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.
Vad som hände var att Avelings älskarinna var Marxs dotter, och när både Darwin och Marx var döda, blandades Marx papper ihop med Avelings. och ett brev från Darwin som sa, "Min gode herre, tack så hemskt mycket men jag önskar inte att du dedicerar din bok till mig," förmodades av misstag vara adresserat till Marx, och det upphov till denna hela myt, vilken ni troligen hört om.