<I responded to your original post which as we all know didn't have such provenance included.>
That's false.  Either you've confused me with the original poster in this part (conversation) of the thread, or you're being deliberately deceptive.  You made the original accusation of plagiarism in the "arc of history" quote without providing any citation or evidence to support it. Ironically, it was I -- not you -- who provided a citation to an article that traces the history of the quote; and that citation appears in my first post to you in this thread.
Your writing is either careless or dishonest; in this instance, anyway, it appears to be careless. I don't suppose you want any suggestions from me -- but Civil Comments does a pretty good job of laying out a "conversation" within a thread so that you can review it for context before posting your response.
Indeed: Peace.