The disciplines of biology and the social sciences share common roots in history and yet have drifted so far apart that the demarcation line between them has become a contested boundary. The boundary shift between the natural and the social is becoming permanent: moves in either direction are subject to ideological rhetoric. Yet there is continual exchange across the line: metaphors are moving freely between biology and the social sciences. As messengers of meaning they become agents of change, forever undermining any attempt at fixing similarities and differences. This work offers an insight into the function of metaphors in mediating between two disciplinary cultures which represent and mould our views about nature and society, and the boundary between them. For professionals and students of history, philosophy and sociology of science, biology, and literary science alike.