A settlement of barnacles on a shore crab.

In November 1845, Charles Darwin wrote to his friend and confidant Joseph Dalton Hooker: ‘I hope this next summer to finish my S. American geology; then to get out a little zoology & hurrah for my species-work’. 

In the event, the ‘little zoology’ turned out to be a study of barnacles that spanned eight years and resulted in two monographs, each of two volumes, describing all the known forms of that neglected and hitherto confusing sub-class of Crustacea, Living Cirripedia (1851, 1854) and Fossil Cirripedia (1851, 1854).