Appendix. Mendel’s Marginalia in Darwin’s Origin of Species.

In this appendix, we provide Mendel’s complete marginalia from his personal copy of the Origin, including the notes in script and the marked passages. For each marked passage, we give first the German page number and passage and then the corresponding page number and passage from the English third edition. We also include commentary and pertinent quotations from Mendel’s "Versuche". In each case, we have quoted full sentences from the marked passages, even though Mendel’s marginalia do not always span full sentences. Citations for all references can be found in the "Literature cited" section of our paper.

Editor’s Preface (Vorrede des Verfassers)

Page 1 German:

"1.2 Lfg 3 4 & 6" written in script.

"Pag 302" written in script.

Commentary: Aside from the numbers on the end leaf, this page contains the only notes in script. The first note reads "1.2 Lfg 3 4 & 6." The abbreviation Lfg is for "Lieferung" (singular) or "Lieferungen" (plural) and refers to installments or fascicles. When Mendel’s copy of the Origin was published, books were often printed and delivered to libraries or book vendors as fascicles which were then bound by a local bookbinder after all the fascicles had been received. This mark probably is a bookbinders note for organization of the fascicles for binding.

The second note, "pag 302," clearly is Mendel’s note. We discuss it in the text of our paper.

Chapter 1: Variation Under Domestication

Page 17 German: Single vertical line in margin and first word of sentence bracketed.

"Es scheint ferner ganz klar zu seyn, dass die organischen Wesen einige Generationen hindurch neuen Lebens Bedingungen ausgesetzt seyn müssen, ehe ein bemerkliches Maass von Veränderung in ihnen hervortreten kann, und das, wenn ihre Organisation einmal abzuändern begonnen hat, diese Abänderung gewöhnlich durch viele Generationen fortwährt."

Page 7 English:

"It seems pretty clear that organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation; and that when the organization has once begun to vary, it generally continues to vary for many generations."

Commentary: We discuss this passage in the text of our paper.

Chapter 2: Variation Under Nature

Page 56 German: Single vertical line in margin.

"Was anderseits aber die kultivirten Pflanzen betrifft, so ist in den wenigen bekanten Fällen, wo eine Varietät Blüthen oder Früchte von zweierlei Beschaffenheit hervorzubringen pflegt, die Entstehung dieser Vareität eine plötzliche gewesen."

Page 46 English

"With cultivated plants, in the few cases known of a variety habitually bearing flowers or fruit slightly different from each other, the production of the variety has been sudden."

Pages 64—65 German: Single vertical line in margin.

"Aus diesen Bemerkungen geht hervor, dass ich den Kunstausdruck >>Species<< als einen nur willkürlich und der Bequemlichkeit halber auf eine Reihe von einander sehr ähnlichen Individuen angewendeten betrachte, und dass er von dem Kunstausdrucke >>Varietät<< nicht wesentlich, sondern nur insofern verschieden ist, als dieser auf minder abweichende und noch mehr schwankende Formen Auswendung findet. Und eben so ist die Unterscheidung swischen >>Varietät<< und >>individueller Abänderung<< nur eine Sache der Willkür und Bequemlichkeit."

Pages 54—55 English:

"From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitraily, and for mere convenience sake."

Page 68 German: Single vertical line in margin.

"Diese Thatsachen sind von klarer Bedeutung für die Ansicht, dass Arten nur streng ausgeprägte und bleibende Varietäten sind; denn wo immer viele Arten in einerlei Sippe gebildet worden sind oder wo, wenn der Ausdruck erlaubt ist, die Arten-Fabrikation thätig betrieben worden ist, müssen wir gewöhnlich diese Fabrikation noch in Thätigkeit finden, zumal wir alle Ursache haben zu glauben, dass das Fabrikations-Verfahren ein sehr langsames seye."

Page 58 English:

"These facts are of plain signification on the view that species are only strongly marked and permanent varieties; for whenever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing of new species to be a slow one."

Page 71 German: Single vertical line in margin.

"Endlich haben dann Varietäten auch die nämlichen allgemeinen Charaktere, wie Species; denn sie können von Arten nicht unterschieden werden, ausser, erstens, durch die Entdeckung von Mittelgleidern, und das Vorkommen solcher Glieder kann den wirklichen Charakter der Formen, welche sie verketten, nicht berühren, - und ausser, zweitens, durch ein gewisses Maass von Verschiedenheit, indem zwei Formen, welche nur sehr wenig von einander abweichen, allgemein nur als Varietäten angesehen werden, wenn auch verbindende Mittelglieder noch nicht entdeckt worden sind; aber dieser Betrag von Verschiedenheit, welcher zur Erhebung zweier Formen zum Arten-Rang nöthig, ist ganz unbestimmt."

page 61 English:

"Finally, then, varieties cannot be distinguished from species, — except, firstly, by the discovery of intermediate forms linking them together, and the occurrence of such links cannot affect the character of the forms which they connect; and except, secondly, by a certain amount of difference, for two forms, if differing very little, are generally ranked as varieties, notwithstanding that intermediate linking forms have not been discovered; but the amount of difference considered necessary to give to two forms the rank of species is quite indefinite."

Commentary: In the three preceding passages, Darwin expressed his view that the distinction between species and varieties is often an arbitrary one. Mendel agreed with Darwin on this point. Regarding the difficulty of distinguishing varieties from species, Mendel wrote in the Verusche about the pea varieties he used:

Pages 71-72 German: Single vertical line in margin.

"Aber durch nachher zu erläuternde Abstufungen streben auch die grösseren Sippen immer mehr in kleine auseinander zu treten. Und so werden die Lebensformen auf der ganzen Erde in Gruppen und Untergruppen weiter abgetheilt."

page 62 English:

"But by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, the forms of life throughout the universe become divided into groups subordinate to groups."

Chapter 3: Struggle for Existence

Page 73 German: Single vertical line in margin.

"Wir haben gesehen, dass der Mensch durch Auswahl zum Zwecke der Nachzucht grosse Erfolge sicher zu erzielen und organische Wesen seinen eigenen Bedürfnissen anzupassen im Stande ist durch die Häufung kleiner aber nützlicher Abweichungen, die ihm durch die Hand der Natur dargeboten werden. Aber die Natürliche Auswahl ist, wie wir nachher sehen werden, unaufhörlich thätig und des Menschen schwachen Bemühungen so unvergleichbar überlegen, wie es die Werke der Natur überhaupt denen der Kunst sind."

pages 64-65 English:

"We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art."

Commentary: This passage aptly summarizes Darwin’s views on the importance of natural selection in evolution. However, Mendel hardly mentioned natural selection in his writings, focusing instead on hybridization.

Chapter 4: Natural Selection

Page 111 German: Single vertical line in margin.

"Andrerseits aber haben viele Blumen ihre Befruchtungs-Werkzeuge sehr enge umschlossen, wie die Schmetterlingsblüthigen z. B.; aber in den meisten solchen Blumen ist eine sehr merkwürdige Anpassung zwischen dem Bau der Blume und der Art und Weise, wie die Bienen den Nektar darous säugen, indem sie alsdann entweder den eignen Pollen der Blume über ihre Narbe wischen oder femden Pollen mitbringen. Zur Befruchtung der Schmetterlingsblüthen ist der Besuch der Bienen so nothwendig, dass, wie ich durch anderwärts veröffentlichte Versuche gefunden, ihre Fruchtbarkeit sehr abnimmt, wenn dieser Besuch verhindert wird."

Page 102 English:

"Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but in several, perhaps in all, such flowers, there is a very curious adaptation between the structure of the flower and the manner in which bees suck the nectar; for, in doing this, they either push the flower's own pollen on the stigma, or bring pollen from another flower. So necessary are the visits of bees to papilionaceous flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if these visits be prevented."

Commentary: Here Darwin indicated that insect-assisted pollination is essential in leguminous plants, an assumption most probably based on his experiments in Phaseolus. Mendel was certainly aware that insect-assisted pollination was not the case in pea which self-fertilizes before the flower is open and available to insects. Mendel grew several of his pea plants within the greenhouse specifically to exclude insects that might cause contaminating cross pollination.

Chapter 8: Hybridism

Page 285 German: Single vertical line in margin.

"In einer nämlichen Familie können zwei Sippen beisammen stehen, wovon die eine wie Dianthus viele solche Arten enthält, die sehr leicht zu kreutzen sind, während die der andern, z. B. Silene, den beharrlichsten Versuchen eine Kreutzung zu bewirken in dem Grade widerstehen, dass man auch noch nicht einen Bastard zwischen den einander am nächsten verwandten Arten derselben zu erzielen vermöchte."

Page 279 English:

"In the same family there may be a genus, as Dianthus, in which very many species can be most readily crossed; and another genus, as Silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid."

Commentary: Mendel mentioned Dianthus several times in his paper. In one instance, he referred to it as one of several species that produce very fertile hybrids. He did not mention Silene in his paper.

Page 287 German: Single vertical line in margin.

"Dagegen kommen aber auch unter denjeningen Bastarden, welche zwischen ihren Ältern das Mittel zu halten pflegen, zuweilen abnorme Individuen vor, die einer der reinen Stammarten ausserordentlich gleichen; und diese Bastarde sind dann gewöhnlich auch äusserst steril, obwohl die mit ihnen aus gleicher Frucht-Kapsel entsprungenen Mittelformen sehr fruchtbar zu seyn pflegen."

Page 281 English:

"So again amongst hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility."

Page 294 German: Two single vertical lines in margin.

"Wenn Bastarde fähig sind sich unter sich fortzupflanszen, so übertragen sie von Generation zu Generation auf ihre Abkommen dieselbe Vereinigung zweier Organisationen, und wir dürfen daher nicht erstaunen, ihre Unfruchtbarkeit, wenn auch einigem Schwanken unterworfen, selten abnehmen zu sehen."

"Wir müssen jedoch bekennen, das wir, von haltlosen Hypothesen abgesehen, nicht im Stande sind, gewisse Thatsachen in Bezug auf die Unfruchtbarkeit der Bastarde zu begreifen, wie z. B. die ungleiche Fruchtbarkeit der zweierlei Bastarde aus der Wechselkreutzung, oder die zunehmende Unfruchtbarkeit derjenigen Bastarde, welche zufällig oder ausnahmsweise einem ihrer beiden Ältern sehr ähnlich sind."

Page 288 English:

"When hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, rarely diminishes."

"It must, however be confessed that we cannot understand, except on vague hypotheses, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent."

Commentary: Of interest in the three preceding passages is Darwin’s conclusion that hybrids resembling closely one of their parents tend to be sterile. Mendel reported loss of fertility in Phaseolus and Hieracium hybrids, and in the case of Hieracium, observed hybrids that had intermediate forms as well as hybrids that appeared like one of the parents from the same cross. He selected Pisum because (in his words) plants of this genus "yield perfectly fertile hybrid offspring from reciprocal crosses." He made no attempt to correlate loss of fertility with resemblance of pure parental type in these cases or in any of his known writings. His observation and interpretation of dominance of various characters in Pisum provided an explanation for hybrids that resemble the parents, with no reason for a loss in fertility. In the Verusche, Mendel warned against making premature assumptions based on the similarity of hybrids to parents:

Page 295 German: Single vertical line in margin.

"So scheint es mir denn, dass einterseits geringe Weschsel der Lebens-Bedingungen allen organischen Wesen vortheilhaft sind, und dass anderseits schwache Kreutzungen, nämlich zwischen verschiedenen Stämmen und geringen Varietäten einer Art, der Nachkommenschaft Kraft und Stärke verleihen."

Page 289 English:

"Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same species which have varied and become slightly different, give vigour and fertility to the offspring."

Commentary: In this passage, Darwin recognized heterosis, which was a well-documented phenomenon in the 19^th Century. Mendel noted in the "Versuche" that the hybrids of plants with long and short stems displayed heterosis: "With respect to this last trait [stem length] it must be noted that the stem of the hybrid is usually longer than the longer of the two parental stems, a fact which is possibly due only to the great luxurience that develops in all plant parts when stems of very different lengths are crossed."

Page 296 German: Double vertical lines in margin.

"Auch wenn wir uns zu den erwiesener oder vermutheter Maassen im Kultur-Zustande erzeugten Varietäten wenden, sehen wir uns noch in Zweifel verwickelt."

Page 290 English:

"If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in doubt."

Commentary: Prior to this passage, Darwin claimed that varieties and species produced under nature may be distinguished from one another on the basis of their interfertility, different varieties of the same species being fully interfertile and different species lacking interfertility. He also discussed some of the problems associated with this form of classification for naturally produced species. The passage marked by Mendel introduces a section on domesticated species in which Darwin explained that domestic varieties may differ substantially in appearance yet be fully interfertile, which is precisely what Mendel observed in Pisum. This is one of only two passages that Mendel marked with double lines.

Page 301 German: Single vertical line in margin.

"Wenn Blendlinge oder fruchtbarere Bastarde einige Generationen lang in sich fortgepflanzt werden, so nimmt anerkannter Maassen die Veränderlichkeit ihrer Nachkommen bis zu einem ausserordentlichen Maasse zu: dagegen lassen sich einige wenige Fälle anführen, wo Bastarde sowohl als Blendlinge ihren einförmigen Charakter lange Zeit behauptet haben."

Page 295 English:

"When mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in their offspring is notorious; but some few cases both of hybrids and mongrels long retaining uniformity of character could be given."

Commentary: The "law" that Mendel devised from his Pisum experiments explained the variation observed in the offspring of hybrids. However, Mendel was especially interested in those species that produced true-breeding hybrids. After completion of his Pisum experiments, Mendel initiated hybridization experiments with species that supposedly produced true-breeding hybrids.

Page 302 German: Double vertical lines in margin.

"Der geringere Grad von Variabilität bei Bastarden aus erster Kreutzung oder aus erster Generation im Gegensatze zu ihrer ausserordenlichen Veränderlichkeit in späteren Generationen ist eine eigenthümliche und Beachtung verdienende Thatsache; denn sie führt zu der Ansicht, die ich mir über die Ursache der gewöhnlichen Variabilität gebildet, und unterstützt dieselbe, dass diese letzte nämlich aus dem Reproducktions-Systeme herrühre, welches für jede Veränderung in den Lebens-Bedingungen so empfindlich ist, dass es heidurch oft ganz unvermögend oder wenigstens für seine eigentliche Funktion, mit der älterlichen Form übereinstimmende Nachkommen zu erzeugen, unfähig gemacht wird."

Page 296 English:

"The slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention. For it bears on and corroborates the view which I have taken on the cause of ordinary variability; namely, that it is due to the reproductive system being eminently sensitive to any change in the conditions of life, being thus often rendered either impotent or at least incapable of its proper function of producing offspring identical with the parent-form."

Commentary: We discuss this passage in our paper. It is one of only two passages marked with double lines. It is the only marked passage on page 302, the page number that Mendel wrote on page 1 of the book.

Page 303 German: Two single vertical lines in margin.

"Wenn zwei Arten gekreutzt werden, so zeigt zuweilen eine derselben ein überwiegendes Vermögen eine Ähnlichkeit mit ihr dem Bastarde aufzuprägen, und so ist es, wie ich glaube, auch mit Planzen-Varietäten. Bei Thieren besitzt gewiss oft eine Varietät dieses überwiegende Vermögen über eine andre. Die beiderlei Bastard-Pflanzen aus einer Wechselkreutzung gleichen einander gewöhnlich sehr, und so ist es auch mit den zweierlei Blendlingen aus Wechselkreutzungen. Bastarde sowohl als Blendlinge können wieder in jede der zwei älterlichen Formen zurückgeführt werden, wenn man sie in aufeinander-folgenden Generationen wiederholt mit der einen ihrer Stamm-Formen kreutzt."

Page 297 English

"When two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid; and so I believe it to be with varieties of plants. With animals one variety certainly often has this prepotent power over another variety. Hybrid plants produced from a reciprocal cross, generally resemble each other closely; and so it is with mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced to either pure parent form, by repeated crosses in successive generations with either parent."

Commentary: This passage treats several of the phenomena that Mendel described and interpreted in his Pisum experiments. The first and second sentences describe dominance. The third describes parental equivalence, a phenomenon that Mendel observed in the offspring of reciprocal crosses. The third sentence describes a phenomenon known in the 19^th Century as transformation. Regarding transformation, Mendel wrote:

This is the penultimate paragraph of Mendel’s paper and is among the most contested passages in it. Fisher (1936) claimed that in it "Mendel expressly dissociates himself from Gärtner’s opposition to evolution." De Beer (1964) stated that "This passage comes as near to the acceptance of the mutability of species as anyone could wish." Callender (1988), on the other hand, stated that "he [Mendel] gave conditional acceptance to the view, expressed by Gaertner, ‘that species are fixed within limits beyond which they cannot change.’"

Chapter 9 On the Imperfection of the Geological Record

Page 329 German: Portion of sentence underlined.

"Was aber die geologischen Forschungen allerdings nicht enthüllt haben, das ist das frühere Daseyn der unendich zahlreichen Abstufungen vom Range wirklicher Varietäten zur Verkettung aller Arten untereinander; und dass sie Diess nicht bewirkt haben, ist zweifelsohne einer der erten [sic] und gewichtigsten Einwände, die man gegen meine Ansichten vorbringen mag".

Page 324 English

"What geological research has not revealed is the former existence of infinitely numerous gradations, as fine as existing varieties, connecting all known species. And this not having been effected by geology is the most obvious of the many objections which may be urged against my views."

Page 331 German: Single vertical line in margin.

"Wenn daher diese Bemerkungen einiger Maassen begründet sind, so sind wir nicht berechtigt zu erwarten, dass wir in unseren geologischen Formationen eine endlose Anzahl solcher feinen Übergangs-Formen finden werden, welche nach miener Betrachtungs-Weise sicher einmal alle früheren und jetzigen Arten einer Gruppe zu einer langen und verzwegten Kette von Lebenformen verbunden haben."

Pages 326-327 English

"If then, there be some degree of truth in these remarks, we have no right to expect to find in our geological formations, an infinite number of those fine transitional forms which, on my theory, have connected all the past and present species of the same group into one long branching chain of life."

Commentary: Immediately before this passage, Darwin provided an explanation for the lack of transitional forms in a fossil record, indicating that speciation is most probable at the geographical limits of a species’ range and that the transitional forms should be found at this limit. Once a new species had become successful, it can then displace the parent species, an event that eliminates transitional forms in the fossil record. Unlike most of the preceding passages that Mendel marked, this passage had little direct bearing on his work. Nonetheless, it is a notable passage because it illustrates Darwin’s commitment to gradualism with an explanation for the apparent lack of transitional forms in the fossil record.

German, end leaf of book (no page number)

"1.6.7.13.16.48.52.57.62.63.76.78.80" written in script.

Commentary: Iltis (1924) assumed that these numbers referred to "the pages that have especially interested him [Mendel]." The only one of these numbers that corresponds to a page with marginalia is "1," which is the first page of the German editor’s preface and is not found in the English version. Also, nearly half of the passages Mendel marked in the Origin are in Chapter 8 (entitled Hybridism). Given Mendel’s interest in plant hybridization, this is probably the chapter that most interested him. The numbers written on the back cover end with "80" which is well before Chapter 8. Under these circumstances, it is not clear that these numbers refer to pages in the Origin.