Above: Peter Arno's "The Whoops Sisters" in their first appearance (Before they were even called "The Whoops Sisters!") from the April 17, 1926 issue of The New Yorker. Nicked from A New Yorker Frame of Mind blog.
Michael Maslin writes about the cartoonists coming to The New Yorker over its history in his piece titled "The Place Was a Mess After the Weekly Art Meetings."
"[Philip Wylie] was the unofficially titled artists' 'hand-holder' — the link between the editorial staff and the cartoonists. Most importantly to The New Yorker‘s history, and to its success, Wylie is the person who, while looking through twenty-one year old Peter Arno’s portfolio one day in 1925 happened to spot a drawing Arno hadn’t intended to show: a sketch of 'two old bats about to charge obliviously into a trap — made by the rise of a sidewalk elevator. It [the drawing] greatly amused me.' The 'two old bats' came to be called The Whoops Sisters, and also came to be credited as very likely rescuing The New Yorker from an early demise."
Go read it all here.