This cartoon (by Lee Lorenz, a 74-year-old former art editor of The New Yorker) ran in the November 26th issue of The New Yorker
This cartoon (by Gary Larson) ran in The Far Side Gallery, in 1984
I think The New Yorker's cartoon editor, Bob Mankoff, is correct here:
Mankoff explains that the sheer volume of cartoons produced by artists means that there is often overlap of ideas. "Often in the same week different cartoonists will independently come up with identical ideas," he says. "Other times cartoonists generate ideas that have been previously published in the magazine. This is not plagiarism; rather it is the result of very creative people developing many ideas from a few well-established, well-traveled cartoon settings."