August Weismann was a German biologist prominent in the late 19th century. Born in Frankfurt in 1834, he initially pursued medicine before his interest in natural history led him to zoology. Working in an era when Charles Darwin’s ideas were transforming science, Weismann became an important figure in the progression of evolutionary theory. He dedicated his work to tackling a significant question left unanswered by Darwin: the precise mechanism of heredity.
The Germ Plasm Theory
Weismann’s most famous contribution was his germ plasm theory, detailed in his 1892 book, The Germ-Plasm: A Theory of Heredity. He theorized that multicellular organisms are composed of two distinct types of cells: somatic cells and germ cells. Somatic cells are the cells that make up the body’s tissues and organs, such as muscle and skin cells. Germ cells, in contrast, are the reproductive cells—the sperm and eggs—responsible for creating the next generation.
The central idea of his theory is that hereditary information is exclusively carried within the germ cells. He called this hereditary substance the “germ-plasm.” According to Weismann, this material is passed down, unchanged, from one generation to the next through the germ line. The somatic cells, which form the body (the “somatoplasm”), are a temporary vessel for the germ-plasm and do not contribute any hereditary information to the offspring.
This concept established the Weismann barrier. This barrier prevents any changes that occur to the somatic cells during an organism’s lifetime from influencing the germ cells. For instance, a weightlifter developing large muscles or a person acquiring a scar would not pass these traits to their children, as the experiences of the body are walled off from the hereditary material.
An analogy is to think of the germ-plasm as master blueprints for a house. The house itself is the body, built according to the specifications of those blueprints. One can remodel the kitchen or paint the walls, but none of those renovations will alter the original master blueprints.
Disproving the Inheritance of Acquired Characteristics
During Weismann’s time, a popular explanation for inheritance was the theory of acquired characteristics, associated with the French naturalist Jean-Baptiste Lamarck. This theory proposed that traits developed by an organism during its life could be passed on to its offspring. For example, it was believed that a giraffe that stretched its neck to reach higher leaves would have offspring with slightly longer necks.
Weismann challenged this belief with a famous experiment. To test whether an acquired physical trait could be inherited, he focused on mutilations. He systematically removed the tails of white mice, then allowed them to breed, continuing this process for five successive generations and documenting the results.
Over the course of the experiment, he observed 901 offspring produced from these tailless parents. The result was that every mouse was born with a normal, full-length tail.
Weismann concluded that this experiment demonstrated that an acquired characteristic—in this case, the absence of a tail—was not passed down. This result provided powerful evidence against the Lamarckian model of inheritance and lent significant support to his own germ plasm theory. It showed that an injury to the somatic cells of the parent did not alter the hereditary information within their germ cells.
Weismann’s Role in Neo-Darwinism
Weismann’s work was instrumental in forming Neo-Darwinism. This term describes the modern version of evolutionary theory, which integrates Darwin’s concept of natural selection with the scientific principles of genetics. A gap in Darwin’s original theory was the absence of a clear mechanism for how traits were inherited and how variation was maintained in populations.
By proposing the germ plasm theory, Weismann provided a “hard” theory of inheritance, where hereditary information was protected and transmitted independently of the parent’s life experiences. His ideas established that the sole source of heritable variation must come from changes within the germ-plasm itself, not from the environment’s effect on the body. This strengthened Darwin’s theory of natural selection by identifying it as the primary agent of evolutionary change, acting upon the variations present in the germ line.
His work helped clear the way for the later rediscovery of Gregor Mendel’s work on genetics. The synthesis of Weismann’s ideas about heredity with Darwin’s natural selection and Mendelian genetics formed the foundation of the modern evolutionary synthesis. His legacy is that of a theorist who solidified Darwin’s ideas by providing a testable mechanism for heredity, setting the stage for the genetic discoveries of the 20th century.