August Weismann

German naturalist

• Born: January 17, 1834
• Birthplace: Frankfurt am Main (now in Germany)
• Died: November 5, 1914
• Place of death: Freiburg im Breisgau, Germany

Weismann is most noted for his development and refinement of the theory of the continuity of the germ plasm, for his devout support of Darwinism and the principle of natural selection, and for his discrediting the idea of the inheritance of acquired characteristics.

Early Life

August Friedrich Leopold Weismann (VIS-mahn) was the son of Johann Konrad August Weismann, a classics teacher at the gymnasium in Frankfurt, and Elise Eleanore Lübbren Weismann, a musician and painter. He was the eldest of four children, and his home life was simple and happy. As a young boy, Weismann showed an active interest in nature. He collected butterflies, caterpillars, beetles, and plants, and he assembled a herbarium. He was a lover of art, literature, and music (especially that of Ludwig van Beethoven). These interests continued throughout his life. He became an accomplished pianist. He attended and did well at the gymnasium where his father was a teacher.

Weismann was interested in chemistry and physics as a young adult and wanted to pursue studies in that direction. His father and friends of the family, however, suggested that he pursue medicine, because a career in medicine would be more lucrative. To this end, he entered the University of Göttingen in 1852, where he studied with Friedrich Henle and Friedrich Wöhler in an atmosphere that emphasized research rather than broader problems. He received his medical degree in 1856.

Life’s Work

Following graduation, Weismann continued his research while working as an assistant in the medical clinic at Rostock. In 1857, he transferred to the Chemical Institute so that he could pursue his interest in chemistry. This was followed by a tour of four German universities and a more extensive stay in Vienna.

Weismann entered private medical practice in Frankfurt in 1858. His practice allowed him sufficient time to pursue studies on heart muscle fibers. His private practice was interrupted in 1859 by the war between Austria and Italy, at which time he entered the German army and served as a surgeon at the field hospital in Italy. He resumed private practice in 1860.

In 1861, Weismann abandoned medicine to pursue what had become his main interest, the biological sciences. He attended the University of Giessen for two months in 1861 and was profoundly influenced by Rudolf Leuckart, under whom he began his studies in insect embryology. He considered the two months he spent with Leuckart to be the most important and inspiring time of his career.

Following his stay at Giessen, Weismann became the private physician of Archduke Stephan of Austria. While in this position, from 1861 to 1863, Weismann had ample time to pursue his interest in insect development and completed his first major work, Die Entwicklung der Dipteren (on the development of the diptera in the egg), in 1864. He also had time to read Charles Darwin’s On the Origin of Species by Means of Natural Selection (1859). Like so many scientists of the time, Weismann was profoundly influenced by Darwin’s book. Along with Ernst Haeckel and Fritz Müller, Weismann became one of Germany’s staunchest supporters of Darwinian theory.

In 1863, Weismann became a privatdocent at the University of Freiburg and taught zoology and comparative anatomy. In 1866, he was appointed extraordinary professor and, in 1874, professor. He was the first to occupy the chair of zoology at Freiburg. He soon became director of the Zoological Institute at the university. He was a well-respected teacher, who always attracted large numbers of students.

Weismann’s first research papers examined insect histology and embryology. Several papers on these subjects were published between 1862 and 1866. One important discovery he made was that, during metamorphosis, tissues completely dedifferentiate and then redifferentiate during the formation of the adult. Weismann was also interested in the origin and fate of the germ cells of hydrozoans. The germ cells of multicellular organisms such as hydrozoans are set aside from the somatic cells early in development and provide for the continuity of the organism through the sperm and the egg. The somatic cells will eventually die, but the germ cells live on in a new individual. Only the reproductive cells have the capacity to form a complete, new individual. From these observations, Weismann developed the theory for which he is most noted, of the continuity of the germ plasm.

In 1864, Weismann’s eyesight failed, and he had to cease work on the microscope. Although he turned to more general problems, his microscopic work was continued by his students, his assistants, and Marie Dorothea Gruber, whom he married in 1867. His wife read to him constantly so that he could keep abreast of the latest scientific developments. His eyesight became so poor that he took a leave of absence from his teaching position from 1869 to 1871. During that time, his eyesight improved, and he resumed lecturing in 1871 and active research in 1874. His eyesight again failed in 1884.

Work on the theory of the continuity of the germ plasm occupied the last thirty years of Weismann’s life as an active scientist. The theory encompassed many areas but primarily focused on heredity and evolution. He first published on these topics in 1883. Although he was not the first to suggest the principle of the continuity of the germ plasm, he did develop the idea to its fullest. He contended that the germ plasm was to be found on the chromatin threads, the “idants” (chromosomes) in the nucleus of the cell. He hypothesized that the idants were composed of smaller units, the “ids,” which in turn were composed of the “determinants,” the individual hereditary units, which he correctly envisioned as being linearly arranged. The determinants, he thought, were composed of still smaller, more basic units, the “biophors.”

Based only on a priori knowledge, Weismann reasoned that the chromosome number must be halved during the formation of the reproductive cells and hypothesized that a “reduction division” must occur during the process. This, he thought, was to prevent doubling of the germ plasm at each generation. This is considered by many to be his most significant and effective scientific contribution. Weismann believed that during fertilization individual ancestral germ plasms, each carrying variations, were combined. He thought that, as well as introducing new variations, this process created new combinations of variations.

Weismann’s ideas on the continuity of the germ plasm put him in direct conflict with many other scientists of the time, because many had come to discredit natural selection as a mechanism of evolution and advocated Lamarckism, the inheritance of acquired traits, as an alternative. Even Charles Darwin proposed the theory of pangenesis, where each body part was thought to produce a gemmule, which could be modified by the environment and eventually passed to the germ cells. Weismann investigated many cases of reported inheritance of acquired characteristics and could find no authenticated instance of such inheritance. His own classic experiments, in which he cut off the tails of mice over several generations but found no tendency for the tail to shorten in succeeding generations, were instrumental in challenging Lamarckism. Weismann thought that the only way acquired characteristics could be passed to the offspring was if the germ cells were affected. He became more devoted to the theory of natural selection than did Darwin. Weismann did not believe that the environment in any way affected heredity.

Weismann extended Darwin’s theory of natural selection to the germ cells in a new theory called germinal selection. He thought that the determinants struggled with one another for nutriment and that the stronger ones would triumph and eliminate the weaker ones. Thus, only the stronger ones would survive in the germ plasm and be passed to the offspring. This, he thought, could account for the loss of organs during evolution. Later work failed to support his idea of germinal selection. Weismann extended his germ plasm theory to development. He correctly thought that the determinants directed differentiation in individual cells but incorrectly envisioned that this was a result of the distribution of different determinants to different cells during cell division. He therefore thought that mitosis could be qualitatively unequal while being quantitatively equal.

Weismann’s main works on heredity and evolution were published in Studien zur Descendenztheorie (1875-1876; Studies in the Theory of Descent , 1882), Essays upon Heredity and Kindred Biological Problems (1889-1892), Das Keimplasma: Eine Theorie der Vererbung (1892; The Germ-Plasm: A Theory of Heredity , 1893), On Germinal Selection as a Source of Definite Variation (1896), and Vorträge über Descendenztheorie (1902; The Evolution Theory , 1904). The Evolution Theory became an important and widely read book. It has been said that since Weismann’s theoretical contributions to science were so important, his experimental and observational work was often overshadowed.

Weismann retired from the faculty of the University of Freiburg in 1912. He died peacefully at Freiburg im Breisgau, Germany, on November 5, 1914, at the age of eighty.

Significance

August Weismann was one of the most respected biologists of the latter part of the nineteenth century and the early part of the twentieth century. His ideas stimulated considerable discussion and research. His theories on heredity and development were far-reaching. He correctly recognized that the hereditary material was contained within the nucleus of the sperm and egg and that the hereditary material of the germ cells is reduced to one-half during the maturation of the sperm and egg. In a single theory, the germ plasm theory, he explained the meiotic reduction division, sexual reproduction, development, and natural selection. It has been said that Weismann’s “ingenious synthesis helped prepare the way for twentieth-century genetics.” Weismann was elected to the Bavarian Academy of Sciences and, as a foreign member, to the Linnean Society, the American Philosophical Society, and the Royal Society of London. He received numerous honorary degrees and medals, including the Darwin/Wallace Medal of the Linnean Society and the Darwin Medal of the Royal Society.

Bibliography

Bowler, Peter J. Evolution: The History of an Idea. 3d ed. Berkeley: University of California Press, 2003. Chapter 7, “The Eclipse of Darwinism: Scientific Evolutionism, 1875-1925,” describes Weismann’s germ plasm theories.

Churchill, Frederick B. “August Weismann and a Break from Tradition.” The Journal of the History of Biology (1968): 91-112. This brief article discusses Weismann’s most significant contribution, the theory of the continuity of the germ plasm. Also shows how his work related to that of Ernst Haeckel and others and how it influenced the development of modern biological thought.

Coleman, William. “The Cell, Nucleus and Inheritance: An Historical Study.” Proceedings of the American Philosophical Society 109 (1965): 126, 149-154. Like the article by Churchill, this article analyzes the impact of the work of Weismann and others on modern biological thought. It is one of the few modern works that analyze Weismann in this light.

Conklin, Edwin. “August Weismann.” Proceedings of the American Philosophical Society 54 (1915): iii-xii. This is a brief summary of Weismann’s contribution to science, written by a friend on the occasion of Weismann’s death.

Gottlieb, Gilbert. Individual Development and Evolution: The Genesis of Novel Behavior. New York: Oxford University Press, 1992. Reprint. Mahwah, N.J.: Lawrence Erlbaum Associates, 2002. Explores the interrelationship of heredity, individual development, and evolution by reviewing the ideas of Weismann and other scientists.

Steele, E. J., and Robert Blanden. “The Evidence of Lamarck.” Quadrant 44, no. 3 (March, 2000): 47. Explores the hypotheses of Weismann and others that characteristics acquired during an individual’s life time may be passed on genetically to offspring.

Weismann, August. Essays upon Heredity and Kindred Biological Problems. 2 vols. Oxford, England: Clarendon Press, 1891-1892. This collection contains some of Weismann’s most important theoretical contributions on heredity, the continuity of the germ plasm, sexual reproduction, and evolution.

‗‗‗‗‗‗‗. The Germ-Plasm: A Theory of Heredity. London: Walter Scott, 1893. This is the most significant book written by Weismann. It addresses the most important theoretical contributions Weismann was to make to science.