Clay's Ark

Butler's main theme in this book is material determinism: the body determining behavior. It's a theme that recurs in some of her other stories as well. It isn't a question of nature, nurture or genes in Clay's Ark however, and that adds to the novel's strength. The microscopic aliens serve as the perfect metaphor for molecules influencing human behavior, and Butler doesn't get stuck in the quagmire of trying to categorize causal chains and drawing moral conclusions from that.