Take a look at this page (click) which quotes Leonard Bernstein: "There are enough themes in An American In Paris for two symphonies."

The themes from American in Paris have always intrigued me. I'd swear that many of them must be from other sources. This page quotes Gershwin:

This new piece, really a rhapsodic ballet, is written very freely, and is the most modern music I've yet attempted. The opening part will be developed in a typical French style, in the manner of Debussy and the Six, though the themes are all original. My purpose here is to portray the impression of an American visitor in Paris, as he strolls around the city, and listens to various street-noises and absorbs the French atmosphere.

...but I still think that not all the themes in the piece can be original.

-Joe Offer-