Abstract: The theory of evolution should answer the question of how to start and how to continue to progress. The evolution of the legal system is carried out in the process of the closed operation of the system and the maintenance of normative expectations. For this reason, the evolution of the legal system is disturbed but not determined by the environment. The evolution depends on the factual process of the system itself. Luhmann's system theory was inspired by Darwin's biological evolution theory by comparing the legal system to biological systems and emphasizing the dynamic nature of evolution itself. However, Luhmann weakened the importance of the environment in the process of natural selection and focused on the system in the evolution process. The mechanism of mutation, selection, and re-stabilization ultimately emphasizes the non-purpose, non-determinism, and non-necessity of evolution. Specifically, the theory of legal evolution includes the three main issues as why the evolution is possible, how to unfold the progress, and the path to move toward the future.