In the front of the book Porter says "The title of this book is a translation from the German of Das Narrenschiff, a moral allegory by Sebastian Brant (1458?-1521).... When I began thinking about my novel, I took for my own this simple almost universal image of the ship of this world on its voyage to eternity. It is by no means new--it was very old and durable and dearly familiar when Brant used it, and it suits my purpose exactly. I am a passenger on that ship."
This reminded me of something, though it took a while to track down the quote. It's from the Venerable Bede, describing what was said to King Edwin to convince him (and through him the whole of Northumbria) to convert to Christianity:
"The present life of man, O King, seems to me, in comparison of that time which is unknown to us, like to the swift flight of a sparrow through the room wherein you sit at supper in winter, with your commanders and retainers, and a good fire in the midst, whilst the storms of rain and snow prevail abroad; the sparrow, I say, flying in at one door, and immediately out at another, whilst he is within, is safe from the wintry storm; but after a short space of fair weather, he immediately vanishes out of your sight, into the dark winter from which he had emerged. So this life of man appears for a short space, but of what went before, or what is to follow, we are utterly ignorant. If, therefore, this new doctrine contains something more certain, it seems justly to deserve to be followed."
It's an unforgettable image, and the whole book seems like that to me. We are watching the sparrow briefly flashing through our hall, a segment in the middle, and we don't know the beginning or end.