I don't know of any comparison between white and brown ferrets in ancient
texts - but I don't know that there aren't any.  But remembering the
ferret is derived from African stock - most common theory I've heard and
that the polecat is in the English Isles and other Northern areas I would
venture to guess the cross breeding came with the moving North out of
what is now Greece and Italy.
 
bil and diane killian
zen and the art of ferrets
[Posted in FML issue 1216]