Phillip Sloan has thoroughly documented the importance of Darwin's general invertebrate research program in the period from 1826 to 1836 and demonstrated how it had an impact on his conversion to transformism. Although Darwin later spent eight years of his life (1846-1854) investigating barnacles, this period has received less treatment in studies of Darwin and the development of his thought. The most prominent question for the barnacle period that has been attended to is why Darwin "delayed" in publishing his theory of evolution. A related but distinct question concerns the variety of earlier events and influences that led Darwin to the study of Cirripedia in 1846, apart from its role in the trajectory that led to On the Origin of Species (1859). In this paper I focus on four specific episodes prior to 1846 that inform a picture of why Darwin had an antecedent interest in barnacles: (1) the orientation to collecting strange and curious invertebrate organisms, as well as the strong affinities of Darwin's invertebrate collecting on the Beagle voyage with the work of John Vaughan Thompson; (2) the critical role of marine invertebrate fossils in Darwin's geological reasoning aboard the Beagle and exemplified in his Geological Observations of South America; (3) the strange absence of a Zoology of the Beagle volume on invertebrates and Darwin's original intent to publish some of the descriptions himself; and (4) the noteworthy presence of barnacles in Darwin's transformation theorizing between 1837 and 1839. There is a wealth of support for the thesis that Darwin had a strong interest in cirripedes prior to the formal barnacle research, blunting arguments that it was psychological aversion or a feeling of inferiority about his taxonomic abilities that drove Darwin to the cirripedes.