German botanist. Trained as a lawyer, he soon left the profession to study natural science. He and T. Schwann developed the cell theory, which states that organisms are composed of cells or substances made by cells; they were thus the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He also recognized the importance of the cell nucleus and sensed its connection with cell division. He was one of the first German biologists to accept C. Darwin's theory of evolution.