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.
Tosiasiassa Avelingin rakastajatar oli Marxin tytär, ja sekä Darwinin että Marxin kuoltua Marxin paperit sekaantuivat Avelingin papereihin ja Darwinilta tullutta kirjettä: "Parahin sir, kiitoksia paljon, mutta en halua teidän omistavan kirjaanne minulle." pidettiin virheellisesti Marxille osoitettuna, mikä synnytti koko myytin, jonka luultavast olette kuulleetkin.