"bush league"

I'm looking for the origin of the phrase "bushleague" Does anyone know?

bush league (small "b") -- "In baseball a bush league is a small minor league of low or mediocre quality, 'bush' here taking on the meaning of 'countrified' and 'unimportant.' The term, in use since the early 1900's, was transferred to other enterprises soon thereafter..." From "Southpaws & Sunday Punches and other Sporting Expressions" by Christine Ammer (Penguin Books, New York, 1993).