First published in 1859, Charles Darwin's On the Origin of Species introduced the scientific theory of evolution by means of natural selection, a revolutionary idea at the time. Fiercely debated on its release, Darwin's famous work was revised several times over the following decade, partly to refute the scientific and religious objections of the author's contemporaries. In this sixth edition of On the Origin of Species, Darwin lays out his theory by citing his studies done in the Galápagos aboard the HMS Beagle in the 1830s, as well as his follow-up research over the ensuing two decades.