The anthology of myths commonly known as America rests upon the notion that history is linear. In the past people in this country ignorantly did bad things to other people. But thanks to the passage of time, we can now "let the past to be the past," because today we live in a time when things have gotten much better. Furthermore, any problem that our society faces in the present will inevitably be solved as "the old guard" dies off and a new generation of better people takes their place.
Of course this story isn't told so simply or explicitly. But the assumption lurks beneath the other stories we, as Americans, tell ourselves and each other. The myth certainly undergirds the notion that racism is a thing of the past, and that today we inhabit a "post-racial" world in which all people, regardless of race have equal access to betterment, dignity and happiness. We are lulled into beliving that at some point in the mid to late 1960's, a wise reverend implored the nation to judge others by the content of their character rather than the color of their skin, and America heeded his call, dropped racism from our social fabric, and we all moved forward into the light.
Time heals all wounds, or so the story goes.