Since Darwin's recompiled Library is in the news,it's a good time to reconsider the question.
Let me quote first from prof. Ian Johnston's lecture on Lucretius;
QuoteThe poem’s influence, according to Stuart Gillespie and Donald Mackenzie, can be linked to a range of twentieth-century poets and philosophers. So pervasive is its presence in the intellectual climate that for one critic at least (Stuart Gillespie) Charles Darwin’s claim that he had not read Lucretius is rather like Milton’s claiming that he had not read Genesis.
I believe he is referencing the Cambridge Companion to Lucretius, and as Stuart Gillespie's quote makes clear, whether Darwin even read the early atomists is far from certain. The catalogue of his reconstructed library contains over seven thousand titles, and only six of these titles relate to materialism. Lucretius' poem is absent, but one of the texts (in good DeWittean style) is an address contrasting the systems of Epicureanism and Christianity;
Thompson, Joseph Parrish. 1875. "Lucretius or Paul: materialism and theism tested by the nature and needs of man" . Berlin: A. Asher and Co.
Since this address was published 16 years after The Origin of Species, it cannot be construed to establish even an interest in Epicureanism on Darwin's part; it's possibly he picked this volume up just to see what all of the chatter was about after he had been more or less accused of plagiarism.
And as I've said before, if Epicurus was right about nature, if the universe was, in fact, a well ordered cosmos, if the laws governing both mundane and celestial mechanics were predictable in their operations, then there is no obstacle for a Darwin to rediscover what a Greek thinker or Roman poet had already learned.