The paintings of Henri Rousseau (1844-1910), particularly his astonishing jungle dreams, are now so popular that it is difficult to realize how they were originally greeted with ridicule and incomprehension. It was not until Rousseau was championed by the young avant-garde--Picasso, Delaunay, and Kandinsky, among others--that he came to be recognized at his true worth. One of the most significant of these early admirers was the dealer and...