Mencken's chosen qualifiers are right there in the text: "I make much of this improvement because it is of an order not witnessed in American writers; and seld

Mencken's chosen qualifiers are right there in the text: "I make much of this improvement because it is of an order not witnessed in American writers; and seldom, indeed, in those who start off with popular success." Emphasis mine. American writers and the one-hit wonders of the literary world tend to get worse rather than better, he's saying. The first is probably a justifiable statement at the time—decent-to-good American literature was rare (though not entirely absent) in the English corpus before the moderns (many of whom preferred to live in Europe, anyway).