Richardson,M. K., Hanken,J., Gooneratne, M.L.,Pieau, C., Darwinianbiology. Raynaud, A., Selwood, L. & Wright, G.M. (1997). There is no highly conserved embryonic stage in the Darwin did not rely on Haeckel,but ratheron von vertebrates:implications for current theories of evolu- Baer.von Baer'sstance against"evolution" is irrelevant. tion and development. Anatomyand Embryology,196, Behe (1998) and Wells (1999, 2000) are deeply con- 91-106. fused or intentionallyconfusing regardingthe history Richardson,M.K. (1999). Vertebrateevolution: the develop- and significanceof this well-knownfield, an area they mental origins of adult variation.BioEssays 21, 7.

claim has special meaningin their politicalmovement. Downloaded from http://online.ucpress.edu/abt/article-pdf/67/5/275/52346/4451840.pdf by guest on 29 September 2021 Sedgwick,A. (1894). On the law of developmentcommonly known as von Baer'slaw; and on the significanceof ances- References tral rudiments in embryonic development. Quarterly Journalof MicroscopicScience, 36, 35-52. Behe, Mj. (1998). Embryologyand evolution. Science,281, 347-351. von Baer, K.E. (1828). Entwicklungsgeschichteder Theire: Beobachtungund Reflexion.Konigsberg: Borntrager. Darwin,C. (1859). On theOrigin of Speciesby Meansof Natural Selection.London: John Murray. Wells,J. (1999). Haeckel'sembryos & evolution:Setting the de Beer, G. R. (1951). Embryos and Ancestors.Oxford: recordstraight. The American Biology Teacher, 61(5), 345- ClarendonPress. 349. Gould, S. J. (1977). Ontogenyand Phylogeny.Cambridge, Wells,J. (2000). Iconsof Evolution.Washington, DC: Regnery London:Belknap Press. Publishing,Inc.

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282 THEAMERICAN BIOLOGY TEACHER, VOLUME 67, NO. 5, MAY2005