Commentary Par XI 82

The adjective ferace (literally, fertile, fecund), a hapax in the poem (as is, more surprisingly, the parallel adjective that precedes it, ignota [unknown]), contrasts with the notion of ordinary life, based as it is in acquiring wealth and possessions, not in living one's faith.  Like its unique status among the words of the poem, its unusualness in ordinarily sterile human experience sets it apart.