Chapter 4 shows how The Four Zoas, as an unfinished manuscript, formally registers Blake’s troubled fascination with evolutionary models of the mind. The first section of the chapter compares the images of fluidity associated with Tharmas, who continually emerges from and dissolves into the waves of the unconscious, against Erasmus Darwin’s poetic descriptions of liquid ontogeny. The next section examines how the sexual drive appears in the text as a disruptive fluid force, illustrating and criticising the materialist argument (found in Mandeville and Malthus) that love and altruism are merely the evolutionary products of libidinal self-interest. The final section returns to the textuality of The Four Zoas and shows how the nervous mind and the sinuous text work together to give unreliable body to thought. Comparing Blake’s poetics to that of Erasmus Darwin and Edward Young, the chapter discusses the mimetic qualities of Blake’s revisionary verse and ends with an analysis of the poem’s fantasies of symbolic liberation through physical destruction.