The Collaborative International Dictionary of English v.0.48:

Scratch \Scratch\, a.
   Made, done, or happening by chance; arranged with little or
   no preparation; determined by circumstances; haphazard; as, a
   scratch team; a scratch crew for a boat race; a scratch shot
   in billiards. [Slang]
   [1913 Webster]

   Scratch race, one without restrictions regarding the
      entrance of competitors; also, one for which the
      competitors are chosen by lot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scratch race
    n 1: a race in which all contestants start from scratch (on
         equal terms)