Probably in example sentence
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.