August Weismann

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(b. Frankfurt am Main, Germany, 17 January 1834; d. Freiburg im Breisgau, Germany, 5 November 1914), zoology.

Weismann’s most influential contribution to biological thought was his theory of the continuity of the germ plasm, an explanation of heredity and development. He maintained that the germ plasm, the substance of heredity, was transmitted from generation to generation, distinguishing it from the somatoplasm; he also was foremost in his day in denying that acquired characters were inherited. keeping abreast of current researches on the cell and the growing understanding of the role of the nucleus and the chromosomes in inheritance, Weismann modified and develoed the theory of germinal continutiy. Cytology repeatedly confirmed phenomena the existence of which he had presumed from theoretical coniderations. He was a storng defender of Darwin’s theory of evolution and a leader among Neo-Darwinists arguing for the sufficiency of natural selection.

Weismann was the son of Johann Konrad August Weismann, a classics professor at the Gymnasium in Frankfurt, and Elise Eleanore Lübbren, a talented musician and painter, who understood her son’s love of nature. At an early age Weismann gathered butterflies adn beetles, bred caterpillars, and assembled an impressive herbarium of the plants in the vicinity of Frankfurt; his pleasure in his butterfly collections and in music was lifelong. He attended the a Gymnasium at Frankfurt, and then–despite his preference for physics, chemitry, and botany, and his hopes perhaps to becomme a chemist–he began the study of medicine at the University of Göttingen; Friedrich Wöhler, a family friend, advised that Weismann take up medicine first, and his father thought he ought to have a practical means of earning a living. Among his professors at Göttingen were Wöhler, Siebold, Lotze, and Henle,who was an especially stimulating teacher to Weismann and one who was wary of the speculation that characterized Naturphilosophie. In 1856 Weismann received his medical degree with a disertation on the formation of hippuric acid in the organism. When he looked back at his education, he felt that detailed research had been emphasized, with little attempt to interrelate the facts and subject matter of the various disciplines, or to deal with broader problems. He was there fore particulary sensitive to the impact of Darwin’s Origin of species, which he first read at one sitting in 1861. Thereafter he was a proponent of Darwin’s theory of evolution by means of nwtural selection, and like Fritz Müller and Ernst Haeckel, through his classes and writings, as well as his lectures to his classes, he was able to draw the theory of descent to the attention of the public.

After graduation Weismann continued his researches, first while he served as an assistant in a clinical hospital in Rostock, then as private assistant to Schulze. A paper on hippuric acid won him a prize at Rostock, as did a chemical invetigation of the salt content of the water of the Baltic Sea. He visited Vienna, and late in 1858 he took his examinations and entered the practice of medicine in his native Frankfurt. Medicine did not take up all of Weismann’s time, for he now turned to histological investigations stemming from his association with Henle at Göttingen and studied the minute structure of the muscle fibers of the heart. During the summer of 1859 he was a field doctor in Italy but soon returned to zoology, visiting the Jardin des Plantes in Paris, and hearing Geoffroy Saint-Hilaire lecture. Early in 1861 he spent two months at the University of Giessen under Leuckart, who, even during this brief period, exerted a strong influence upon Weismann. It was Leuckart who interested the young physician in developmental studies of insect, the Diptera in particular.

Weismann again practiced medicine for a short while in Frankfurt. Then he served from 1861 to 1863 as the private physician of the Archduke Stephan of Austria at Schaumburg Castle, and meanwhile studied insect development and metamorphosis and pursued researches in his spare time. Then Weismann gave his full attention to zoology. Strongly attracted to the beauty of Freiburg, he habilitated at its university in 1863 and tauhgt comparative anatomy and zoology as a privatdozent. Weismann wsa appointed extraordinary professor in 1866 and full professor in 1874. The first to hold the chair in Zoology, he spent the rest of his career at the University of Freiburg. In 1864 his eyesight became seriously affected, but he continued his work. From 1862 to 1866 six memoirs on insects and another memoir on heart muscle appeared, even when he himself could not use the microscope. Weismann was helped immeadurably by his wife, marie Dorothea Gruber of Genoa, whom he had met during his 1859 stay in Italy. She read to him and aided in countless ways over the years. They had six children and the family shared many interest: music was a part of their life, and their son became musician and composer.

It was ten years before Weismann could again use his eyes to make his own observations with the microscope, although a sojourn in Italy from 1869 to 1871, during which his family accompanied him, proved beneficial to his eyesight. Throughout this period and afterward, he was well informed, how ever, and his lectures continued, although perhaps he was forced more to theorize and to delegate certain researches to others. From insects and their embryology he turned to the small crustaceans, daphinids and ostracods; and with the improvement of his eyesight a series of publications appeared. He then took up the examination of the Hydrozoa and folloed the origin of the sexual cells through generations of Hydromedusae. The culmination of this work was Die Entste lu ng der Sexual:ellen bei den Hydromedusen (Jena. 1883), and his conclusions, as he traced the fate of the primitive germ cells and those after them, were strong evidence to Weismann that there was a continuity of the germ plasm. When in 1884 his eyesight was again severely limited, his students and other assistants shared his work, his wife helped him, and the research continued under his guidance. Weismann approached his theoretical determinations from an evolutionary standpoint, but he correlated his theories with cytological investigations, and was always alert to the interpretations and arguments of his colleagues. He was not rigid, but, heeding the growing knowledge of the cell, was willing to develop and change his ideas.

In the intervening years Weismann had given the Darwinian theory of evolution his full support, and he called his inaugural lecture, in “justification” of Darwin’s ideas, Über die Berechtigung der Darwin’schen Theorie(Leipzig, 1868), a “kind of confession of faith,” when the trouble with his eyesight restricted his work, Weismann gave Darwin’s theory more of his attention, and in his Studien zur Descendenztheorie(Leipzig, 1875-1876)–with a preface by Darwin himself ion the English translation (London, 1882)–Weismann treated the seasonal dimorphism of butterflies and questions of evolution and herdity. Although he remained one of the foremost defenders of the Darwinian theory of evolution through natural selection, Weismann —a strict selectionist, more so indeed then Darwin—proceeded to construct his own theory of heredity rather than accept Darwin’s hypothesis of panfenesis.

Weismann had come to an early conclusion that the “direction of development,” the same as that of the parent, was transferred by means of the protoplasm of the sperm and of the egg cell (1868). Studying the Diptera, he found that the sexual glands had originally been derived from pole cells that were fromed by segmentation of the egg. But most crucial to him was the evidence of the Hydromedusae for a continuity of the germ plasm.

Through evolutionary considerations and in probing the question as to whether acquired characters could be inherited, Weismann came to develop his theory of the germ plasm. He decided that acquired characters could not be inherted, for to become inheritable, changes would have to affect the germ plasm itself. Weismann was not the first to conceive of a continuity of the substance of heredity. Galton had outlined it (1872); and Gutav Jäger had actually written of a “continuity of the germ-protoplasm” in 1875, although this was at the time unknown to Weimann. Moritz Nussbaum and August A. Rauber later claimed to have originated the theory; still, it is Weismann with whom the theory of the continuity of the germ plasm is associated, for he first developed it into a coherent explanation of inheritance and brought it into agreement with the new understanding of the cell. While the phenomena of the cell, and specifically of the chromoomes, were being followed in the laboratory, he modified his theory, and in later years it was a presupposition of his views on the sources of variation in evolution.

In 1881 Weismann gave a lecture(“The Duration of Life”) before the Deutsche Naturforscher Gesammlung. In the lecture he contrasted the “immortality” of one-celled organisms, which reproduce by division to form two organisms of the same age—this potential immortality was, of course, abrogated by accidents and other vicissitudes— with the division of labor that natural selection had brought about in the more complex forms of life. In the latter forms there was an early separation of the elements that were to form the “immortal” reproductive cells from the elements that were to form the body cells that perished in each succeeding generation. In “On Heredity” (1883) Weismann still conceived of the germ cells as containing configurations of molecules that led to the reproductive cells, as well as other configurations for the somatic cells. The germ cells contained the Arda,ten–the concept of Anlagen was current then, and to Weismann, these were hereditary tendencied or predispositions for certain characters to develop. The Anlagen were not affected by the outer conditions that affected the organism, but they were subject to natural selection. Weismann was still dealing with heredity in terms of the entire cell.

In the following years Weismann’s concept of the germ plasm changed, and he developed his theory further. He borrowed the term “idioplasm” from the botanist Naegeli, who in 1884 described a continuity of the idioplasm, the protoplasm concerned with inheritance (as distinguished from the rest of the protoplasm) and thereby accounted also for variation. The idioplasm that Naegeli described had a structure of parallel orws of “micelles,” which sometimes branched, and formed a network that coursed through the body; cell structure bore little importance in his theory. Since he did not subscribe to the structure Naegeli claimed for the idioplasm, Weismann continued to develop his own concept, but kept in mind the cytological researches then under way.

Following the independent observations (1873) of cell division–and Schneider’s illustrations of the stages of division–the previous decade had brought many new discoveries dealing with the cell in division. In 1875 Oscar Hertwig had seen the appartent fusion of the nuclei during fertilization; and in 1879 Walther Flemming had clearly shown that the threads seen in the nucleus split longitudinally, and had pointed to the possible significance of this segmentation. Wilhelm Roux’s conclusions influenced Weismann especially; Roux remarked how complicated the process of mitosis was: It seemed to be the means of accomplishing more than a division of the quantity of the substance of the nucleus between the two resulting daughter cells; the mechanism could divide the qualities equally between the new cells that were formed in division. Roux went on to propose that after the first equal segmentation there might possibly be unequal divisions in which the quantity might be the same in each cell, but the distribution of the qualities unequal.

In 1884 several scientists including Weismann independently came to attribute the main role in heredity to the nucleus (as Haeckel had proposed for theoretical reasons in 1866). After studying plant cells and following the reproduction of an-giosperms, Strasburger arrived at this conclusion. Shortly thereafter Hertwig published a paper on his researches (Weismann had reached similar views before he read it). Koellker decided also that the nucleus was involved in inhritance, al-though he later criticized other tenets of Weismann’s theory.

In the essay “The continuioty of the Germ-Plasm” (1885) Weismann reflected upon his new views, for he now located the germ plasm more precisely within the nucleus, and he took up other questions as well. He still had some reservations as to whether the idioplasm was definitely to be identified with the portions of the nuclear filaments that took up chromatin stain, although he had decided that the nucleus indeed contained the idioplasm of Naegeli. Weismann made his stand on the germ plasm clear: Germ cells did not necessarily lead directly to other germ cells, forgerm plasm might be transmitted through a series of cells—its particles remaining discrete—before reproductive cells were again formed .

Weismann was already attempting to find the significance of the process of meiosis that his colleague Edouard Van Beneden had been investigating. He was dissatisfied with the interpretations of Van Beneden, Minot, and Balfour of the apparent casting-out from the egg of som of the hereditary material, for he did not agree that the egg cell was hermaphroditic, shedding the male element preparatory to fertillzation. Weismann struggled to solve the problem. At first he maintained that in the maturation of the egg the formation of the polar bodies separted the germ plasm from a “hitogenic” plasm in the nucleus, and that this resulated in a necessary preponderance of germ plasm in the reproductive cell. But by 1887 he had decided instead that this was a means ofpreventing the indefinite increase of ancestral plasms, which each fertilization would otherwise double.

Weismann believed that sexual reproduction led to variation, through the ever-new combinations of Anlagen. In the essay “On the Number of Polar Bodies” (1887) he reported on the researches he had undertaken, aided by his pupil and collaborator C. Ishikawa (who later became professor of Zoology at Tokyo), when his sight was again severely affected. Although Weismann has for some time assumed that the hereditary qualities were linearly arranged along the “nuclear loops,” as he described the chromosomes, and that differentiation occurred by means of qualitative divisions, such as Roux had proposed, he inferred—on theoretical grounds–that there must be a reduction division. He described the mounting complexity of the ancestral plasms, and it was his theory that there were to kinds of division: an equal division or Aequations-theilung, whereby the nuclear threads in splitting longitudinally divided the germ palsm equally; and a “reduction division,” Reduktions-theilung, with half of the loops simply distributed to each ofthe daughter nuclei. Not only did Weismann theorize that reduction division takes place, but he suggested also that a similar process to that in the maturation of the ovum could occur during spermatogenesis; this was confirmed by cytological investigation.

Meanwhile, Weismann’s interest in the problem of the possibility of the inheritance of acquired characters persisted, and he experimented to determine whether environmentally produced changes were inherited. He followed the results of mutilations, cutting off the tails of hundreds of mice, but without any apparent hereditary effects.

Weismann was aware of the attention the threadlike structures in the nucleus—first called chromosomes in 1888 by Waldeyer—were receiving. They were frequently described as rods or loops and seemed to be present in characteristic numbers in certain organisms and tissues. Rabl remarked that the chromosomes must persist throughout the various changes during cell division (1885), and in 1887 he advanced the concept of chromosomal individuality, which was basic to chromosome theory, and which Boveri brought to clearer development and a more definitive statement in the course of his cytological studies. In 1889 de Vries outlined his theory of “intracellular pangenesis” and his concept of the “pangen,” an ivisibly small vital unit concerned in heredity; this affected Weismann’ thinking on the germ plasm, although they disagreed on important points (such as the necessity of a reduction division).

Comparable to de Vries’s pangenes were Weismann’s “biophors,” as hedeveloped his theory of the germ plasm. The biophors, however, did not mix freely as did the hereditary factors that de Vries described. Weismann then postulated the existence of larger, although still submicroscopic units, the “determinats.” Weismann then proceeded to describe the progression of units that were the “bearers of heredity.” In accordance with the appearance of the visible chromosome. there were next the “ids,” made up of determinants (the ancestral plasms of his germ plasm theory and linked together in definite groupings). The ids were the disks or microsomes that had been observed beneath the microscope and were linearly arranged on the chromosomes. in Weismann’s terms, the “idants.” Weismann thought that each id carried the entirety of the ancestral plasms that the individual inherited. Weismann gave his theory with his views of the significance of the laboratory investigations of the cell and of contemporary hypotheses. in Das Keimplasma. Eine Theorie der Vererbung (Jena, 1892; English translation, The Germ-plasm [London, 1893]). The theory of the continuity of the germ plasm was now more completely developed and formulated in terms of the cell theory, although necessarily in his own interpretation, for phenomena that were being observed remained to be more clearly understood.

Through these units Weismann also explained the process of differentiation during embryogeny. He continued to maintain the the existence of the eternal germ plasm and the mortal somatoplasm, but, in addition, described a sort of breakdown of the complexes of the hereditary units by which specific determinants eventually direct development in the cells concerned. This was predicated on Weismann’s belief, based on Roux’s opinions years before, that both equal and unequal divisions were possible. But Weismann later made some minor compromises in his theory when eventually he allowed that some changes might occur in the germ plasm itself, although he continued to think in terms of variation, natural selection, and a struggle for existence that took place on various levels.

Weismann’s theories, elicited praise from some his contemporaries, criticism from others. Whatever his interpretation of the actual mechanism of the processes he described (the reduction division, for example, which drew much further research and dispute in the 1890’s), Weismann brought important questions to active discussion and was especially influential thought his stress on chromosome theory and his repudiation of the inheritance of acquired characters. No doubt, too, many of the disagreements in his day were due to the different materials and life histories that were investigated by botanists and zoologists. There seemed to be too many difficulties and exceptions for broad theories to explain them in a way that was completely acceptable. To Oscar Hertwig, for example, Weismann’s theory smacked of preformation, and questions of the importance of the relationship between the individual and the environment in determining the direction of development arose.

Early in his career, in advocating the Darwinian theory in Germany, Weismann had joined its most famous popularizers, Müller and Haeckel; but each of the three overstepped the bounds of Darwin’s own propositions. According to Darwin’s theory, evolution results from the natural selection of heritable, favorable variations that are always occurring by chance. New varieties grow up, and, in time, species are formed by this means. But even Darwin did not rule out some effect of external conditions and use and disuse; in his later years he granted a greater role to these factors than he had in the Origin of Species in 1859.

Even as natural selection drew wide attention. naturalists who applied it to specific instances in their experiences and investigations found difficulties and came to carious conclusions. Opinions divided on the way in which evolution actually takes place, and differed as to the sources of variation and their relative importance: on whether external changes and acquired characters play a part in inherited variation; and on the role of slow, cumulative change as opposed to saltatory changes. Even if natural selection were accepted, it was difficult to explain the degeneration and loss of organs, and to account for continuing changes below the level at which natural selection could be expected to act at all.

Weismann, who was the most notable of the Neo-Darwinists, claimed that natural selection alone could provide for the formation of varieties and, in time, species, although the Neo-Lamarckians found support on Darwin’s own admitted doubts as to the degree to which natural selection alone acted. In addition, there were other variations of the Lamarckian or Darwinian positions, and some naturalists departed from both.

In 1893 and 1894 the Contemporary Review carried Weismann’s controversy with Herbert Spencer. Weismann contended that natural selection acting on innate variation was the sole factor in establishing varieties and species, and that acquired characters were not transmitted, and therefore were not a source of evolutionary change; Spencer took a Lamarckian view. Weismann’s own experiments supported his arguments, and he could point also to the general lack of proof for such inheritance. He became widely known for the stand he took; indeed, it was cited in his day as on eof his great contributions to biology.

In his 1894 Romanes lecture, “The Effect of External Influences Upon Development,” Weismann dealt with the difficulties of explaining by means of the “all sufficient principle” of natural selection not only the progressive development of some variations, but degenerative changes and the disappearance of useless organs. He maintained that heritable peculiarities occurred among the biophors and cells, and that the peculiarities were actually variations in the primary cell constituents and might be acted upon by natural selection.

Weismann believed that natural selection was adequate even beyond Darwin’s hopes—in the light of further researches—but he found it necessary to fortify the theory with some additional, although subsidiary, explanations. Indeed, Weismann was forced to support his arguments. for natural selection, first with his theory of “panmixia,” then with his further theory of “germinal selection.” He thereby extended what he had come to refer to as the “Darwin-Wallace principle of natural selection” ; but both panmixia and germinal selection were also based upon Weismann’s own theory of the germ plasm, for he took for granted the theory of idants, ids, determinants, and the smallest units, biophors.

In defending the Darwinian theory, dealing with apparent difficulties or omissions, Weismann “extended” it and expressed his concepts in new terms. He had maintained that sexual reproduction, or to use his term, “amphimixis,” was the source of variation; but he also found that he had to explain the disappearance of organs. Arguing that external influences could act only in a selection way on changes that had already occurred in the germ plasm, he ascribed regressive changes to variations that occurred in the primary constituents. But the problem remained. To account for the degeneration and ultimate disappearance of organs or parts, Weismann modified the Darwinian concept of natural selection with his own theory of “panmixia.” Resisting Lamarckism, and trying to broaden the usefulness of the theory of natural selection, Weismann now inferred that natural selection not only brings about the development of the part but must actively cause it to be retained. Unless selection continued, the organ that lost its usefulness or specialized importance and became disused would tend to diminish and, in time, to disappear.

But how to explain its complete disappearance? To Weismann, panmixia was still not satisfactory, and his views changed as he looked next to what he called “germinal selection” as a further auxiliary to the selection theory. He went quite beyond Darwin’s original account of the way in which natral selection acts and sought the selections process even in the germ plasm itself. Weismann came to see selection as taking place on many levels and in terms of his own later description of the hereditary units. There were fluctuations in nourishment and in conditions of life even among the biophors and determinants, and natural selection determined their chance to reach development and expression in the individual. Thus Weismann had come to conceive of a struggle of parts within the germ plasm. This enabled him to continue to deny that there was a direct effect upon heredity, through external conditions, and to insist that, although external causes might eventually lead to variations, this would be through selections acting on the variations that occurred in the internal and innate hereditary tendencies and within the germ plasm itself. Nevertheless, it was quite a change of view for Weismann to admit that internal causes could intensify variational tendencies; and that these were “simply accumulated by natural selection in an evergrowing majority of ids in a germ-plasm through the selection of individuals” (The Evolution Theory, II, 332). It was almost impossible at the time for Weismann to separate his considerations of evolution and heredity. De Vries’s mutation theory made it necessary for Weismann to define variation more completely in terms of natural selection, and he concluded that no lines could be drawn between variation and mutation, and that ultimately the difference was a matter of the number of similarly varying ids.

For decades Weismann drew a constant stream of students of his laboratory, and they returned to foreign countries with descriptions of his laboratory, reporting him as a teacher of impressive stature. Many students had been drawn to him through hearing of his views; one of his prominent pupils recalled his desire to go to study under Weismann and to hear his theories after having been well schooled by the opposition. At the University of Freiburg, Weismann saw the Zoological Institute, of which he was the director, installed in a new building with the expansion of the facilities.

Weismann’s wife died in 1886, and his second marriage at the age of sixty to Willemina Tesse of Holland lasted but six years. His five surviving children. often with him during summers at his home at Lake Constance, became his greatest comfort. One of his daughters, Hedwig. married the English zoologist W. Newton Parker, who translated Das Keimplasma (with Harriet Rönnefeldt). After retiring in 1912 as professor emeritus, Weismann gave his attention to the third edition of his Vorträge über Descendenztheorie (1913), which provides an overview of his ideas in an era in which cytology had developed as a science, Mendel’s work had become known, and questions of evolution and heredity had been the subject of research and controversy. Following the outbreak of Word War I, Weismann was ill and deeply unhappy. with members of his family on both sides during the conflict.

Weismann belonged to the Bavarian Academy of Sciences and was a corresponding member of the Academy at Vienna. He was a foreign member of the Linnean Society and the Royal Society of London, and of the American Philosophical Society.

Weismann traveled throughout Europe, and once to Constantinople. He was often in Italy and on the Riviera, and his work on the Hydromedusae was largely the result of a stay at the Naples zoological station during the winter of 1881-1882. Weismann took three trips to England but he never met Darwin, although they corresponded.

In Germany the government designated Weismann Wirklicher Geheimer Rat, and the king of Bavaria bestowed the Grand Cross of the Zähringer Löwensorden. The University of Freiburg gave him the ph. D. honoris causa in 1879. The University of Oxford made him doctor of common law, and the University of Utrecht made him doctor of botany. He received wide recognition when biologists from many countries honored him at the celebration of his seventieth birthday and a bust was given to the Zoological Institute at Freiburg, while the Zoologische Jahrbücher marked the anniversary with a Festschrift.

Weismann’s influence was well summarized in the citations he received in London in 1908. Accompanying the medal awarded him bythe Linnean Society at the Darwin-Wallace celebration (which his duties at Freiburg unfortunately prevented his attending) was the following:

Professor Weismann has played a brilliant part in the development of Darwinian theory, and is indeed the protagonist of that theory in its purest form, retaining all that was the peculiar property of Darwin and Wallace and eliminating the traces of Lamarckism which still survived…his profound knowledge of cytology enabled him to base his theory of heredity on a firmer foundation of face than had been possible in the case of previous speculations.

The following citation was given him with the award of the Darwin Medal at the anniversary meeting of the Royal Society: “The fact remains that he has done more than any other man to focus attention on the mechanism of inheritance.”


I. Original Works. Weismann’s writings are comprehensively listed in the biography by Gaupp (see below). pp. 290–297. His collected essays are in Edward B. Poulton, Selmar Schönland, and Arthur E. Shipley, trans., Essays Upon Heredity and Kindred Biological Problems (Oxford, 1889); and in a two-volume 2nd ed. (Oxford, 1891-1892). For the continued development of his thory of the germ plasm, see Das Keimplasma or its translation, noted above. See also Vorträge über Descendenztheorie (Jena, 1902, 1904, 1913), trans. by J. Arthur Thomson and Margaret R. Thoams as The Evolution Theory 2 vols. (London, 1904), which provides some account of Weismann’s change in thinking over the years. A short autobiography is Herbert Ernest Cushman, trans., “Autobiography of Professor Weismann,” in Lamp (new series of Book Buyer), n.s. 26 (1903), 21–26. For a further personal glimpse of Weismann, see his correspondence with Haeckel in Georg Uschmann and Berhard Hassenstein, “Der Briefwechsel zwischen Ernst Haeckel und August Weismann,” in Manfred Gersch, ed., Kleine Festgabe aus Anlass der hundertjährigen Wiederkebr der Gründing des Zoologischen Institutes der Friedrich-Schiller-Universität Jena im Jahre1865 durch Ernst Haeckel Haeckel (Jean, 1965), 7–68.

II. Secondary Literature. The major biography of Weismann is Ernst Gaupp, August Weismann, sein Leben und sein Werk (Jena, 1917. Among the articles on his life and work are Edward G. Conklin, “August Weismann,” in Proceedings of the American Philosophical Society, 54 (1915), iii-xii, and in Science, n.s. 41 (1915), 917–923; F. Doflein, “August Weismann,” in Münchener medizinische Wochenschrift, 61 (1914), 2308-2310; and V. Haecker, “August Weismann,” in Deusches Biographisches Jahrbuch, 1914-1916, I (Berlin, 1925), 897–103. See also R. v. Hanstein, “August Weismann,” in Naturwissenschaftliche Wochenschrift, n.s. 14 (1915), 113–120, 129–136; and the obituary by R. Hertwig, in the Jahrbuch der bayerischen Akadenmie der Wissenschaften (1915), 118–127; Prof. and Mrs. W. N. Parker, “August Friedrich Leopold Weismann,” in Proceedings of the Linnean Society of London, 129th session (1914–1915), 33–37; and for the recollections of one of Weismann’s students, Alexander Petrunkevitch, “August Weismann, Personal Reminiscences,” in Journal of the History of Medicine and Allied Sciences, 18 (1963), 20–35. Further accounts of his life include E. B. poulton, “Prof. August Weismann,” in Nature, 94 (1914), 342–343, and “August Friendrich Leopold Weismann,” in Proceedings of the Royal Society, ser. B. 89 (1916), xxvii–xxxiv: and H. E. Ziegler, “August Weismann,” in Neue Rundschau,26 (1915). 117–124.

Because Weismann’s work drew so much attention and controversy, a list of discussions of his views and contributions is necessarily incomplete; contemporary assessments include George John Romanes, An Examination of Weismannism (Chicago, 1893); Vernon L. Kellogg, Darwinism To-Day (New York-London, 1908), 45–46, 77–78, and passim, for Weismann’s position as a Neo-Darwinist and his colleagues’ stands. and his theories of panmixia and germinal selection; and Yves Delege and Marie Goldsmith, The Theories of Evolution, André Tridon, trans. (New York, 1913), 134–162. A later résumé of the course of Weismann’s work and of his theories is W. Schleip, “August Weismann’s Bedeutung für dew Entwicklung der Zoologie und allgemeinen Biologie,” in Naturwissenschaften, 22 (1934), 33–41. A miore recent evaluation of Weismann’s work in the context of the growing understanding of cytological phenomena, and his views and those of his contemporaries, is William, Coleman, “Cell, Nucleus, and Inheritance: An Historical Study,” in Proceedings of the American Philosophical Society, 109 (1965), 126, 149–154. The origins and the development by Weismann of the germ plasm theory, and its implications for him, are discussed in Frederick B. Churchill, “August Weismann and a Break From Tradition,” in Journal of the History of Biology, 1 (1968), 91–112. For Weismann’s theory of the germ plasm against the background of theories of heredity from Darwin’s pangenesis, see Gloria Robinson, A Prelude to Genetices (Lawrence, Kans., 1976). For an analysis and discussion of Weismann’s views on the problem of the reduction divion, see Frederick B. Churchill, “Hertwig, Weismann, and the Meaning of Reduction Division Circa 1890,” in Isis, 61 (1970), 429–457.

Gloria Robinson

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(b. Frankfurt am Main, Germany, 17 January 1834;

d. Freiburg im Breisgau, Germany, 5 November 1914), zoology, heredity, evolution. For the original article on Weismann, see DSB, vol. 14.

The necessary starting point for evaluating Weismann’s career remains his zoological writings. This is not a large corpus in comparison to those of some of his contemporaries, say, Charles Darwin or Ernst Haeckel. It consists, however, of professionally solid and challenging monographs and books, including Das Keimplasma(1892), that were influential in their own day and still referred to in the early twenty-first century. It also consists of many elegant essays and monographs on evolution and heredity. Finally, there exist three editions of Weismann’s comprehensive advanced textbook, Vorträge über Descendenztheorie (1902, 1904, 1913) devoted to a neo-Darwinian view of evolution. This comprised the first such modern textbook that framed an elaborate mechanistic model for evolution, heredity, and development. Most, but not all, of this literature was translated into English during Weismann’s lifetime.

The primary account of Weismann’s work remains the biography written by the Freiburg anatomist, Ernst Gaupp (1917). As a younger colleague of Weismann, Gaupp knew Weismann’s family and after Weismann’s death in 1914 had access to some of Weismann’s papers. World War I, dramatic changes in the heredity and developmental sciences during these and subsequent years, the emergence of biometrics and endocrinology, and a disciplinary shift from German universities, marine laboratories, and independent museums to American biology, which was fostered not only by the same kind of institutions but supported by agricultural programs and philanthropic foundations dramatically diminished the need of revisiting Weismann’s work. Not until the second half of the twentieth century were serious historical efforts begun to review Weismann on his own terms.

Since the publication of Gloria Robinson’s informative and judicious assessment of August Weismann in the DSB (1976) and her more ample discussion of 1979, new material and new scientific concerns have emerged that indicate how important it is to again reevaluate Weismann’s entire career. This is reflected not only by the collection of Weismann’s Selected Letters and Documents assembled by the author and his colleague Helmut Risler (1999) but by analytical essays about Weismann’s career by Ernst Mayr (1985), Rasmus Winther (2001), Ida Stamhuis (2003), and the author. Above all, a conference held in Freiburg in 1984 (Sander, 1985) with contributions by eighteen European and American scholars has demonstrated how complex and multifaceted Weismann’s zoology had been at a time when general zoology was coming into its own as an independent field of research at German universities. It is also clear that Weismann’s contributions continue to be invoked by twenty-first century biologists in multiple ways to justify or denigrate positions that have not been in keeping with the spirit of Weismann’s work and ideas. (See Berrill and Liu [1948], Blacher [1971], Gould [1977], Buss [1987], and Griesemer and Wimsatt [1989].) This entry does not attempt to duplicate or replace what Robinson has already presented in her original article, but surveys some of the historio-graphical advances that have been made since that was written.

Relationship to Haeckel. One important question that is much clearer now is Weismann’s relationship with Ernst Haeckel. The publication of what appears to remain of the Haeckel-Weismann correspondence, cited but not closely examined by Robinson, has made this possible (Uschmann and Hassenstein, 1965; see bibliography in original article for citation.). Born only a month apart in early 1834, the two zoologists became identified toward the end of their careers as the foremost evolutionists of the immediate post- Origin generations. Both read and were captivated by Darwin’s Origin when it appeared in the Heinrich Georg Bronn translation. Both shared a belief that the evolution of life must be explained in mechanical terms. Both soon sensed the importance of the germ-layer theory for providing a mechanism for explaining development and phylogeny.

Weismann encouraged Haeckel in his early publications, especially his study of calcareous sponges and the development of his biogenetic law. Haeckel sympathized with Weismann’s devastating retinal affliction, which for the ten years between 1864–1874 and again after 1884 curtailed his microscopic research. Furthermore, Haeckel assisted his friend, as the latter returned to study and write major monographs on cladocerans and hydroids. Both spent their entire careers at small universities at the periphery of German academic life, and both founded and turned their zoological institutes into major research and teaching centers for developmental and evolutionary biology. They shared information about and specimens of organisms and about the details of the construction of their respective new institutes. They celebrated each other’s birthdays and the birth of children; they consoled the other when each lost his first wife.

Notwithstanding the warm scientific and personal ties, the research and commitments of these two Darwinian warhorses diverged. Never a proponent of the Lamarckian mechanism of the inheritance of acquired characters, Weismann found by the early 1880s in his demonstration of a continuity of germplasm in hydroids and in its conjectured segregation from the differentiating cells of the soma an a priori argument against such inheritance. As a consequence natural selection appeared to him the “all sufficient” mechanism for evolution. Haeckel to the contrary became ever more supportive of the inheritance of acquired traits, a process that he had redesignated progressive inheritance.

Added to this fundamental theoretical difference was a divergence in research styles. At first both zoologists had approached evolution as natural historians and descriptive embryologists. Continuing within this framework, Haeckel constructed further taxonomies and proposed ever more phylogenies. He occasionally supported his conclusions with the microscopy of germ-layer formation.

During the 1870s and 1880s, however, morphology was undergoing fundamental changes. By tracing the formation and lineages of germ cells, Weismann refocused his attention on questions of fertilization and germ-cell maturation. He was soon swept up by the new wave of nuclear cytology promoted by the Hertwig brothers, Otto Bütschli, Walther Flemming, Eduard Strasburger, Edouard van Beneden, and many more, most of them younger biologists. From the early 1880s till the end of the century, and with the assistance of advanced students,

such as Eugen Korscheldt, Ernst Ziegler, Chiyomatsu Ishikawa, and Valentin Haecker, Weismann expanded his institute to include the descriptive study of polar bodies and their chromosomes. These latter structures, mobile and complex, provided an empirical foundation upon which Weismann constructed his germ plasm theory.

Haeckel could not tolerate either this plunge into the mysteries of cellular mechanics or a provisional explanation for evolution that seemed to undercut the central causal role of his biogenetic law and the need for progressive inheritance. He publicly attacked Weismann’s germplasm theory, but shortly thereafter sent Weismann a letter of reconciliation. The two continued to exchange birthday greetings for the rest of Weismann’s life, and agreed to disagree on fundamental principles. When in 1908 Haeckel was attacked by the conservative Protestant Keplerbund for misrepresenting many of his illustrations, Weismann, along with other biologists, signed the so-called Leipzig Declaration supporting his friend.

Importance of Experiment. Another side of Weismann’s science to receive increased attention concerns his use of experimentation to explore and explain natural phenomena. This is particularly evident in his study of seasonal and geographical dimorphism in butterflies where there is often a dramatic difference in the color and patterns in the wings of spring and summer broods. These phenomena could plausibly be explained by differences in the temperature of the seasons and locations, and the Austrian naturalist Georg Dorfmeister had already published an account of exposing butterfly larvae and pupae to abnormal temperatures in the effort to induce variations. His concerns, however, were simply taxonomic.

Weismann began his experiments in 1866, before learning of Dorfmeister’s results, and it is clear from the outset that he was concerned about the evolutionary implication of temperature-induced changes. He carried out his first set of experiments at a time when he could not use the microscope because of his eye illness. Instead he used ice chests and cold rooms and traveled to Sardinia to study southern populations. The work enabled him to explore the value of natural cyclical patterns in alternating generations. In the end he recognized that the spring broods were more stable than the summer, and this in turn led him to assume that they expressed an older and more stable constitution. Newer colors and patterns, he explained, reflected warmer temperatures associated with a general warming after the last ice age.

He picked up temperature experiments again at a time when he was developing his germplasm theory. They helped him find ways of confirming what became known as parallel induction of soma and germplasm without resorting to a Lamarckian-type inheritance. In both of these periods of experimentation Weismann showed an awareness of the importance of a large number of tested organisms and of sets of controls taken from the same brood, but his statistical analysis remained simply a comparison of averages. Similar designs characterized Weismann and Marie von Chauvin’s experiments on axolotl and the three years of studies he and his students carried out in collaboration with the apiarist Ferdinand Dickel in 1897 to 1900 to determine whether drone eggs did, in fact, develop parthenogenetically.

Concept of Germplasm. When Weismann introduced the term amphimixis in 1891, he was building on two decades of research in nuclear cytology that demonstrated that the process of fertilization in metazoans and plants and of conjugation in protozoans and other protists, “has no significance except the union in the single offspring of the hereditary substance from two individuals” (1891 [1892, vol. 2, p. 113]). It is this “union” of the chromosomal constitutions of the germ cells of two individuals that inspired Weismann to fashion his notion of the germplasm into a complex model of nested units. These, in turn, accounted for reduction division, transmission, parental and ancestral inheritance, differentiation, expression of sexual traits, and regeneration, and, in his opinion, provided the grist for natural selection. It was his model of the germplasm that undergirded this first of neo-Darwinian “syntheses.”

With the rise of classical genetics after Weismann’s death there was a tendency on the part of historians and biologists to assume that Weismann interpreted somatic variations as simply a product of recombinations within the germplasm. More recent research has pointed out that from the start Weismann considered the changing composition of the germplasm to be a response to nutritional and external conditions. It must be emphasized that such variations were not adaptive responses on the part of the organism, but the result of nonadaptive molecular reactions.

Throughout his career Weismann held a nominalistic view of the nature of species and remained committed to what later would be called sympatric speciation. Whether he was examining the paleontological records of the snail Planorbis, determining the phylogenetic relationship of saturniid moths, or responding to Moritz Wagner’s migration theory of evolution, he never felt the species category was unique in contrast to the variety, on the one hand, and genus, on the other.

At the end of the century, partially in response to criticisms by George John Romanes and Herbert Spencer, Weismann resorted to what he called a germinal selection between different homologous determinants to explain the results of intra- and interspecific hybridizations and the decline and disappearance of morphological traits in phylogeny. Moreover, germinal selection became for him a Darwinian-like mechanism for effecting additional change in the germplasm itself. Influenced by the physical anthropologist Otto Ammon, Weismann also attempted to describe evolution in terms of a changing biometrical curve of somatic traits. The bottom line, however, in his understanding of evolution remained the continual change in the germplasm of a species wrought by germinal and natural selection, possible asymmetries in the mating process, and the perceived randomness in the maturation divisions of gametes. When Paul and Fritz Sarasin detailed “Formenketten” or chains of forms in populations of land snails of the island of Celebes, Weismann found in their descriptions further documentation of the arbitrariness of the species category.

Given his model of the germplasm, Weismann had a difficult time adjusting to the discovery of Mendel’s laws, the increasing recognition that chromosomes possessed individuality, and the demonstrations by Theodor Boveri and Walter Sutton that Mendelian factors correlated with chromosomal segregation at meiosis. With the help of his assistant, Waldemar Schleip, Weismann attempted to make appropriate adjustments in the second and third editions of his Vorträge. By the last edition, however, Weismann was seventy-nine years old and nearly blind. The task of reconstructing the germplasm model was beyond him, or for that matter, anyone else.


A comprehensive bibliography of Weismann’s publications may be found in Gaupp (1917) and an updated version in Churchill and Risler (1999).


“Amphimixis or the Essential Meaning of Conjugation and Sexual Reproduction” (1891). In Weismann’s Essays upon Heredity and Kindred Biological Problems. 2 vols. Edited and translated by Edward B. Poulton and Arthur E. Shipley. Oxford: Clarendon Press, 1892.

Das Keimplasma: Eine Theorie der Vererbung. Jena, Germany: Fischer, 1892.

Vorträge über Descendenztheorie. 2 vols. Jena, Germany: Gustav Fischer, 1902. Revised 1904 and 1913.

“Der Briefwechsel zwischen Ernst Haeckel und August Weismann.” In Kleine Festgabe aus Anlass der hundertjährigen Wiederkehr der Gründung des Zoologischen Institutes der Friedrich-Schiller-Universität Jena im Jahre 1865 durch Ernst Haeckel, edited by Manfred Gersch. Jena, East Germany: Friedrich Schiller Universität, 1965. Cited but not discussed in Robinson’s DSB entry on Weismann.

August Weismann: Ausgewählte Briefe und Dokumente, Selected Letters and Documents. Edited by Frederick B. Churchill and Helmut Risler. 2 vols. Schriften der Universitätsbibliothek, Freiburg im Breisgau, vol. 24, parts 1 and 2. Freiburg im Breisgau, Germany: Universitätsbibliothek, 1999. Volume 2 includes the essay by Churchill, “August Weismann: A Developmental Evolutionist.”


Berrill, N. John, and C. K. Liu. “Germplasm, Weismann, and Hydrozoa.” Quarterly Review of Biology 23 (1948): 124–132.

Blacher, Leonid I. The Problem of the Inheritance of Acquired Characters: A History of a Priori and Empirical Methods Used to Find a Solution. Edited by Frederick B. Churchill. Translated by Noel Hess. New Delhi: Published for the Smithsonian Institution Libraries and the National Science Foundation, by Amerind Publishing Co., 1982. Originally published Moscow: Nauka, 1971.

Buss, Leo W. The Evolution of Individuality. Princeton, NJ: Princeton University Press, 1987.

Churchill, Frederick B. “Rudolf Virchow and the Pathologist’s Criteria for the Inheritance of Acquired Characteristics.” Journal of the History of Medicine 31 (1976): 117–148.

———. “The Weismann-Spencer Controversy over the Inheritance of Acquired Characters.” In Human Implications of Scientific Advance: Proceedings of the XV International Congress of the History of Science, Edinburgh, 10–15 August1977, edited by Eric G. Forbes. Edinburgh: Edinburgh University Press, 1978.

———. “Weismann, Hydromedusae, and the Biogenetic Imperative: A Reconsideration.” In A History of Embryology, edited by Thomas J. Horder, Jan A. Witkowski, and C. C. Wylie. Cambridge, U.K.: Cambridge University Press, 1986.

———. “From Heredity Theory to Vererbung: The Transmission Problem, 1850–1915.” Isis 78 (1987): 337–364.

———. “Life before Model Systems: General Zoology at August Weismann’s Institute.” American Zoologist 37 (1997): 260–268.

———. “August Weismann and Ferdinand Dickel: Testing the Dzierzon System.” In Science, History, and Social Activism: A Tribute to Everett Mendelsohn, edited by Garland Allen and Roy MacLeod. Dordrecht, Netherlands: Kluwer Academic, 2001.

Gaupp, Ernst Wilhelm Theodor. August Weismann: Sein Leben und sein Werke. Jena, Germany: G. Fischer, 1917.

Gould, Stephen Jay. Ontogeny and Phylogeny. Cambridge, MA: Harvard University Press, 1977.

Griesemer, James, and William Wimsatt. “Picturing Weismannism: A Case Study of Conceptual Evolution.” In What the Philosophy of Biology Is: Essays for David Hull, edited by Michael Ruse. Dordrecht, Netherlands: Kluwer Academic, 1989.

Mayr, Ernst. “Weismann and Evolution.” Journal of the History of Biology 18 (1985): 295–329.

Robinson, Gloria. A Prelude to Genetics: Theories of a Material Substance of Heredity, Darwin to Weismann. Lawrence, KS: Coronado Press, 1979.

Sander, Klaus, ed. August Weismann (1834–1914) und die theoretische Biologie des 19. Jahrhunderts. Freiburger Universitätsblätter, Hefte 87/88, Jha. 27 (1985). This contains essays on Weismann and his school by Sander, Risler, Schwoerbel, Mayr, Robinson, Sitte, Cremer, Churchill, Danailow, Jahn, Schott, Uschmann, Groeben, Sorensen, Müller, Lücke, Regelmann, and Harwood.

Stamhuis, Ida H. “The Reactions on Hugo de Vries’s Intracellular Pangenesis: The Discussion with August Weismann.” Journal of the History of Biology 36 (2003): 119–152.

Winther, Rasmus. “August Weismann on Germ-Plasm Variation.” Journal of the History of Biology 34 (2001): 517–555.

Frederick B. Churchill

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August Weismann (ou´gŏŏst vīs´män), 1834–1914, German biologist. He taught zoology at the Univ. of Freiburg from 1866 to 1912. He is known as the originator of the germ-plasm theory of heredity. His doctrine, formerly called Weismannism, stresses the unbroken continuity of the germ plasm and the nonheritability of acquired characteristics. His works include The Germ-Plasm (1892, tr. 1893) and a series of essays translated into English as Essays upon Heredity and Kindred Biological Problems (2d ed., 2 vol., 1891–92).

See G. J. Romanes, An Examination of Weismannism (1903).

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Weismann, August (1834–1914) A German biologist who established the improbability, if not impossibility, of the inheritance of acquired characteristics, as required by Lamarck's theory of adaptation.

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Weismann, August (1834–1914) A German biologist who established the improbability, if not impossibility, of the inheritance of acquired characteristics, as required by Lamarck's theory of adaptation.

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Weismann, August (1834–1914) German biologist. His essay discussing the germ plasm theory, The Continuity of the Germ Plasm (1885), proposed the immortality of the germ line cells as opposed to body cells. It was influential in the development of modern genetic study.