The idea of the title “Things Fall Apart” is that the old tribal ways are supplanted by new ones, and that the new ones are not as full of integrity and purpose, and don’t have the same sense of values; a treasure is being lost. The world is disintegrating. The phrase, as was said, comes from Yeats’s poem “The Second Coming” (note spelling of Yeats–it’s “Yeats’s poem,” not spelled as previous responder does). As in “The Second Coming,” the violent replacement of former customs may be part of a cycle, a broader sweep of history; the book is ambivalent about what happens but fearful at the same time.