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Wilhelm Johannsen is a standard reference in the history of genetics. He clarified the distinction between genotype and phenotype, and introduced the term ‘gene’. He also carried out the famous experiment of selection within pure lines of beans, an experiment that became a paradigmatic demonstration of the stability of genotype. Arguably Johannsen’s experimental and theoretical development of the distinction between the phenotype — which depends on variation in environment, — and the genotype — which remains stable through generations, — provided the basis for genetics as an exact science, experimentally and theoretically.

Johannsen’s magisterial treatise Elemente der exakten Erblichkeitslehre [Introduction to an exact science of heredity] profoundly influenced the development of genetics in the early decades of the 20th century. The original publication of 1909 was followed by thoroughly revised editions in 1914 and 1926. Johannsen published only a couple of relatively short and specialized genetics papers in English (in particular, Johannsen 1907, 1911, 1923). The popular 1903 article on Darwinism and heredity that is translated below gives an insight into the background and context of his developing theory of genotype. The article was written the same year that he published his classical bean selection experiment (Johannsen 1903), and shows how Johannsen at that point related his ideas about heredity to running debates on evolution, systematics and plant breeding.

Wilhelm Johannsen (1857–1927)

As the son of an army officer, Wilhelm Johannsen was born into the class of civil servants. Higher education was the mainstay of this class which had a key role in the cultural development of Denmark and other Nordic countries in the 19th century. Wilhelm’s older brother went to university to study natural sciences and engineering, but poor family finances forced Wilhelm into a more roundabout route. After finishing the preliminary exam that qualified for university entrance the bright and knowledge-seeking boy became an apprentice in a pharmacy at the age of fifteen.

In a short autobiography[1] Johannsen describes his own upbringing and education in characteristic Johannsen style, precise and sober but not without humour and a lyrical touch. He tells how his grandmother fascinated the little boy with “stories about historical events and her own experiences during the Napoleonic era”. And how his mother’s love for plants and animals and “her perhaps slightly excitable way of seeing their symbolic meaning, awakened at an early age my interest for the life cycles of nature especially in springtime”. His father was a more distant and ambivalent figure, having what Johannsen described as a “strict sense of order and eye for small things[2], his good taste and demands for proper form and behaviour was often more than a little annoying to us children and young people”. As an adult however, Wilhelm reflected that what made him a good scientist was his “father’s eye for small things and mother’s imagination” — together with a third element, “the understanding of verification as necessary for the health of scientific research.” The latter he learned through his studies, it was not “part of my nature.”

Working in Danish and German pharmacies, Wilhelm successfully pursued his education and took the final exam to qualify as a pharmacist in 1880. Through this period, he made diligent use of the libraries and other facilities of friendly and learned apothecaries. The education programme for pharmacists included a last year of study at the University of Copenhagen, where Wilhelm studied botany and chemistry following the lectures of Eugenius Warming (1841–1924). He later recalled, “Never have I known more stimulating and informative teaching than Warming’s as it was in those years”, and the two quickly became good friends. Warming was to become one of the founding fathers of ecology as a scientific discipline. Johannsen also attended lectures in plant physiology by Rasmus Pedersen whom he was to succeed as Professor of plant physiology in 1905.

Johannsen’s career as a scientist began in earnest in 1881, when he became an assistant in the chemical department of the Carlsberg Laboratories headed by Johan Kjeldahl (1849–1900), father of the Kjeldahl Method of analysing nitrogen in organic compounds. The neighbouring physiology department was headed by Emil Chr. Hansen (1842–1909), famous for isolating single cells as a starting point for breeding of yeast of constant quality for beer production. Johannsen’s assignment was to study “questions relating to barley”. In fact, during his six years at Carlsberg, Johannsen enjoyed great freedom to pursue his own scientific interests, recalling that the actual duties and tasks required of an assistant were carried out by “another young man”.

During the six-year appointment at Carlsberg Laboratories, Johannsen read broadly in biological and general scientific literature. He studied for two semesters in Tübingen with the plant physiologist Wilhelm Pfeffer (1845–1920), whom he found to be at this time to be “dried out and disgruntled, tired and nervous”, and he also spent a few months in Paris. Claude Bernard’s (1813–1878) book Leçons sur les phénomènes de la vie communes aux animaux et aux végétaux made an specially strong impression and inspired Johannsen’s life-long passion for “the general problems in biology, periodicity, variation, heredity, etc.”

In 1887 he went abroad again with a stipend to visit a number of German scientific institutions in preparation for a teaching position at the Landbohøjskolen (The Royal Veterinary and Agricultural College). But public decision-making was slow and from 1888 to 1892 Johannsen had to make his living from a variety of activities, including agricultural research, administration, and information service. In 1892 he was finally appointed as a Lecturer at the Landbohøjskolen, and thus obtained a permanent position with opportunities for sustained long term research. His first claim to international fame was the discovery that ether and other narcotic substances could break the winter dormancy of plants like lilacs and willows. This procedure was soon widely applied in the gardening industry to produce flowers in wintertime.

The study of barley at Carlsberg had drawn Johannsen’s attention to questions of plant breeding, turning variation and heredity soon into his main research interest at the Landbohøjskolen. The work of Francis Galton (1822–1911) was a major source of inspiration, not least the quantitative statistical methods that Galton had developed. The pamphlet Om Arvelighed og Variabilitet (about heritability and variability), published in 1896, testifies to Johannsen’s rather advanced understanding of the central issues in this then rapidly developing field of research. He was well informed on the latest developments in cytology as is apparent from the textbook Almindelig Botanik (General Botany) co-authored with Warming. Johannsen’s contribution includes among other things four pages on “The law of segregation” where he discusses dominance and segregation in hybridization, referring to Mendel, Millardet, De Vries, Correns, and Tschermak (Warming and Johannsen 1900: 679–683).[3]

Johannsen’s little monograph on the hereditary effect of selection in pure lines of beans was published in Danish in February 1903, and later that year in German translation, Über Erblichkeit in Populationen und reinen Linien. Ein Beitrag zur Beleuchtung schwebender Selektionsfragen. The popular scientific paper “Om Darwinismen, set fra Arvelighedslærens Standpunkt”, published in the June (?) 1903 issue of the monthly journal Tilskueren (The Spectator), presents Johannsen’s general view of evolution and heredity just as he was publishing this classical contribution to genetic science. The paper surveys the scientific status of four main theories of the evolution of species, or more precisely four theories about the hereditary variation that provides the material for natural selection: 1) the modification theory, 2) the mutation theory, 3) the theory of successive selection, 4) the hybridization theory. These theories represent four different types of causation which are not mutually exclusive. His primary focus is the mutation theory, and he finds that its claim to playing a major role in natural selection is much strengthened by his new experimental results.

The modification theory (neo-Lamarckism) is discussed at length. Johannsen finds much of the evidence for this theory and its claims to be untenable or weak. His old colleague and patron Eugenius Warming did not appreciate this criticism and as a consequence, their friendship cooled somewhat. In spite of this critical attitude, however, Johannsen continued to take very seriously the possibility of a significant role for modification in the evolution of species. This was a widespread view, especially in German biology, and it received extensive treatment in his later publications. The main target of Johannsen’s criticism was the third of these theories — that of successive selection. He took the trouble to emphasize the great service of the biometricians in developing precise statistical methods for measuring hereditary variation in populations. But having acknowledged that such methods are essential in testing whether hereditary variation and change is discontinuous or continuous, he then went on to show how it is precisely the use of such statistical methods that demonstrates the falsity of the biometric view that hereditary variation is continuous in degree as well as through time.

Johannsen’s presentation of the hybridization theory referred to the two volumes of Hugo de Vries’s Die Mutationstheorie (1900–1903). No doubt hybridization can produce new stable species, wrote Johannsen, but how often this happens in Nature is an open question; it appears, however, that mutation and hybridization “are so closely related that that they can be unified into one, the theory of stepwise change in types.” And he went on to add that “The steps can be fairly small and close to continuity for that matter”. Johannsen’s general view of the evolutionary mechanism was thus quite similar to that of the neo-Darwinism developed a few decades later.

Johannsen’s 1903 monograph is dedicated to “The creator of the exact science of heredity Francis Galton F.R.S. in veneration and gratitude”. This was primarily a tribute to Galton’s quantitative statistical methods. But Johannsen also appreciated his open attitude toward mutation theory. Johannsen came to see Galton’s idea of the stirp, an underlying entity which was transmitted in stable form from one generation to the next, and August Weisman’s (1834–1911) theory of the germ-line, as precursors to his own genotype theory. In 1903 it is the development of Galton’s ideas by the mathematician Karl Pearson (1857–1936) and biologist Frank Raphael Weldon (1860–1906) which is Johannsen’s target. He finds their insistence on prioritising mathematical analysis whilst neglecting adequate biological theory and classification to be highly misleading. However, the sharp controversy that developed between Johannsen and Pearson/Weldon did not represent a general conflict between Johannsen and the Biometricians. George Udny Yule (1871–1951) was a prominent mathematical statistician and collaborator of Pearson who defended Johannsen against unfair and misconceived criticism, in return for which he received Pearson’s scorn

Johannsen’s 1903 article in Tilskueren presents the broader scientific context and background to his famous bean selection experiment, as he saw it himself at the time. Among other things the paper shows just how much his idea of inherited biological types, which was to be the starting point of his genotype theory, owes to ongoing discussions about taxonomy, systematics, and reproduction in the field of botany. At this time, the traditional Linnean concept of species was being undermined by the efforts of botanists to subdivide taxonomic systematization into ever more refined categories such as subspecies, and varieties. One question was the existence of some kind of minimum taxonomic unit, an elementary or smallest species. According to Johannsen the mutation theory of Hugo de Vries assumed a stepwise “intermittent appearance of new small species” (translation, p. 7). Weldon’s criticism of de Vries’ mutation theory (Weldon 1902) provided Johannsen with a starting point. In the bean selection experiment, he used the statistical approach of the biometricians to make a demanding test of de Vries’ theory and found it confirmed.

In July 1903 Johannsen sent copies of his booklet, Über Erblichkeit in Populationen und reinen Linien, to Galton, de Vries, Yule and Pearson. Galton replied courteously that he was flattered by the dedication of the pamphlet to himself. He hoped that “the original observ. will be accessible to qualified inquirers”, and suggested that Johannsen had not correctly understood the biometric theory of regression: “Pearson’s wider generalisations are the true conceptions, but frightfully tedious to work at arithmetically, a hard reading mathematically”. De Vries was happy for the support to his mutation theory and for the clarification of Galton’s law of regression, which had always given him so much trouble. It appeared to de Vries that Johannsen’s pure lines were “nothing else” than his own elementary species (‘elementare Arten’). Yule answered politely that “I am waiting in keenest interest for the successive generations of your beans”. He thought it “an exceedingly difficult matter to devise a real experimentum crucis as to continuity or discontinuity of variation [in germ-plasma structure] […] continuity of variations seems to me more likely and probable, and therefore in the absence of strong proof I assume continuity — that is my ‘working hypothesis’” (Müller 1971).

Yule expressed his high appreciation of Johannsen’s experimental work as well as his skepticism toward the mutation theory in a thorough review (Yule 1904b). And when, in 1906, Johannsen presented the results which showed that continuing the selection within pure lines for three more generations confirmed his theory, Yule was still not convinced.[4] Pearson’s review of Johannsen’s 1903 pamphlet dismissed his experiment as completely faulty due to lack of statistical competence. As far as Pearson and Weldon could see, Johannsen’s results confirmed the biometricians’ view rather than the mutation theory (Pearson 1903, Weldon 1903). And as mentioned previously, Pearson responded with scorn when Yule pointed out that he had misunderstood Johannsen’s theoretical view (Yule 1904a, Pearson 1904).[5] In essence Pearson and Weldon neglected Johannsen’s idea of a fundamental distinction between biological type and concrete individual appearance and its decisive role in his analysis of the experimental material. Johannsen’s genotype theory with the distinction between genotype and phenotype was more fully developed in his classical treatise Elemente der Exakten Erblichkeitslehre (Johannsen 1909).

Half a century ago Leslie Dunn, American geneticist and leftist activist in the politics of science, characterized Johannsen as a bridge between nineteenth and twentieth century biology — “a bridge over which nineteenth-century ideas of heredity and evolution passed to be incorporated, after critical purging into modern genetics and evolutionary biology” (Dunn 1973). Staffan Müller-Wille and Marsha Richmond have recently argued for the very important contributions of William Bateson and Wilhelm Johannsen as two “’co-rediscoverers’ of Mendel’s laws” in the founding of classical genetics (Müller-Wille and Richmond 2016). One was a zoologist and the other a botanist, and although their scientific experience and theoretical outlook differed in a number of respects, they nevertheless both had deep roots in nineteenth-century biology. A better understanding of the impulses and assumptions coming from classical theorizing of sub-disciplines like natural history, taxonomy, systematics and ecology can improve greatly our understanding of the origins of classical genetics. To this end, Johannsen’s popular article of 1903 is a very valuable source, as it highlights the role of classical botanical theory in his thinking.[6] Johannsen developed a strong interest in the history of biology and linked his own genotype to the Aristotelian concept of form. In recent years our current concept of “the gene” as a particulate unit of heredity and genetic science has come under scrutiny (Rheinberger and Müller-Wille 2017), and as it was Johannsen who first coined this term, it may therefore be an opportune time to take a closer look at his continuing criticism of the gene concept that came to dominate the chromosome theory (Johannsen 1923, Moss 2003, Roll-Hansen 2014).

[1] Such an autobiography was a requirement when he was decorated with the knight’s cross of the Royal Dannebrog Order in 1909. The document written in Johannsen’s own hand is kept in the archive of the order.

[2] The Danish word ‘smaating’ also has the connotation of ‘trifles’.

[3] The fourth edition of this book is from 1900, but its preface was dated June 1901.

[4] See discussion section following Johannsen 1907.

[5] For more information on Johannsen’s genotype theory and on his controversy with the Biometricians see Roll-Hansen 1978, 2009.

[6] The enduring influence of Linnean theory of species in the work of Mendel is thoroughly discussed by Müller-Wille and Orel (2007) suggesting a continuation well through the rediscovery of Mendel in 1900. Closer attention to the formative influence of Johannsen’s experiments and theories can be a useful antidote to the constricting influence of the modern synthesis and neo-Darwinism on the history of genetics (Müller-Wille 2007).


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  • Dunn, Leslie C. (1973). “Johannsen, Wilhelm Ludvig.” Dictionary of Scientific Biographies, 7 (New York 1973), p. 115.
  • Johannsen, Wilhelm (1903). Über Erblichkeit in Populationen on reinen Linien. Ein Beitrag zur Beleuchtung schwebender Selektionsfragen. [On heredity in pure lines and populations. A contribution to pending questions of selection]. Jena: Gustav Fischer.
  • Johannsen, Wilhelm (1907). “Does hybridisation increase fluctuating variability?“ Report on the Third International Conference on Genetics, London, Spottiswood 1907. Pp. 98–111 & 112–113.
  • Johannsen, Wilhelm (1911). “The Genotype Conception of Heredity.” American Naturalist, 45: 129–159.
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Translation by Nils Roll-Hansen

The translation can be downloaded as PDF here (and the original article as well), or otherwise you can read it here: