The poem begins with an invitation: look at how much everything turns. A door swings on its hinge, water circles and flows, stories change shape, and whole ages give way to new ones. Change is the one constant, and the speaker wants us to notice it.
Then the tone darkens a little. The world is "laboring / to eclipse us," as if its slow motion is quietly working to cover us over, to outlast human lives and concerns. It's a humbling thought, though the speaker doesn't sound panicked.
The last image softens everything. A dove and a wren are "pleased with our chimney," happily making use of a human structure. The title, "Natural selection," seems to hint at this: we may be eclipsed, but the world goes on making homes in what we leave behind. The mood is calm, observant, and gently accepting, with a touch of wry humor about our small place in things.