O'Neill's Desire Under the Elms is indeed an example of naturalism in theater--or--naturalistic theater. In fact, critics acknowledge this play as O'Neill’s last naturalistic play. The characteristics of naturalism in theater are several. Naturalism is built from Darwinian principles which accord animalistic and subconscious forces to humanity and human behavior. Originated by Emil Zola, naturalism endeavors to reveal the affects of determinism so that a person's fate is seen as governed by naturalistic heredity and environment. Naturalism eschews (i.e., avoids) the psychological manipulation of symbolic characters who are created to be sympathetic and prefers characters whose instincts and drives lead them into encounters with ungovernable fate, the objective being to reveal how believable characters behave and react to unknown or novel or extreme situations.
Desire Under the Elms fulfils each of these defining characteristics. The characters, from Eben to Abbie, all act based on animalistic and subconscious drives: There are no higher motives to temper desire and instinct. Darwinian subconscious forces are at work in each instance of action, such as Eben's manipulation of Ephraim and Abbie's infanticide. The underlying desire for revenge that Eben claims at the start of the play ("her vengeance ... so’s she kin rest quiet in her grave") and the remote setting, devoid of any civilizing influence, underscore the idea that Ephraim's fate of eventual exile on the farm is the result of hereditary and environmental forces. It can realistically be argued that the characters in this play are not made to appear sympathetic through psychological or symbolic treatment of their development--they appear in their crudest reality and work out their animalism-driven fates without a blush or a sigh. O'Neill shows how individuals operating at a naturalistic Darwinian level behave without higher order motivations in the extremely compelling and novel circumstances on Ephraim's farm.