Charles Darwin's On the Origin of Species, in which he writes of his theories of evolution by natural selection, is one of the most important works of scientific study ever published. This unabridged edition also includes a rich selection of primary source material: substantial selections from Darwin's other works (Autobiography, notebooks, letters, Voyage of the Beagle, and The Descent of Man) and selections from Darwin's sources and contemporaries (excerpts from Genesis, Paley, Lamarck, Spencer, Lyell, Malthus, Huxley, and Wallace).