Both Sigmund Freud and Erasmus Darwin had important effects on the evolution of the modernist movement in the arts in the early twentieth century. While many early modern artistic genres had often treated humans as unified subjects, located at the center of the universe, created in the image of the God the Creator, and acting according to rational free will, Freud and Darwin both undermined this vision, one on the level of the individual and one on the level of the species, influencing the development of modernism.
Freud's challenge to pre-modernist thought was his concept of the subconscious, the notion that many of our actions are not motivated by the rational thought processes accessible to our conscious minds but rather based on the primal needs of the id, the socially constructed superego, and the forgotten traumas of our childhoods. Rather than having free will and making decisions as rational agents, we are at the mercy of forces within our own psyche which we perceive at best only dimly.
What Freud accomplished on the level of the individual, Darwin accomplished on the level of the species, positing that we are all the unwitting agents of evolution, acting to preserve our species.
Modernism reacted to these notions by creating artistic forms in which individuals were no longer portrayed as rational agents employing free will, but rather as the victims of forces they did not fully understand, inexorably swept along in a tide of evolutionary necessity or hidden psychological motives. In light of this, modernists engage in various formal innovations, breaking down the appearance of rational structure and causation, and de-centering the human subject.