Source : Webster's Revised Unabridged Dictionary (1913)

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]

   {Scratch race}, one without restrictions regarding the
      entrance of competitors; also, one for which the
      competitors are chosen by lot.

Source : WordNet®

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