English: The only illustration in Charles Darwin's 1859 On the Origin of Species, showing the divergence of species. Caption: Diagram of Divergence of Taxa. The letters A–L represent distinct descents. Each horizontal line represents 1000 generations. Descent A has 3 existent species after 10000 generations. Descent I has 2. Descents E, F have 1 each. The other descents have gone extinct. As Darwin summarized: "Thus, the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species."