Natural Selection is a manuscript written by Charles Darwin, in which he presented his theory of natural selection and its role in biological evolution. He did not publish the work while he was alive, but wrote an abstract, titled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, which he published in 1859.
Darwin regarded Natural Selection as his main work, while On the Origin of Species was written for a wider audience. He always intended to finish Natural Selection, but because of frail health, the publicity and work involved in publishing six editions of On the Origin of Species, plus other research and publications, he never got around to finish it.
Natural Selection was transcribed after Darwin's death, and first published in 1975.