It is commonly held that Darwin discovered natural selection in a flash of insight after reading Thomas Robert Malthus’ Essay on Population in September 1838. Additionally, most people think this was the missing piece that completed his theory of evolution. However, the Eureka moment is a myth, as is the claim that its purported insight finalized his theoretical understanding. Darwin not only slowly worked out his idea of natural selection over many months but he also took another twenty years to formulate the theory presented in On the Origin of Species. Most notably, his thinking changed radically in 1856 with the introduction of the principle of divergence. Analyzing this history demonstrates that there are a variety of misunderstandings about what exactly Darwin’s theory was, how it was structured, and whether it changed over time. Overall, this casts doubt on the claim that Darwin’s theory was ever “essentially complete,” even after contemplating several different meanings for this phrase. Rejecting this myth yields a richer understanding of the way scientific inquiry operates, especially how its methods and outcomes are justified, while augmenting our confidence in the importance of natural selection for explaining adaptive evolution.