Any serious ethical discussion of the enhancement of human nature must begin with a reasonably accurate picture of the causal-historical structure of the living world. In this Comment, I show that even biologically sophisticated ethical discussions of the biomedical enhancement of species and speciel natures are susceptible to the kind of essentialistic thinking that Lewens cautions against. Furthermore, I argue that the same evolutionary and developmental considerations that compel Lewens to reject more plausible conceptions of human nature pose equally serious problems for some prominent critiques of biomedical enhancement that presuppose the existence of a "given" biological potential that can be distorted by agentic cultural influences.