Naturalism is a style of narrative which features a realistic approach to storytelling. In this realistic approach life, human nature, and behaviors are depicted with increased intensity and detail. This is because the purpose of naturalism is to look at life in a deterministic way; in a way where our environment (nature) has such a strong effect on us all that it renders us as its victims.
Lily Bart is an example of such a victim. Her environment, glitzy, rich and fashionable, contrasts dramatically with the reality of her life. She, who once was a rich heiress with every attribute for social success, is now a poor woman who still hangs around the fancy clique. Her only salvation is that of being still beautiful and still fun to be with. As a result, Lily is at their mercy, and at the mercy of her tragic flaws: she still loves money, she wants the lifestyle, but every chance for marriage is ruined by her personal pride.
Another example of naturalism in the novel is the exposure of injustice as a reality of life. In naturalist novels nobody gets their "just deserts". This is because, by describing the crudest aspects of life, a naturalist novel will show that life is never going to be fair. This is the reason why we have a mean and backstabbing character like Bertha Dorset enjoying life to the fullest at all times.
Do look at Mrs. George Dorset's pearls--I suppose the smallest of them would pay the rent of our Girls' Club for a year
This is while Lily, whose life went from riches to rags unfairly, never gets to fully enjoy the life that she dreams of and still shows more dignity than Bertha.
Naturalism is also evident in the description of Lily's life after the falling out with Bertha. Once completely removed from the fashionable clique, Lily endures for the first time the "dingy" life that her mother detested so much: living in a boarding home, working as a hat maker, living in a tiny room...all of this leading to her spiraling depression and, eventually, her addiction.
She had never been able to understand the laws of a universe which was so ready to leave her out of its calculations.
This is a view of humanity at its darkest hour. This is what makes it real; natural.
Perhaps the most evident example of naturalism comes at the ending of the novel when the ultimate tragedy of life takes place: Lily accidentally (or not?) overdoses on sleeping medication after a long battle with deception, depression, and unfulfilled wishes. All she wanted was a second chance at excelling at what she did best, which was to be ornamental. Yet, even this shallow dream proved to be too high to reach. Ironically, it is through Lily's slow but consistent social, physical, and psychological death that her true character is finally born. This does not take away, however, from the fact that she had a mundane and dingy death. This merely makes the tragedy of her life even more evident.