Mendel's Opposition to Evolution and toDarwinB. E. Bishop

Although the past decade or so has seen a resurgence of interest in Mendel's rolein the origin of genetic theory, only one writer, L. A. Callender (1988), has concludedthat Mendel was opposed to evolution. Yet careful scrutiny of Mendel's Pisum pa-per, published in 1866, and of the time and circumstances in which it appearedsuggests not only that It Is antievolutlonary In content, but also that it was specif-ically written in contradiction of Darwin's book The Origin of Species, published in1859, and that Mendel's and Darwin's theories, the two theories which were unitedin the 1940s to form the modern synthesis, are completely antithetical.

Address reprint requests to the author at 6 BarbadosRd., Federation Park, Port of Spain, Trinidad, Trinidadand Tobago, West Indies.

Journal of Heredity 1996^7^05-213; 0022-1503/96/J5.00

Mendel does not mention Darwin in his Pi-sum paper (although he does in his lettersto Nlgeli, the famous Swiss botanist withwhom he initiated a correspondence, andin his Hieracium paper, published In 1870),but he states unambiguously in his intro-duction that his objective is to contributeto the evolution controversy raging at thetime: "It requires a good deal of courageindeed to undertake such a far-reachingtask; however, this seems to be the onecorrect way of finally reaching the solu-tion to a question whose significance forthe evolutionary history of organic formsmust not be underestimated" (Mendel1866). So It is not plausible that Mendelcould have been writing In ignorance ofDarwin's ideas, which had aroused theworldwide furor.

Of course, neither Darwin nor Mendelused the word "evolution." It Is a termthat came into vogue as a synonym forDarwin's "descent with modification"shortly after the publication of The Originof Species, when it assumed application tofully developed organic forms, In contrastto its earlier embryological connotation.However, Gavin de Beer has observed thatit is in exactly this sense that Mendel em-ploys the German expression "Entwick-lungs-Geschichte," and he comments:"Here, therefore, was Mendel, referring toevolution and laying down an experimen-tal programme for its study by means ofresearch in breeding hybrids" (de Beer1964). Some years later, Robert Olby(1979) also emphasized the evolutionaryorientation of Mendel's paper: "The lawsof inheritance were only of concern to him

[Mendel] in so far as they bore on his anal-ysis of the evolutionary role of hybrids."

Olby's (1979) article, entitled "MendelNo Mendelian?," led to a number of revi-sionist views of Mendel's work and his in-tentions, although no agreement has beenreached. For instance, some writers, un-like de Beer, Olby, and Callender, overlookor minimize the evolutionary significanceof Mendel's paper, while others maintainthat Mendel had little or no interest in he-redity, an interpretation that has been op-posed by Hartl and Orel (1992): "We con-clude that Mendel understood very clearlywhat his experiments meant for heredity."If there is any consensus at all about Men-del, it is, quite extraordinarily, that he wasan evolutionist, but it is inconceivable thata priest could have been openly support-ing a theory that Darwin had been hesi-tant to publish because of Its heretical re-ligious and political implications.

Mendel does not make a definite state-ment about his stance, but it is arguedthat there is evidence, both from the his-torical background and in Mendel's paperitself, that indicates that the latter was anad hoc attempt to refute Darwin's ideasand that Mendel's position was one of the-ological orthodoxy. (Mendel was a well-in-tegrated member of his monastic com-munity and a zealous defender of the faith,not a dissident.) Furthermore, if Mendel'spaper Is seen in this light, the many incon-gruities (or apparent incongruities) thathave been the subject of discussionthroughout the twentieth century disap-pear.

205

Evidence From the HistoricalBackground

It is known that Mendel possessed a copyof the second German edition of The Ori-gin of Species published in 1863, many pas-sages of which he marked (de Beer 1964;Orel 1971), and his paper distinctly re-flects certain important aspects of Dar-win's thought, as de Beer (1964) has point-ed out. Also, Mendel was most probablyfamiliar with the earlier German editionowned by the natural history society towhich he belonged (Orel 1971). In 1861,the parent body of this society had dis-cussed The Origin, a year after the publi-cation in a German journal, to which Men-del could have had access, of the "word-by-word translation" of Darwin's chapterentitled "On the Geological Succession ofOrganic Beings," in which Darwin clearlyformulated his theory of descent withmodification through natural selection(Orel 1971). [Mendel was obviously inter-ested in Darwin's ideas. Hugo Iltis, Men-del's first biographer, notes that Mendelbought all Darwin's later works as soon asthey were published in German, adding:" . . . and not Darwin's books alone, for al-most all the Darwinian literature of the'sixties and 'seventies is to be found in themonastery library at Brunn" (Iltis 1932).]

This would have given Mendel at least 4years in which to carry out his experi-ments and prepare his paper for presen-tation in February and March 1865. Evi-dently this would not be sufficient time ifthe experimental program was conductedas Mendel records, but recently doubt hasbeen cast on the veracity of Mendel's ac-count. Federico Di Trocchio (1991), in anarticle entitled "Mendel's Experiments: AReinterpretation," observes: "To under-stand Mendel's work we are forced to ad-mit, as was tentatively suggested by Wil-liam Bateson, that most of the experi-ments described in Versuche are to be con-sidered fictitious." Di Trocchio (1991)claims that Mendel did not perform theseven monohybrid experiments that he re-ports, the basis of the law of segregation:" . . . Mendel never carried out these ex-periments in the garden, but rather onlyon the pages of his notebooks." He sug-gests that this could also be true of boththe dihybrid experiment and trihybrtd ex-periment, from the results of which thelaw of independent assortment was de-rived, and he states: " . . . we are to-dayforced by a series of anomalies and incon-gruities to admit that Mendel's account ofhis experiments is neither truthful nor sci-

entifically likely, and that the strategy hereally followed must have been complete-ly different" (DI Trocchio 1991). Di Troc-chio argues that the numerical data for themono-, di-, and trlhybrid experimentswere obtained by progressively disaggre-gating those from polyhybrid crosses con-ducted for about three hybrid genera-tions, that Mendel may have used the 3:1ratio as his criterion for the selection ofthe seven traits, and that he did not reportall his data "because he was well awarethat other Pisum characters did not followthe law, having already ascertained thatthey produced strange ratios (i.e., thatthey were linked)" (Di Trocchio 1991). DiTrocchio adds: "Had he [Mendel] de-scribed the real course of his experimentshe would have had to admit that his lawworked for only a few of the hundreds ofPisum characters—and it would thus havebeen considered more of an exceptionthan a rule" (Di Trocchio 1991). (The pos-sibility that Mendel reported marginal to-tals without saying so was first raised byFisher in 1936.)

Therefore, 4 years would have been ad-equate time for Mendel to perform what-ever experiments were necessary and towrite his paper, especially as he was fa-miliar with the prior work in the field, ofwhich, however, he makes no acknowledg-ment, although many of the earlier inves-tigators had also used garden pea and re-ported very similar results: "It is wellknown that practically all the pointsproved by Mendel in his main paper hadbeen made by others before—J. Goss andA. Seton in 1822 reported the results oftheir pea-crossing experiments, indicatingdominance in the first hybrid generationand reappearance of recesslves in the sec-ond. Knight verified their results in 1824,adding such observations as the 'scatter-ing' of 'indivisible' characters in the gen-erations following hybridization. [Knightalso reported backcrosses and a 2 year tri-al period, as Mendel was later to do in hispaper.] A. Sageret (1826) had further sub-stantiated the independent reappearanceof Knight's 'indivisible' characters in suc-cessive generations; only some charactersreappeared in all generations (the domi-nants). This work was known to Mendelwhen he planned his experiments" (Geddaand Milani-Comparetti 1971). Mendel's at-tention had been drawn to these papersby Gartner's book Experiments and Obser-vations upon Hybridization in the PlantKingdom, which was published in Germanin 1849 and which extensively reviewed in-vestigations into plant crossing. Mendel

possessed a copy, and his marginalia andnotes on the flyleaf suggest that he stud-ied it in detail several times (Orel 1984).Olby (1985) comments: "These notes areimportant because they show Mendel atwork, hunting for clearly-marked charac-ter differences between the various formsof peas."

There is no record of any previous ex-periments by Mendel that might have ledto the initiation of the famous Pisum pro-gram. ["But despite all that has been dis-covered and preserved we have no directinformation on the sources of Mendel's In-spiration . . ." (Olby 1985).] Also, Mendeldoes not provide a time schedule for hisexperimental program or even state thedate of commencement of the project, ob-serving in his paper only that it occupied"a period of eight years." However, in hissecond letter to Nageli, obviously a defen-sive response to the latter's criticism ["Iam not surprised," Mendel wrote, "to hearyour honor speak of my experiments withmistrustful caution" (Stern and Sherwood1966)], Mendel elaborates on several is-sues, stating categorically that the exper-iments were conducted from 1856 to 1863,when "they were terminated in order toobtain space and time for the growing ofother experimental plants" (Stern andSherwood 1966). Nevertheless, through-out this period, Mendel maintained a live-ly correspondence with his brother-in-law,Leopold Schindler, and yet he never allud-ed to his scientific work, although makingfrequent references to the events of theday (Iltis 1932). And it is clear from Na-geli's reply to Mendel's first letter, withwhich Mendel had enclosed his paper,that Nageli considered the latter to be nomore than an outline: "I shall not remarkon any other points in your communica-tion, for without a detailed knowledge ofthe experiments on which they are basedI could only make conjectual comments"(quoted by Stubbe 1972).

It was in May 1856, when he was pur-portedly setting out on his experimentalprogram, that Mendel failed in his secondattempt to become a certificated teacher,an event that was preceded and followedby illness. Olby (1985) writes: "EvidentlyMendel was very ill and it was presumablythe same illness which had plagued him attimes of stress when he was a schoolboyand a student. Dr. Joseph Sajner, who hasmade a special study of Mendel's Illness,calls it an 'unstable psychological consti-tution'. Mendel had already been ill earlierin the year, no doubt owing to the stressof his studies, and when faced with the 'Vi-

206 The Journal erf Heredity 1996:87(3)

enna ordeal' again his nerves gave way."[It was in 1856 also that the controversyover procreation (whether only one par-ent or both parents make a material con-tribution to the embryo), which was thegreatest biological conflict before the Dar-winian revolution, was only just "comingto a head" (Orel 1982).] At that time, Men-del was a supply teacher at Brunn ModernSchool, which had just recently beenfounded. In 1857 and 1858, Mendel re-ceived commendations from the educa-tion authorities for his "zealous and suc-cessful endeavours" (Iltis 1932) in pro-moting the working of the school, and heremained in his office until 1868. Thus,Mendel could have had very little time todevote to his botanical pursuits, particu-larly in the important early stages.

The myth has prevailed that Mendelworked in isolation, the bibulous gardenerusually being cited as the only helper andheld responsible for the "too good" re-sults, but it appears that the experimentswere a communal enterprise, to which themonastic hierarchy must have been verykindly disposed to allow such appropria-tion of time and space usually set aside forother purposes. Abbot Napp, the head ofthe monastery, was keenly interested innatural history, and, like Mendel and othermonks, he was a member of the agricul-tural society that had discussed The Ori-gin of Species in 1861. Orel (1984) ob-serves: "Perhaps it is as well that, duringhis studies and early experiments, Mendelenjoyed both the moral and material sup-port of Abbot Napp, who understood hismotivation if anyone did."

Hugo Iltis, a teacher in Brunn like Men-del before him, names two other monksinvolved in the experimental program andrefers to a senior member of the staff atthe school where Mendel taught, Alexan-der Makowsky, the noted natural historian"who from 1860 onwards collaboratedwith Mendel" (Iltis 1932) (i.e., after thepublication of The Origin of Species). Ali-pius Winkelmayer, one of the monks whoassisted Mendel, was Mendel's contem-porary, having entered the monasterysoon after Mendel, and he, like severalother monks, had a taste for botany. Forinstance, Thomas Bratranek and Mat-thaeus Klacel, two of the monastery'sleading scholars, "though not professionalbotanists, were distinguished amateurs ofthat branch of science" (Iltis 1932). Cor-cos and Monaghan (1990) comment: "His[Mendel's] colleagues were highly intellec-tual and, like him, were teachers and mem-bers of learned societies."

So Mendel's fellow monks must have notonly understood his work but regarded itwith great approval, for 2 years after Men-del's paper was published they electedhim abbot of the rich and influential mon-astery to succeed an outstandingly goodadministrator. His priorities necessarily re-aligned, Mendel spent the rest of his lifein an office of power and privilege. It wasa position to which he had aspired. In aletter to his brother-in-law, 4 days beforethe election, Mendel had written: "It is stilluncertain which of us will be the luckyone. Should the choice fall on me, which Ihardly venture to hope, I shall send you awire on Monday afternoon" (quoted by Il-tis 1932).

After becoming spiritual leader of hismonastery, Mendel journeyed to Rome tobe received by the Pope (Sootin 1959). In1872, four years after his election, he wasmade Commander of the Order of FranzJoseph, the published citation for theaward referring not only to Mendel'steaching career but also to his politicalservice (Orel 1984).

Some of Mendel's notes for addresses asabbot have survived. Zumkeller (1971)writes: "For precisely because these ser-mon outlines are but little polished, theygive us a clearer idea of the personal relig-ious sentiment and thought of the preach-er. One fact imposes itself at the very firstreading of these texts: Here speaks a manof true faith and a pastor who thinks withthe Church. He endeavors zealously to ac-quaint his audience with the unadulterat-ed Christian doctrine . . . Decades ago,some writers endeavored to present Men-del as a freethinker, who searched fortruth 'without deference to dogmas.' It haslong been recognized that this opinion isincorrect. The discovery of these sermonoutlines of the Abbot Mendel should cutthe ground forever from under such im-putations."

In 1870, there had been a proclamationof the infallibility of the Pope, and laterencyclicals announced the infallibility ofthe Bible. It was not until 1951 (almost acentury after the publication of The Originof Species), when Pope Pius XII relaxed thedogmatic interpretation of the Bible, thatopen discussion of evolution was officiallypermitted by the Catholic Church (George1982).

Evidence From Mendel's Paper

The Origin of Species was an onslaught onthe doctrine of special creation, whichDarwin (1859) referred to as the belief that

"at innumerable periods in the earth's his-tory certain elemental atoms have beencommanded suddenly to flash into livingtissues," and Darwin's themes were evo-lution, population, and heredity, a concep-tual framework that is mirrored in its en-tirety by Mendel's paper. ["As a conse-quence of his evolutionary approach,Mendel adopted, as Thoday (1966) hascorrectly pointed out, the method of pop-ulation analysis . . . " (Mayr 1982).] Hered-ity was a vital component of Darwin's the-ory, for he reasoned that for variation tobe evolutionarily important it must be her-itable, although he was forced to admit:"The laws governing inheritance are quiteunknown" (Darwin 1859). Thus, it is high-ly significant that only a few years laterMendel elaborated his very definite at-omistic theory of heredity—a theory,moreover, that results only in stasis: "Thepure process of Mendelian heredity doesnot produce any evolutionary change atall: the population stays the same" (Ridley1985).

The populational orientation of Men-del's paper is also significant. Populationthinking was Darwin's most original andrevolutionary concept, a concept thatmade the introduction of natural selectionpossible (Mayr 1972), and its genesis hasbeen well documented, but it was not untilthe publication of Darwin's iconoclasticbook that it was unveiled to public view.Therefore, Mendel must have either inde-pendently adopted a populational ap-proach from the outset or planned his ex-periments with knowledge of Darwin'sideas, for it is obvious from the design ofthe reported experimental program thatthere could have been no change in hisresearch objective after its initiation.

At that time, the existence of constant(true-breeding) hybrids, which several ex-perimentalists had reported, was of greatinterest, as Mendel emphasizes in his Pi-sum paper: "This feature is of particularimportance to the evolutionary history ofplants, because constant hybrids attainthe status of new species" (Mendel 1866;emphasis in original). And it was Mendel'sconcern with constant hybrids thatspurred him to enter the fray again a fewyears later with his Hieracium paper: "Thequestion of the numerous and constant in-termediate forms has recently acquired nosmall interest since a famous Hieraciumspecialist has, in the spirit of the Darwin-ian teaching, defended the view that theseforms are to be regarded as [arising] fromthe transmutation of lost or still-existingspecies" (Mendel 1870). But Mendel him-

Bishop • MendeTs Opposition to Evolution and to Darwin 207

self does not report constant hybrids inhis Pisum paper: " . . . he [Mendel] did notfind any constant hybrids" (Monaghanand Corcos 1990). In fact, Mendel's papercan be seen as an attempt to provide atheoretical explanation for the phenome-na of reversion and transformation, bothof which were held to support the ortho-dox doctrine of special creation. [L. A. Cal-lender (1988) has differentiated betweenwhat he calls the orthodox doctrine ofspecial creation (which asserted that allexisting species were directly created bythe hand of God and which denied the ex-istence of constant hybrids) and what hecalls the modified version proposed byCarolus Linnaeus in the middle of the eigh-teenth century, which claimed that con-stant hybrids could arise.] Ernst Mayr(1982) remarks: "It is highly significantthat, as in the case of Darwin, it was thespecies question which inspired Mendel inhis work on inheritance."

However, Mendel's experimental mate-rial did not fulfill the criteria considerednecessary by theoretical biologists of thetime for hybridization pertaining to the or-igin of species (Gasklng 1959): Mendelchose domestic varieties, not wild spe-cies, although he often uses the word"species" in his paper, obviously tenden-tiously, in reference to his garden peas. L.C. Dunn (1965a) comments: "Kolreuterand Gartner, to whom he [Mendel] refers,worked with true species hybrids differingin many variable intergrading charactersfrom which such a rule as Mendel envis-aged could not have been derived." AndCorcos and Monaghan (1993) write:"Hence, what Mendel was discussing asthe transformation of one species into an-other was really the transformation of onevariety into another." It has been arguedthat the classification of peas was uncer-tain at the time and that Mendel states inhis paper that their position in a classifi-catory system was "completely immaterialto the experiments in question," an atti-tude that has been labeled "cavalier"(Haiti and Orel 1992). However, the im-portant point is that Mendel is not consis-tent, referring to his experimental plantsas "varieties" in some contexts and as"species" in other contexts, as Corcos andMonaghan (1993) have noted: "In thisparagraph [detailing the selection ofplants for his experimental program] Men-del uses the word 'variety' for his breedinglines, and yet in his 'Introductory Re-marks' he wrote about hybrids betweenspecies." Mendel also called his peas "va-rieties" in his first letter to Nageli, in

which he enclosed his paper (Stern andSherwood 1966).

The late eighteenth- and early nine-teenth-century experimentalists in plantcrossing are seen to fall into two distinctgroups: those who investigated the "es-sence of the species as a whole" (Mayr1982) and those (such as Gallesio, Knight,Goss, Seton, and Sageret) who followedthe course of individual plant charactersand who, therefore, like Mendel, worked ofnecessity with varieties. Nevertheless,Mendel obviously felt that his conclusionshad relevance to the former group, severalof whom he mentions in his paper (Kol-reuter, Gartner, Herbert, Lecoq, Wichura),while making no acknowledgment of thepioneers in his own field. Mayr (1982) ob-serves: "Even though Mendel occasionallycalls himself a hybridizer and in his paperoften refers to Kolreuter, Gartner, and oth-er plant hybridizers, he himself does notat all belong to that tradition."

Ever since Kolreuter's work with the to-bacco plant in the 1760s, hybridizers hadsought to settle the question of immuta-bility and creation by experiment, and, asCallender (1988) has suggested, a properunderstanding of such important theoret-ical categories as reversion and transfor-mation, two phenomena that Kolreuterclaimed to have discovered in the courseof his experimental investigations that re-futed the existence of constant hybrids, isessential to an interpretation of Mendel'spaper. In this context, reversion was thereturn of a hybrid by self-fertilization tothe two original types crossed, whiletransformation was the conversion of onespecies into another already existing spe-cies by repeated backcrossing of the hy-brid with one or the other of the two orig-inal parental forms. The purpose of suchexperiments was not to disprove the fixityof species, as so it may appear today, butto support the orthodox doctrine of spe-cial creation, since no new species wasproduced.

Joseph Gottlieb Kolreuter (1733-1806)and his successor Carl Gartner (1772-1850), to whom Mendel repeatedly refers,were firm believers in the orthodox doc-trine of special creation ["Constant hybridplant forms, they maintained, could notand did not exist" (Callender 1988)], andDarwin had cited them in The Origin ofSpecies as adversaries. [Darwin describedGartner as "so good an observer and sohostile a witness" (Darwin 1859).]

The first half of Mendel's (1866) paper(pp. 1-23) is devoted exclusively to theacquisition of his "generally applicable

law" quantifying reversion (in which thepopulation reverts to the pure parentforms, while the hybrid represents anever-decreasing proportion of the proge-ny), while the final pages (pp. 44—48) aredevoted exclusively to transformation ex-periments. Mendel (1866; emphasis inoriginal) writes: "Finally, the experimentsperformed by Kolreuter, Gartner, and oth-ers on transformation of one species intoanother by artificial fertilization deservespecial mention. Particular importancewas attached to these experiments; Gart-ner counts them as among 'the most dif-ficult in hybrid production.' " Mendel thenproposes the application of his "law validfor Pisum": "If one may assume that thedevelopment of forms proceeded in theseexperiments in a manner similar to that inPisum, then the entire process of transfor-mation would have a rather simple expla-nation." And in the penultimate paragraphof his paper, Mendel (1866) states: "Thesuccess of transformation experiments ledGartner to disagree with those scientistswho contest the stability of plant speciesand assume continuous evolution of plantforms. In the complete transformation ofone species into another he finds unequiv-ocal proof that a species has fixed limitsbeyond which it cannot change. Althoughthis opinion cannot be adjudged uncon-ditionally valid, considerable confirmationof the earlier expressed conjecture on thevariability of cultivated plants is to befound in the experiments performed byGartner." The last sentence obviously in-dicates that Mendel thought that the "gen-erally applicable law" he had acquiredsupported Gartner's position. However, ithas been interpreted as meaning the exactopposite, as L. A. Callender has pointedout: "Despite its clarity this paragraph hasbeen a source of endless confusion in theliterature. If this statement is to be takenliterally, as Mendel most assuredly intend-ed it to be taken, then it says quite simplythat he gave conditional acceptance to theview, expressed by Gartner, 'that species arefixed within limits beyond which they can-not change'. Nothing could be clearer. Nev-ertheless, interpretations of this passagehave been given which are remarkable fortheir extreme departure from accepteduse in both the German and English lan-guages" (Callender 1988; emphasis in orig-inal).

Thus, Mendel's concerns can be seen asconsistent with the traditional science ofhis day (Max Wichura's book on true-breeding willow hybrids had been pub-lished as recently as 1865) and also with

2 0 8 The Journal of Heredity 1996 87(3)

the tumultuous times in which his paperappeared, when the Darwinian revolutionwas demanding a complete reassessmentof man's concept of himself and his worldin a way that no earlier revolution in thephysical sciences had. David L. Hull(1973), discussing the impact of Darwin'sbook, writes: "Seldom in the history ofideas has a scientific theory conflicted soopenly with a metaphysical principle asdid evolutionary theory with the doctrineof the immutability of species." And Au-gustine Brannigan (1979), in an article en-titled "The Reification of Mendel," com-ments: "If anything, Mendel's reputationwas modest not because he was so radi-cally out of line with his times but becausehis identity with his contemporaries wasso complete!" One of Mendel's contempo-raries was Gustave Niessl [in fact, HugoIltis records that Mendel "owed a gooddeal to Niessl's insight and clarity" (Iltis1932)], who was secretary of the naturalhistory society to which Mendel had pre-sented his paper in 1865, and he was stillan active official at the turn of the centurywhen Mendel's paper was revived. Bran-nigan (1979; emphasis in original) ob-serves: "In 1902, he [Niessl] suggestedthat it was believed in the 1860s that Men-del's work was in competition with, as op-posed to complementary to, that of Darwinand Wallace." [The problem of hybridiza-tion and its relationship to evolution hadbeen a frequent theme since the foundingof the society by Mendel and his friendsand acquaintances in December 1861(Brannigan 1979).]

Mendel also compares his Pisum "law"with the reversion experiments reportedby Kolreuter and Gartner. After statingthat the monohybrid seeds and plantshave been followed through four to sixgenerations (and just before presentinghis formula extrapolating to any numberof generations), Mendel (1866) writes:"The observation made by Gartner, Kol-reuter, and others, that hybrids have a ten-dency to revert to the parental forms, isalso confirmed by the experiments dis-cussed."

Therefore, no constant hybrids arise inthe monohybrid experiments, and Mendelconcludes that the same "law of develop-ment" applies in the dihybrid and trihy-brid experiments: "Therefore there can beno doubt that for all traits included in theexperiments this statement is valid: Theprogeny of hybrids in which several essen-tially different traits are united represent theterms of a combination series in which theseries for each pair of differing traits are

combined. This also shows at the sametime that the behavior of each pair of dif-fering traits in a hybrid association is inde-pendent of all other differences in the twoparental plants" (Mendel 1866; emphasisin original). So no constant F, hybrids oc-cur in any of Mendel's experiments. Al-though true-breeding forms appear amongthe progeny of the hybrids (Fj) in the di-hybrid and trihybrid experiments, theyare only the result of a recombination ofexisting alternatives. Nothing new comesinto being at either the phenotypic or ge-notypic level, for Mendel's abstract sym-bolic notation implies that Darwin's "ele-mental atoms" are discrete entities that re-main immutable throughout both time andspace, irrespective of how many genera-tions or character pairs are involved. [In-terestingly, Mendel refers to the internaldeterminants he postulated in order to ac-count for his "doctrine of hybridization"(Sinoto 1971) as "elements": ". . . those el-ements of both cells that cause their dif-ferences" (Mendel 1866). Meijer (1983)writes: "Whenever Mendel discusses thematerial composition of the germ-cells, heuses the word 'elements' ('Elemente')."]This is in marked contrast to Darwin, whoargued that there is a continual produc-tion of small heritable variations uponwhich his mechanism of natural selectioncould act. Callender (1988), in his articleentitled "Gregor Mendel: An Opponent ofDescent with Modification," observes: ". . .Mendel, of course, denied the very basisof the evolutionary process, modificationof hereditary characteristics." And Callen-der elaborates: ". . . it is a striking fact thatthe multitude of commentators who haveso consistently held that Mendel was inessential agreement with the theory ofevolution have singularly failed to dem-onstrate in his theory of heredity anymechanism by which descent with modi-fication might have come about. Indeed,only the merest handful have ever drawnattention to the fact that a concept of he-reditary mutation is entirely absent fromthe whole of Mendel's published work."

Of particular significance is the singletrihybrid experiment, from which Mendelobtains the expected result despite the re-calcitrant data. In this experiment, Mendelincludes the color of the seed coat withthe familiar traits of seed shape and cot-yledon color that he had used for the di-hybrid experiment, although the seed coatis maternal tissue and therefore its ap-pearance following each artificial fertiliza-tion is determined solely by the maternalgenetic constitution and not by an equal

contribution from both parents (Mayr1982). (William Bateson, in his book Men-del's Principles of Heredity, first publishedin 1909, wrote: "The seeds of course aremembers of a generation later than that ofthe plant that bears them. Thus when across is made the resultant seeds are F,,showing the dominant character yellow-ness or roundness, but the seed-skins arematernal tissue.") Consequently, the ap-pearance of the seed coat relevant to Men-del's formulae is not observable in com-bination with the two embryonic traits inthe same plant or in the same year. Men-del does not mention this, nor does hestate the very complex procedures nec-essary to overcome the serious difficul-ties, reporting only that this "experimentwas conducted in a manner quite similarto that used in the preceding one" (Men-del 1866), all of which cast doubt onwhether he actually performed the exper-iment, thus substantiating Di Trocchio's(1991) claims.

Mendel says nothing at all about a tet-rahybrid experiment, although his popu-lational generalizations immediately fol-lowing the trihybrid experiment are pro-jections from four pairs of character dif-ferences: "For instance, when the parentaltypes differ in four traits the series con-tains 3" = 81 terms, 44 = 256 individuals,and 24 = 16 constant forms; stated differ-ently, among each 256 offspring of hybridsthere are 81 different combinations, 16 ofwhich are constant" (Mendel 1866). Men-del then extrapolates to any number oftraits, even those he had excluded at theoutset because they did not fulfill his cri-teria for selection: "The complete agree-ment shown by all characteristics testedprobably permits and justifies the assump-tion that the same behavior can be attrib-uted also to the traits which show less dis-tinctly in the plants, and could thereforenot be included in the individual experi-ments. An experiment on flower stems ofdifferent lengths gave on the whole a rath-er satisfactory result, although distinctionand classification of the forms could notbe accomplished with the certainty that isindispensable for correct experiments"(Mendel 1866). This suggests that Mendelacquired his seven traits by prior experi-ment, which again correlates with themethodology Di Trocchio argues Mendelmust have used to obtain his data. Mayr(1982) comments: "The 22 varieties[which Mendel (1866) reports he "plantedannually throughout the entire experimen-tal period"] differed from each other byfar more than the seven selected traits,

Bishop • Mendel's Opposition to Evolution and to Danvin 209

but Mendel found the other traits unsuit-able because either they produced contin-uous or quantitative variation not suitablefor the study of the clear-cut segregationhe was interested in, or else they did notsegregate independently." It also supportsFisher's (1936) evaluation of Mendel's pa-per, of which L. C. Dunn (1965b) wrote:"The impression that one gets from Men-del's paper itself and from Fisher's studyof it is that Mendel had the theory in mindwhen he made the experiments reportedin the paper. He may even have deducedthe rules from a particulate view of hered-ity which he had reached before beginningwork with peas. If so, the outcome of hisexperiments constitutes, in Fisher'swords, not discovery but demonstration."

In his conclusion, Mendel (1866) dis-cusses the combination series he has ac-quired, in which "the members of the se-ries tend equally toward both original par-ents in their internal makeup" [converse-ly, Darwin (1859) had argued that when"hybrids are able to breed inter se, theytransmit to their offspring from generationto generation the same compounded or-ganization"], and he declares: "One seeshow risky it can sometimes be to drawconclusions about the internal kinship ofhybrids from their external similarity."

As Arnold Ravin (1965) has pointed out,Mendel's theory was based upon the as-sumption of equality throughout all stagesof the life cycle: equal gametes that uniteat random to form equal zygotes that growinto equal plants, reproducing equallygeneration after generation. This, ofcourse, is the antithesis of Darwin's theo-ry of differential survival and differentialreproductive success. Also, Mendel fo-cused on discontinuous variation: "Men-del's concept of discrete characters wascompletely opposed to Darwin's idea ofcontinuous variation" (Orel and Kuptsov1983). Mayr (1982) remarks: "Even thoughDarwin acknowledged the existence of dis-continuous variation as a separate cate-gory, he considered it evolutionarily un-important."

When Mendel's paper was revived at thebeginning of the twentieth century, the pu-rity of the gametes was highlighted by Wil-liam Bateson (1901; emphasis in original)in his introduction to the first Englishtranslation: "The conclusion which standsout as the chief result of Mendel's admi-rable experiments is of course the proofthat in respect of certain pairs of differ-entiating characters the germ cells of a hy-brid, or cross-bred, are pure, being carri-ers and transmitters of either the one

character or the other, not both." Thesame point had been made by Mendel inhis second letter to Nageli: "The course ofdevelopment consists simply in this; thatin each generation the two parental traitsappear, separated and unchanged, andthere is nothing to indicate that one ofthem has either inherited or taken overanything from the other" (Stern and Sher-wood 1966). Eiseley (1958) comments ofthis passage: "Heredity and variation inthe old Darwinian sense could, therefore,not be synonymous. The unit factors hada constancy which the Darwinians hadfailed to guess."

Darwin and Heredity

Darwin, of course, believed throughout hislife in blending inheritance, like all his con-temporaries ["In Darwin's time everybodyexcept Mendel . . . thought that inheri-tance was blending" (Dawkins 1986)], andin the inheritance of acquired charactersfor both corporeal structures and behav-ior. In fact, Darwin put forward his owntheory of heredity, "Pangenesis," in 1868in order to account not only for the pro-duction of variation but also for the inher-itance of acquired characters. It is gener-ally thought that this was a retreat on Dar-win's part in the face of mounting criticismof his mechanism of natural selection, buthistorians have shown that Darwin's ideason heredity were developed over theyears in concert with his theory of de-scent with modification ["... the hypoth-esis represents the crystallisation of Dar-win's thoughts over a period of a quarterof a century, thoughts that began with hiswonder at the ability of a planarian to re-generate after division" (Olby 1985)], andit is clear from the first edition of The Or-igin of Species that Darwin took the inher-itance of acquired characters for granted.Darwin devotes a whole chapter to in-stinct, and in his discussion of the hive-bee (Mendel, incidentally, also workedwith bees), he states: "The motive powerof the process of natural selection havingbeen economy of wax; that individualswarm which wasted least honey in the se-cretion of wax, having succeeded best,and having transmitted by inheritance itsnewly acquired economical instinct tonew swarms, which in their turn will havehad the best chance of succeeding in thestruggle for existence" (Darwin 1859).

Mendel's theory was obviously con-structed to deny all Darwin's ideas aboutheredity and thus his theory of descentwith modification. George (1982) ob-

serves: "The final blow, or so it seemed—to pangenesis, inheritance of acquiredcharacters, inequality of parental contri-bution and blending inheritance—oc-curred in 1900 when Mendelian and deVriesian theories of inheritance reachedthe scientific world." Mendel's onslaught,however, unlike that of de Vries, wasmounted in the early 1860s, when theworld was agog with Darwin's ideas andwhen biological knowledge and theorywere entirely different.

Darwin's belief in the inheritance of ac-quired characters was increasingly playeddown in the twentieth century—after theacceptance of August Weismann's theoryof the isolation of the germ plasm from so-matic influence—so much so, in fact, thatthe inheritance of acquired characters be-came the central theme of acrimonious de-bate between French and English biolo-gists and philosophers, who lined up onopposing sides behind their heroes. Al-though, as Maynard Smith (1982) haspointed out, it was not Lamarck's theoryof heredity that Darwin rejected but hisidea that evolution could be explained byan inner drive toward complexity.

It was the hereditary aspect of evolu-tionary theory that most fascinated Dar-win. In a letter to T. H. Huxley, some yearsbefore the publication of The Origin of Spe-cies, Darwin had remarked: "Approachingthe subject from the side which attractsme most, viz inheritance, I have latelybeen inclined to speculate" (Burkhardtand Smith 1990). Darwin drew a sharp lineof demarcation between the inheritance ofwhat later came to be known as Mendeliancharacters and those acquired by naturalselection. In the first edition of The Originof Species, he states: "Looking to the caseswhich I have collected of cross-bred ani-mals closely resembling one parent, theresemblances seem chiefly confined tocharacters almost monstrous in their na-ture, and which have suddenly ap-peared—such as albinism, melanism, de-ficiency of tail or horns, or additional fin-gers and toes; and do not relate to char-acters which have been slowly acquiredby selection" (Darwin 1859). This passageappears unaltered in the sixth (and final)edition, published in 1872.

Darwin began The Variation of Animalsand Plants under Domestication (in whichhe presented his theory of heredity) in1860, a year after the publication of thefirst edition of The Origin of Species, and itwas first published in 1868, two years afterthe appearance of Mendel's Pisum paperand a year before Mendel's presentation of

210 The Journal oJ Heredity 1996 87(3)

his Hieracium paper. [Interestingly, Mendelmade numerous marginalia in Darwin'schapter on inheritance (Orel 1971) (theGerman edition of The Variation was pub-lished in the same year as the English edi-tion), and, as we have seen, he mentionedDarwin in his Hieracium paper.] In thismassive, two-volume work, the first part ofthe long-promised addition to The Origin(the second part on the variability of or-ganic beings in a state of nature was neverwritten), Darwin devoted only five pagesto Mendelian-type inheritance. The sec-tion, entitled "On certain characters notblending," concludes: "All the charactersabove enumerated, which are transmittedin a perfect state to some of the offspringand not to others—such as distinct col-ours, nakedness of skin, smoothness ofleaves, absence of horns or tail, additionaltoes, pelorism, dwarfed structure, etc.—have all been known to appear suddenlyin individual animals and plants. From thisfact, and from the several slight, aggregat-ed differences which distinguish domesticraces and species from one another, notbeing liable to this peculiar form of trans-mission, we may conclude that it is insome way connected with the sudden ap-pearance of the characters in question"(Darwin 1875).

In the section of The Variation entitled"Prepotency in the transmission of char-acter," Darwin presented in careful detailexperiments very similar to Mendel's. [AsMayr (1983) has pointed out, Darwin wasnot only a meticulous observer but "alsoa gifted and indefatigable experimenterwhenever he dealt with a problem, the so-lution of which could be advanced by anexperiment."] He writes: "Now I crossedthe peloric snapdragon (Antirrhinum ma-jus), described in the last chapter, withpollen of the common form; and the latter,reciprocally, with peloric pollen. I thusraised two great beds of seedlings, and notone was peloric . . . The crossed plants,which perfectly resembled the commonsnapdragon, were allowed to sow them-selves, and out of a hundred and twenty-seven seedlings, eighty-eight proved to becommon snapdragons, two were in an in-termediate condition between the peloricand normal state, and thirty-seven wereperfectly peloric, having reverted to thestructure of their one grandparent" (Dar-win 1875).

The occurrence of intermediate types isvery significant, for it categorically rulesout the possibility of establishing a defi-nite ratio between only the two types re-sembling the parental forms, as it is well

known that Mendel did in his paper: "Tran-sitional forms were not observed in any ex-periment" (Mendel 1866; emphasis in orig-inal). [Hugo de Vries, one of Mendel's "re-discoverers," later commented: "The lackof transitional forms between any two sim-ple antagonistic characters in the hybridis perhaps the best proof that such char-acters are well delimited units" (de Vries1900).] Thus, the difference between Dar-win's and Mendel's ideas about inheri-tance was not only a question of interpre-tation: Mendel's "facts" were differentfrom Darwin's—and from those of all hiscontemporaries. [For instance, Meijer(1983; emphasis in original) notes: "Men-del's views appear to have been very dif-ferent from Nageli's, and also his factswere different."]

Darwin (1875) began the conclusion tohis theory of heredity: "The hypothesis ofPangenesis, as applied to the several greatclasses of facts just discussed, no doubtis extremely complex, but so are thefacts."

At the time when Darwin and Mendelwrote, it was impossible for a scientist tobe anything but speculative about the na-ture of inheritance, for it was not until1869 that Friedrich Miescher elucidatedthe chemical composition of the cell nu-cleus, and it was even later that it was un-derstood that the nucleus of the zygote isformed by the fusion of the egg nucleusand a male nucleus derived from a sper-matozoon.

A. R. Wallace, the codiscoverer with Dar-win of the principle of natural selection,lived to see the great cytological advancesat the end of the nineteenth century—andthe rise of Mendelism. George (1971)states: "... in 1910, in the 'World of Life'Wallace made his last comments on Men-delism. He still objected to the idea be-cause the Mendelian characters were ab-normalities and, therefore, detrimental tothe species under natural conditions. Hestressed, once more, their rarity: "The phe-nomena on which these theories are found-ed seem to me to be mere insignificant by-products of heredity, and to be essentiallyrather self-destructive than preservative.They form one of nature's methods of get-ting rid of abnormal and injurious varia-tions. The persistency of Mendelian char-acters is the very opposite of what is need-ed amid the ever-changing conditions ofnature.'"

Mendel's Wrinkled Pea SeedsInterestingly, molecular studies have re-cently revealed that the wrinkled-seed

character that Mendel chose for his firstmonohybrid experiment (and also forboth the dihybrid and trihybrid experi-ments) is caused by an insertion sequencethat prevents normal expression of a genefor an enzyme (the presence of whichgives rise to the alternative character,roundness), and this has important impli-cations to Mendelian theory.

Madan Bhattacharyya and his col-leagues point out that the phenotype de-scription of wrinkled pea seeds that Men-del provided fits only two loci describedamong commercial cultivars, r and rb, andthat the latter would have been unavail-able to Mendel, and they comment: "It hasbeen widely accepted that the wrinkled-seed character described and studied byMendel (1865) was an allele of the r locus"(Bhattacharyya et al. 1990). The insertionis thought to be located in an exon of theSBE1 gene, which codes for an enzymecalled the starch-branching enzyme 1, andit is estimated that it would cause loss ofthe last 61 amino acids of the SBE1 pro-tein, resulting in defects in starch, lipid,and protein biosynthesis in the seed(Bhattacharyya et al. 1990). Therefore,wrinkled pea seeds are abnormal peaseeds, the equivalent of Darwin's "sports"and Garrod's "inborn errors of metabo-lism," and round pea seeds are normal peaseeds. There are no instructions "for"wrinkledness—only defective instructionsfor roundness. In other words, there arenot two different units of information, onecoding for wrinkledness and the other forroundness; there is only one unit, the wild-type gene coding for roundness, with orwithout an insertion sequence.

Furthermore, the insertion sequence is0.8 kb long, and, whereas the normal SBE1gene transcript in round seeds is 3.3 kb,the aberrant transcript in wrinkled seedsis 4.1 kb, suggesting that the nuclear DNAof the homozygous recessive wrinkled peaseeds contains the entire information forthe enzyme (i.e., for roundness), althoughit cannot be expressed. [J. R. S. Fincham(1990) writes: "The size difference wasshown to segregate from crosses with ther locus, as if the r mutation were due to aninsertion of an extraneous sequence intothe wild-type R gene."] Thus, the infor-mation for roundness is not being elimi-nated at meiosis (although, of course, dis-junction of chromosomes with their differ-ent complements occurs), and conse-quently all gametes for both phenotypes(and for the heterozygote) must carry theinformation for roundness, a fact that callsinto question the validity of the first Men-

Bishop • Mendel's Opposition to Evolution and to Darwin 211

delian law, the law of segregation. As Do-ver (1986) has observed in a different con-text: "The traditional algebraic represen-tation of the Mendelian segregation ofgenes from one generation to the next is,strictly speaking, a description of the seg-regation of chromosomes." [The secondMendelian law, the law of independent as-sortment, was disproved many years ago,as Mayr (1982) has noted, despite its per-sistence as textbook orthodoxy: "Finally,free assortment is also [in addition to theshort-lived "law of dominance"] not a val-id 'law,' because it was discovered soonafter 1900 that characters could be'linked,' by having their determinants onthe same chromosome."]

Although there is no evidence of exci-sion of the insertion sequence that pro-duces wrinkled pea seeds, there are in-creasing reports of somatic and germ-lineexcision of such elements in fruit flies andother organisms, leading to reversion towild type. Thus, organisms scored as mu-tants in one generation can give rise tonormal progeny in subsequent genera-tions. This, of course, has important im-plications to all aspects of Mendelian the-ory: gametic transmission of information,fixed ratios, predictions of long-term sta-bility, the definition of "rate of mutation,"and even Mendelian terminology. (Most ofthese mutant organisms, like Mendel'swrinkled pea seeds, are labeled "homozy-gous recessive" in the Mendelian system,according to which information for com-plex normal characters has been lost andcannot be reacquired.)

These findings would correlate with Dar-win's observation preluding his report ofhis experiments with normal and peloricsnapdragons: " . . . plants bearing peloricflowers have so strong a latent tendencyto reproduce their normally irregular flow-ers, that this often occurs by buds whena plant is transplanted into poorer orricher soil" (Darwin 1875). [Fincham(1990) points out that transposable DNAelements have been characterized fromseveral plant species, including antirrhi-num, and it is known that excision eventsare strongly dependent upon the environ-ment, occurring, for instance, less fre-quently in plants grown inside a green-house than in those grown outside, wherethe temperature is lower.]

It is possible that other recessive char-acters Mendel chose could also be causedby insertion sequences in wild-type genesand that the departure from the expectedphenotypes that Mendel recorded but ig-nored in formulating his theory could be

explained by somatic excision of the ele-ments. For instance, in the experimentwith yellow/green seeds (which Mendelused for his second monohybrid experi-ment and, like the round/wrinkled char-acter pair, for both the dihybrid experi-ment and trihybrid experiment), Mendelobserved a "partial disappearance of thegreen coloration" (Mendel 1866) in someseeds.

Now, in retrospect, it can be seen exact-ly what Mendel did: he deliberately chosecharacters exhibiting a most unusual pat-tern of inheritance ("this peculiar form oftransmission," as Darwin referred to Men-delian-type inheritance) because he want-ed to demonstrate stasis, formulated ahighly improbable theory, and then ex-trapolated to all other modes of inheri-tance. But it is today known that the ge-nome is extraordinarily fluid, as a conse-quence of a variety of mechanisms of non-reciprocal DNA transfer within andbetween chromosomes, and it is obvious,as the early opponents of Mendelismmaintained (Wallace was by no meansalone in his objections), that the type oftransmission upon which Mendel focused(that represented largely by human dis-eases and laboratory mutants) is the ex-ception and not the rule: "The ubiquity ofgenomic turnover mechanisms both with-in and between genes (single-copy andmultigene families) means that few geneswill be found that are refractory to themechanisms involved. It is conceivablethat strict Mendelian genes and stableMendelian populations in Hardy-Weinbergequilibria do not exist except as observedover short periods of time and amongstsmall numbers of progeny" (Dover 1986).

Conclusion

1. Mendel's sole objective in writing hisPisum paper, published in 1866, was tocontribute to the evolution controversythat had been raging since the publi-cation of Darwin's The Origin of Speciesin 1859.

2. Mendel could have come into contactwith Darwin's ideas as early as 1860,when the first German edition of TheOrigin of Species was published and achapter was printed in a journal thatwas available to him. This would havegiven Mendel a time frame for his ex-perimental program that correlateswith the methodology Di Trocchio(1991) claims he must have used to ob-tain his data.

3. The content of Mendel's paper shows

that he was familiar with The Origin ofSpecies, Darwin's themes of evolution,population, and heredity being echoedby Mendel, and that he was opposed toDarwin's theory: Darwin was arguingfor descent with modification throughnatural selection, Mendel was in favorof the orthodox doctrine of special cre-ation.

4. Darwin's theory was based on differ-ential survival and differential repro-ductive success, Mendel's on equalitythroughout all stages of the life cycle:equal gametes that unite at random toform equal zygotes that grow into equalplants, reproducing equally generationafter generation.

5. Darwin's concepts were continuousvariation, mutation, and "soft" heredi-ty; Mendel espoused discontinuousvariation and "hard" heredity withoutmutation.

6. The theoretical significance of Mendel'sfirst monohybrid experiment (withseed shape) and of both the dihybridand trihybrid experiments (in whichthe same trait was included) is ques-tionable in light of molecular studiesshowing that the wrinkled-seed pheno-type is caused by an insertion se-quence that prevents normal expres-sion of the gene for an enzyme, thepresence of which gives rise to the an-tagonistic character, roundness (Bhat-tacharyya et al. 1990).

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Received December 7, 1994Accepted November 8, 1995

Corresponding Editor Stephen J. O'Brien

Bishop • Mendel's Opposition to Evolution and to Darwin 213