One of the phrases frequently used in the defintion of naturalism is the indifference of nature. The Romantics viewed nature (or Nature, as they would probably write it) as restorative and somehow deeply connected and important to the spiritual health of humans. The naturalist, by contrast, viewed the natural world as a place of Darwinian struggle for survival.
The link cited below quotes from Stephen Crane's short story "The Open Boat." I last read this story years ago, but I remember well that the story contains repeated phrases about nature (esp. the ocean) being "indifferent" toward the men struggling to stay alive.
I'm not as familiar with Thomas Hardy's poem "Hap," but I suspect that naturalism shows itself in at least two places in the short poem. According to the speaker, there is no divine consciousness or design ("If but some vengeful god..."); instead what we have is scientific cause-and-effect ("Crass Causality").