Darwin’s monograph on barnacles comprised four volumes. Two volumes covered living Cirripedia, while two covered fossil cirripedes from Great Britain.
Darwin worked on barnacles for roughly eight years, beginning in 1846. The living-barnacle volumes examined pedunculated forms known as Lepadidae and sessile forms including Balanidae and Verrucidae. The fossil volumes treated corresponding British fossil groups. Together, the books represented a highly detailed taxonomic and anatomical investigation rather than a single short publication.
The project became important to Darwin’s evolutionary thinking because barnacles displayed striking variation in structure, development, and reproductive biology. His comparisons of living and fossil forms helped him study species boundaries, homology, and how classification might reflect relationships. The work also strengthened his reputation: the Royal Society awarded him its Royal Medal in 1853, before every part of the study had appeared.
The dates can cause confusion because some volumes carry earlier imprint dates than their actual appearance dates. Bibliographies commonly list the four-volume series across 1851–1854, with one volume sometimes treated as appearing in 1855.