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.
Bylo to tak, že Avelingova žena byla Marxova dcera, a když byli Darwin i Marx po smrti, Marxovy dokumenty se pomíchaly s Avelingovými, takže dopis od Darwina, kde psal "Vážený pane, děkuji, ale nechci, abyste svou knihu věnoval mně," byl mylně považován za dopis pro Marxe, čímž vznikl celý ten mýtus, o kterém jste nejspíš slyšeli.