What do the opening line and closing line of "Dover Beach" have in common? What’s significant about this?

The last three lines of Matthew Arnold's "Dover Beach" are among the most quoted lines in English poetry. They reflect Arnold's belief—a belief shared by many in late Victorian England—that the slow decline of religion left the people of England on a "darkling plain" of ignorance, on which all were "confused" and filled with a sense of "struggle." The word "darkling" emphasizes the idea that the sun is setting on something; in this case, it is plunging the country into spiritual darkness as science becomes more significant in people's lives and the clear light of religious faith diminishes.