Historical background
What is usually understood as modern theatre began to develop from the middle of the nineteenth century, when new philosophical ideas of realism and naturalism replaced the subjective traditions of the Romantic Movement. As a result, a stage style that had remained virtually unchanged for a century and a half underwent a radical shift.  Romanticism had been the predominant artistic movement in Europe from the late eighteenth century onwards, with an intense focus on the consciousness of the individual in terms of imagination, emotion and an intense appreciation of the beauties of nature. However, by the middle of the nineteenth century, the Romantic emphasis on emotion over reason and of the senses over intellect had given way to a much more objective and scientific way of examining the human condition. A number of factors contributed to this:
A year of revolutions in France, Germany, Poland, Italy, and the Austrian Empire during the so-called Springtime of the Peoples in 1848 showed that there was a widespread desire for political, social, and economic reform , Technological advances in industry and trade led to an increased belief that science could solve human problems . The working classes were determined to fight for their rights, using unionisation and strikes as their principal weapons . Romantic idealism was rejected in favour of pragmatism . The common man demanded recognition and believed that the way to bring this about was through action.These factors helped fuel the development of two major philosophical ideas, realism and naturalism , which resulted in a radical shift in theatrical presentation.
The terms realism and naturalism are closely linked but there are significant differences in what they mean in the theatre: Realism describes any play that depicts ordinary people in everyday situations , Naturalism is a form of realism that particularly focuses on how technology and science affect society as a whole, as well as how society and genetics affect individuals. Beginning in the early nineteenth century, realism was an artistic movement that moved away from the unrealistic situations and characters that had been the basis of Romantic theatre. The playwright Henrik Ibsen is regarded as the father of modern realism because of the three-dimensional characters he created and the situations in which he put them. People in the audience could relate to the activities occurring on stage and the individuals involved. Charles Darwin, in his book The Origin of Species theorised that only the fittest of any natural species would survive to pass on its genetic material. Thus in drama a naturalistic focus addresses subjects in a scientific manner. The writer becomes a disinterested party who observes and studies specimens as though in a laboratory. 
Similarities between realism and naturalism
Realistic and naturalistic plays depict events that could happen in real life, maybe even to members of the audience. Both genres focus on individuals and families in everyday situations. During the late 1800s and early 1900s, playwrights found ample subject matter for both genres as the sciences advanced and people struggled and fought against oppressive governing systems. 
Differences between realism and naturalism , Naturalism approached art in a more scientific, almost clinical, manner than realism . Realistic plays often had characters to whom the audience could relate and sympathise . Naturalistic plays, which were difficult to create and rarely popular, approached every element with the detachment of a scientist . Realistic plays could show characters breaking free from difficult situations and allow the audience to empathise with their plight. Naturalistic works, on the other hand, sought only to study the situation, characters and other factors without interpretation. As time went on, the two genres blended together so as to make it very difficult to classify plays as either completely naturalistic or totally realistic. Elements of both could and did exist alongside one another. 

 Naturalism and realism in France
Zola defined the naturalist movement in the preface to his novel Therese Raquin (1867). He stated that the writer's task was to dissect human nature and the environment with the clinical precision of a scientist. Zola's stage adaptation of Therese Raquin (1873) served as model for theatrical naturalism and influenced other French playwrights such as Henri Becque and Jean Jullien .  Jean Jullien