WHEN Christopher Columbus kissed the ground of the new world in 1492, he was on terra firma; five centuries later, of course, we find the great navigator on shaky ground.  Under the weighty dictates of what is known as political correctness, the monumental nature of Columbus’s epic achievement crumbles as the very meaning of discovery—or, in current parlance, “discovery”—is called into question. Accordingly, Columbus didn’t discover America; the islands that became known as a part of the “New World” were already inhabited. In the shank of this century, to “celebrate” Columbus Day is to be mindful of his ominous significance in the conquest of the continent’s earlier residents.