There is a proof without words in [Etienne Ghys' ICM address.][1] and [a short proof with words by Rubinstein and Gardiner.][2] (Compositio, 1979)


  [1]: http://www.umpa.ens-lyon.fr/~ghys/articles/icm.pdf
  [2]: http://archive.numdam.org/ARCHIVE/CM/CM_1979__39_3/CM_1979__39_3_297_0/CM_1979__39_3_297_0.pdf