At that moment a swallow, one among the last of the season, which had by chance found its way through an opening into the upper part of the tent, flew to and fro in quick curves above their heads, causing all eyes to follow it absently. (1.37)
The swallow (a small bird, for those of you not into bird-watching) is a distraction to the people in the furmity-tent. They absent-mindedly watch it fly around and forget about the wife auction for a few minutes.
Hardy uses a lot of bird imagery in his novels and poems, and the birds almost always mean something. What is this swallow doing? It enters the tent by accident and is trapped there. No one tries to help it, but the people are distracted and momentarily entertained by its struggles. Hmm...this could be a parallel for Susan, couldn't it? Of course, the bird manages to free itself, and Susan does not, so the parallel doesn't work entirely. What do you think?
The sun had recently set, and the west heaven was hung with rosy cloud, which seemed permanent, yet slowly changed. To watch it was like looking at some grand feat of stagery from a darkened auditorium. In presence of this scene, after the other, there was a natural instinct to abjure man as the blot on an otherwise kindly universe; till it was remembered that all terrestrial conditions were intermittent, and that mankind might some night be innocently sleeping when these quiet objects were raging loud. (1.84)
The narrator compares the beauty of the sunset with the ugliness of the wife sale, suggesting that it's only "natural instinct" to feel like humans are a "blot" on the universe for allowing something so horrible to take place. But the narrator then reminds us that nature isn't always so beautiful and peaceful either.
The dense trees of the avenue rendered the road dark as a tunnel, though the corn-land on each side was still under a faint daylight; in other words, they passed down a midnight between two gloamings. (4.25)
Susan and Elizabeth-Jane enter Casterbridge through a darkened avenue of trees, and it's like walking through "a midnight between two gloamings." ("Gloaming" is twilight or dusk.) Sounds like a pretty ominous way to enter the town, doesn't it?
The mellow air brought in the feel of imminent autumn almost as distinctly as if she had been in the remotest hamlet. Casterbridge was the complement of the rural life around; not its urban opposite. (9.1)
Casterbridge is a pretty sizable town, but it's not filled with mills and factories. It doesn't feel like a big city because it's surrounded by nature: rural, agricultural land.
Thus Casterbridge was in most respects but the pole, focus, or nerve-knot of the surrounding country life; differing from the many manufacturing towns which are as foreign bodies set down, like boulders on a plain, in a green world with which they have nothing in common. (9.26)
Casterbridge is connected to the surrounding country as strongly as the "nerve-knot," or brain, is connected to the body. It's not like the industrial cities up north, which share nothing with the countryside around them.
Casterbridge, as has been hinted, was a place deposited in the block upon a corn-field. There was no suburb in the modern sense, or transitional intermixture of town and down. It stood, with regard to the wide fertile land adjoining, clean-cut and distinct, like a chess-board on a green table-cloth. (14.29)
Although Casterbridge might be connected with the surrounding countryside more intimately than most large towns, it still is completely separate and distinct from the farmland that surrounds it.
Among the rest of the on-lookers were Elizabeth and her mother – the former thoughtful yet much interested, her eyes beaming with a longing lingering light, as if Nature had been advised by Correggio in their creation. (16.23)
Elizabeth-Jane is completely natural, as the narrator keeps assuring us, but she's so beautiful that she looks like she could have been painted by Correggio (a Renaissance Italian painter). Which is it: is she like nature or art?
This, she decided, was the best position after all; and thus she remained till a man's step was heard on the stairs. Whereupon Lucetta, forgetting her curve (for Nature was too strong for Art as yet), jumped up, and ran and hid herself behind one of the window-curtains in a freak of timidity. (22.79)
Unlike Elizabeth, Lucetta isn't so natural. She is like an artist herself, arranging her own body to create the greatest possible effect. But she still has enough "Nature" in her to be embarrassed and shy.
It was an odd sequence that out of all this wronging of social law came that flower of nature, Elizabeth. Part of his wish to wash his hands of life arose from his perceptions of its contrarious inconsistencies – of Nature's jaunty readiness to support bad social principles. (44.8)
The narrator suggests that what Henchard did – selling his wife – wasn't a violation of natural or moral law, but a "wronging of social law." What he did was wrong, but it didn't violate any natural laws. And so "Nature" doesn't punish him, but instead creates Elizabeth, "that flower of nature."