He conducted this experiment in the late 19th century to test the theory of inheritance of acquired characteristics. This theory, popularized by Jean-Baptiste Lamarck, proposed that traits acquired during an organism's lifetime could be passed down to its offspring.
Weismann's experiment, however, showed that this was not the case. Despite cutting off the tails of mice for multiple generations, the offspring were still born with tails. This experiment provided strong evidence against Lamarckism and helped to solidify the modern understanding of inheritance through genes.