This essay considers the biologist Thomas Henry Huxley and his twentieth-century grandson Julian Huxley as cosmopolitans. Perhaps their foundational shared question was how to comprehend human unity and human difference, both biologically and politically; how to comprehend humans as one. Both Huxleys insisted on the singularity of the human species, but as evolutionary theorists insisted also on individual biological variation and distinction. For this reason, they offer the opportunity to consider the history of cosmopolitanism alongside the intellectual history of thought on species, and on the species: Homo sapiens. They were both deeply engaged with the idea of human unity—variously biological, cultural, political—while remaining confident about their own epistemological privilege and capacity to pronounce on humanity as a whole. The history of cosmopolitanism is ill-served by attempts to pinpoint the truest, purest, exponents. The Huxleys’ flawed metropolitan cosmopolitanism was perhaps the commonest sort in practice.