August Weismann

August Weismann looked at a mouse's tail and saw a fundamental truth about life. In late 19th-century Germany, at a time when many still believed in the inheritance of acquired characteristics, Weismann proposed a daring separation. He divided the body into the 'soma'—the mortal, disposable physical form—and the 'germ plasm', the immortal line of reproductive cells that carried heredity intact from generation to generation. To prove his point, he famously cut the tails off generations of mice, showing the injury was never passed on. This 'Weismann barrier' was a revolutionary concept, decisively discrediting Lamarckian ideas and providing a robust biological framework for Darwin's theory of natural selection. As a professor at Freiburg, his meticulous research and forceful arguments made him a central figure in evolutionary biology, steering the field toward the genetic understanding that would blossom in the next century.