In a celebration of the 200th anniversary of his birth in 1809, this short essay explores the influence of marine biology on Charles Darwin, and vice versa. Darwin made his first forays into the world of marine biology as a medical student in Edinburgh from 1825 to 1827. He came under the influence there of the Lamarckian Robert Grant, and developed an understanding of the simple organisation of the early developmental stages of marine invertebrates. Yet Darwin balked at Lamarckian transmutation. The voyage of the Beagle led to Darwin’s perceptive theory of the origin of coral reefs, an origin still mainly accepted today. This theory was steeped in the uniformitarianism of the geologist Lyell, depending on the slow, gradual growth of billions of coral polyps keeping pace with slow sinking of land to produce an atoll. From 1846 to 1854 Darwin revolutionised the understanding of barnacles, producing monographs still relevant today. His barnacle studies gave him the credibility to pronounce on the origin of species; he found great variation in morphology, and a series of related species with remarkable reproductive adaptation culminating in the presence of dwarf males. Barnacles showed him an evolutionary narrative laid out before him, and contributed greatly to his qualification and confidence to write with authority on the origin of species.