On the Origin of Species
Darwin's ideas on the connection between natural selection and inheritance were unclear, mainly because the process of genetic inheritance was unknown at that time. This was solved in the mid-20th century by the modern evolutionary synthesis, which showed that Gregor Mendel's genetics was compatible with evolution in small steps. Darwin made almost no mention of the evolution of the human race, though most of the controversy raged around this topic. Darwin eventually published The Descent of Man in 1871.