On 24 November 1859, Charles Darwin published *On the Origin of Species by Means of Natural Selection*, presenting his theory of evolution — the most consequential idea in the history of biology.
Drawing on observations from his 1831–1836 voyage aboard HMS Beagle — especially from the Galápagos Islands, where finch beak shapes varied by island food sources — Darwin argued that species were not fixed divine creations but changed over generations through *natural selection*: individuals with advantageous traits survived and reproduced, passing those traits on. The first edition of 1,250 copies sold out on the day of publication.
The theory unified all of biology under a single explanatory framework and was later integrated with Gregor Mendel's genetics into the Modern Synthesis. It remains one of the best-supported theories in all of science. The book sparked fierce religious and philosophical controversy that, in parts of the world, continues today.