The realism and naturalism movements are extremely similar in that both depict everyday characters in real-life situations in which conflicts are psychologically driven. Both realism and naturalism also make use of realistic costumes and indoor stage settings that are believable. Yet, one way in which they differ is that naturalism is based on Darwinian ideas of evolution from a shared ancestry and of survival of the fittest ("Naturalism," Northern Virginia Community College).
In accordance with Darwinism, naturalistic works focus on themes that show human beings are controlled by their environment and genetics. Plus, since human beings are controlled by their environment, people can take no responsibility for their actions because the environment is a force that is beyond control. To capture the Darwinian theory of survival of the fittest, naturalistic works also develop themes concerning the individual's need to fight against the grain of society in order to progress ("Naturalism"). To capture human beings as prisoners of their environment, fighting against the grain of society in order to progress, naturalistic works depict characters in states of deprivation such as poverty, prostitution, or emotional agonies leading to suicide.
August Strindberg's play Miss Julie can be classified as a naturalistic play because it captures these Darwinian beliefs through the characters of Julie, daughter of a count, and Jean, valet to Julie's father.
Jean was born a commoner but longs to be an aristocrat, as depicted in the recurring dream he relays to Julie of climbing a tree to reach the nest of golden eggs atop. To achieve his desire, he is willing to leave his fiancée Kristin to run off with Julie, open up his own hotel, and purchase a title as a count. Yet, when Julie suggests they commit suicide, Jean changes his mind, tells Julie to run off on her own, and returns to his born destiny of servitude. Jean's inability to climb out of his state of servitude shows he is completely controlled by his environment and genetics, a theme found throughout naturalistic literature.
Just like Jean, Julie is actually descended of commoners. The only reason she is considered an aristocrat is because her family's title was earned through sexual favors performed by one of her ancestors. As Jean explains it, "The founder of [Julie's] family... was a miller whose wife found favor with the king during the Danish War." In other words, as recompense for seducing the miller's wife, the king granted the miller the title of count. The fact that Julie actually comes from common, humble beginnings helps explain why, during the course of the play, she prefers to socialize with the servants. When socializing with the servants, Julie is forced by her environment to return to her humble roots. Plus, through seducing Jean and behaving, as Jean puts it, like a "whore," Julie symbolically returns to her natural class status of commoner. Julie's symbolic fall from the status of aristocrat to the status of commoner shows she, too, is nothing more than a product of her environment and her genes. Plus, characteristic of many naturalistic works, Julie ends by committing suicide to escape the fact that she has fallen in status by giving herself to a servant.