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The principal missing element in Darwin's concept of evolution was a clear understanding of the mechanisms of heredity and the source of variation. Darwin did not know about Mendelian genetics (which was rediscovered after his death) or the molecular basis of inheritance (DNA). While he observed variation and natural selection, he could not explain how traits were inherited or how new variation arose. The other options were either available (fossil records were known, though incomplete) or were not the central missing element.