This paper argues that, for a number of naturalists and lay commentators in the second half of the nineteenth century, evolutionary – especially Darwinian – theory gave new authority to mythical creatures. These writers drew on specific elements of evolutionary theory to assert the existence of mermaids, dragons and other fabulous beasts. But mythological creatures also performed a second, often contrapositive, argumentative function; commentators who rejected evolution regularly did so by dismissing these creatures. Such critics agreed that Darwin's theory legitimized the mythological animal, but they employed this legitimization to undermine the theory itself. The mermaid, in particular, was a focus of attention in this mytho-evolutionary debate, which ranged from the pages of Punch to the lecture halls of the British Association for the Advancement of Science. Crossing social boundaries and taking advantage of a range of venues, this debate arose in response to the indeterminate challenge of evolutionary theory. In its discussions of mermaids and dragons, centaurs and satyrs, this discourse helped define that challenge, construing and constructing the meanings and implications of evolutionary theory in the decades following Darwin's publication.