In this essay, I discuss the origin of Charles Darwin's interest in cirripedes (barnacles). Indeed, he worked intensively on cirripedes during the years in which he was developing the theory that eventually led to the publication of The Origin of Species. In the light of our present knowledge, I present Darwin's achievements in the morphology, systematics and biology of these small marine invertebrates, and also his mistakes. I suggest that the word that sheds the most light here is homology, and that his mistakes were due to following Richard Owen's method of determining homologies by reference to an ideal archetype. I discuss the ways in which his studies on cirripedes influenced the writing of The Origin.
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