August Weismann

August Weismann was a highly influential German evolutionary biologist.

He lived from January 17, 1834, to November 5, 1914. The biologist Ernst Mayr ranked him as the second most important evolutionary theorist of the 19th century. Only Charles Darwin was ranked higher. Weismann held many important positions in science. He served as the Director of the Zoological Institute at the University of Freiburg. He was also the first Professor of Zoology at that same institution. His work helped define how biological information moves through generations.

Weismann is best known for his germ plasm theory.

This theory explains how inheritance works in multicellular animals. He proposed that living things are made of two different types of cells. The first type is the germ cells, which include gametes like eggs and sperm. These germ cells are the only agents of heredity. The second type is somatic cells, which make up the rest of the body. Weismann argued that germ cells produce somatic cells through a one-way process. This means that the somatic cells cannot pass information back to the germ cells.

This one-way flow of information is called the Weismann barrier. The barrier prevents genetic information from passing from the soma, or body, to the germ plasm. Because of this, any abilities or changes an individual acquires during its life cannot be inherited. This concept was a major blow to the ideas of Jean-Baptiste Lamarck. Lamarck had proposed the inheritance of acquired characteristics. He suggested that traits gained through effort or use could be passed to offspring. Weismann’s theory showed that such changes are not possible. The germ cells remain unaffected by the experiences or physical changes of the somatic cells.

Weismann tested this idea through a famous experiment involving mice. He wanted to see if physical injuries could change the traits of future generations. He worked with 68 white mice over five generations. During this time, he repeatedly removed the tails of the parent mice.

He reported that a total of 901 young mice were produced. Despite the mutilations, not a single baby was born without a tail. There were no examples of shorter tails or other abnormalities in that organ. This supported his belief that the germ cells were protected from somatic changes.

Weismann’s scientific views evolved throughout his long career. In his earlier years, he believed that environmental influences caused species to change. He even used the Lamarckian idea of the "use and disuse" of organs. However, his work on the division of cells changed his perspective. He studied the embryology of sea urchin eggs to understand cell processes. During this research, he identified two types of cell division. He named them equatorial division and reductional division. He also noted that mutation in the gametes is the only source of change for natural selection. This helped link his ideas to the broader process of evolution.

Before becoming a biologist, Weismann had a very diverse education. He was born in Frankfurt am Main to a high school teacher and a mayor's daughter. As a child, he took music, drafting, and painting lessons. A piano teacher actually introduced him to collecting caterpillars and butterflies. Because science was expensive, he studied medicine at Göttingen instead. He worked as a physician in several places, including Rostock, Vienna, and Paris. He even served as a Chief Medical Officer during a war between Austria, France, and Italy. Even when his eyesight became too poor for microscopes, he continued his biological inquiries.

Weismann’s legacy is tied to the modern synthesis of evolutionary biology. His ideas about the germ line were central to the scientific developments of the early 20th century. While modern scientists might use different terms, the core of the Weismann barrier remains vital. His work also connected with the later rediscovery of Gregor Mendel’s laws of inheritance. By separating the body from the hereditary material, he provided a clearer map of how life evolves. His research helped move biology toward a more precise understanding of genetics and cell biology.