Martin Luther King Jr., (1929 - 1968) American civil rights leader, clergyman,
youngest recipient of Nobel Peace Prize in 1964 is justly famous for one speech, containing the phrase I Have a Dream. Campaigning for Civil Rights in the US he was soon to be murdered, but this speech and many others live on.

See the transcript and video clip.

The phrase used by commentators describing the way history has eventually come around to having a black or mixed race president, in Barack Obama is "The arc of history is long, but it bends toward freedom."

This seems to be based on a Martin Luther King phrase in his book "Where do we go from here?", August 1967

The wider text reads:

Difficult and painful as it is, we must walk on in the days ahead with an audacious faith in the future. When our days become dreary with low-hovering clouds of despair, and when our nights become darker than a thousand midnights, let us remember that there is a creative force inthis universe, working to pull down the gigantic mountains of evil, a power that is able to make a way out of no way and transform dark yesterdays into bright tomorrows. Let us realize the arc of the moral universe is long, but it bends toward justice.