What happened was that Aveling's mistress was Marx's daughter, and when both Darwin and Marx were dead, Marx's papers became muddled up with Aveling's papers, and a letter from Darwin saying, "My dear sir, thank you very much but I don't want you to dedicate your book to me," was mistakenly supposed to be addressed to Marx, and that gave rise to this whole myth, which you've probably heard.
Nutiko taip, kad Avelingo meilužė buvo Markso dukra, mirus Darvinui ir Marksui, Markso dokumentai susimaišė su Avelingo, o Darvino laiško, kuriame rašyta: "Brangus pone, labai Jums ačiū, bet nenoriu, kad savo knygą dedikuotumėte man", adresatu klaidingai palaikytas Marksas, dėl ko ir kilo visas šis mitas, apie kurį greičiausiai girdėjote.