Doonesbury by Garry Trudeau for July 26, 1972
Transcript:
Phil: Honey, I finally figured out how two fine people like us ended up with a louse like Mark! ...Ol' Fred Simpson's wife had a baby same day as you - you know, Ted, who grew up to be a fine scholar. Well, I figure somewhere along the line the babies were switched! That's got to be it! Mark: How do you feel about that theory, mom?