In the Preface to Leaves of Grass, Whitman accurately conveys a central quality of the American psyche in stating that Americans are not oriented to the past, to "reminiscences." In this he is implicitly contrasting the attitudes of the New World with those of the Old:
As if it were necessary to trot back generation after generation
to the eastern records! As if the beauty and sacredness of the
demonstrable must fall behind that of the mythical! As if men
do not make their mark out of any times!
What was true 160 years ago is true now. Though the peoples of the US, apart from the Native Americans, all trace their roots to other continents, to the Old World, all have become American, have become a new nation, and their old-world roots and myths, though not forgotten, have receded into the background, in Whitman's time and our own. Whitman does mention the indigenous Americans as the "tribes of red aborigines." It is a brief reference, but it shows that he is including them in his predictions for America's future, thus separating himself from other European Americans of the time.
In general, Americans historically, whatever our other faults and wrongdoings, which are many, tend to cut themselves loose from the past. Whitman, however, is not stating that the past should be forgotten:
The American poets are to enclose old and new, for
America is the race of races.
In this melding of old and new, Whitman is actually prophesying the multiculturalism which has become a reality in our time. He anticipates, as well, the dissolving of past hierarchies, including those based on gender, when he refers to "the perfect equality of the female with the male." He knows (in 1855) that slavery will be abolished as well, for he speaks of "the stern opposition to it which shall never cease till it ceases or the speaking of tongues and the moving of the lips cease. In perhaps his most radical statement, Whitman says,
There will soon be no more priests. Their work is done.
Whitman is predicting the brotherhood and sisterhood of all humanity as replacing the older faiths. In all, Whitman's message is that Americans are the ones who will lead the world forward, not forgetting the past, but rejecting (as he observes that they already are doing, or have the potential to do) the old ways that have burdened and inhibited the advance of humanity.