In Hardy's naturalism, nature is not full of signs of God's purpose or wonder, but is largely indifferent to the fate of Hardy's characters. Protagonists and antagonists alike are crushed as randomly as one would an insect. Naturalism tries to be scientific and detached in its depiction of human life, and tends to dwell more on the negative aspects of existence: the misery, betrayals and suffering that can be as much a part of human experience as the happy outcomes novelists often produce. The naturalists found these positive endings phony and contrived. Life doesn't always resolve with a "happier ever after" conclusion.
In Tess, Hardy shows his naturalism by not contriving a happy ending for Tess. We as an audience yearn with her for a happier life and a good marriage to Angel after her early rape and pregnancy at the hands of Alec, but it quickly becomes clear that she has been filled with romantic illusions about who Angel is. He is not able to rise beyond his class and his own illusions about Tess's purity. The couple's mutual self-deceptions lead to tragedy for Tess. The idea that the illusions we hold can destroy us in the face of an indifferent universe is a recurrent theme of Hardy's later novels. A miracle doesn't save Tess, deserving as she is.
Thomas Hardy's novel Tess of the d'Urbervilles reflects naturalistic themes in both its plot structure and its literary techniques.
Naturalism in literature is not just a literary technique, but a more general ideological or philosophical position about the relationship of people to their external environments. In particular, Naturalism tends to portray character as overwhelmed by circumstance, and the individual's life as to a great degree determined by environment.
In terms of literary style, Tess of the d'Urbervilles reflects the themes of naturalism in the way in which it includes rich details of everyday life, with long, minute descriptions of everyday activities rather than just selected important moments. In setting, it reflects another major theme of Naturalism, namely an emphasis on the lives of people living in poverty and of the lower classes rather than focusing on the upper and middle classes. Finally, it shows Tess (and to a lesser degree Angel) being very much the victims of external circumstances, unable to escape the patterns of life associated with social class and environment. Rather than external factors and coincidences operating in a positive fashion, in Tess of the d'Urbervilles they tend to have negative impacts, grinding characters back down just as it appears that they may be able to escape their fates.