Haldane's Rule of the Right Size

J. B. S. Haldane wrote "On Being the Right Size" for Harper's Magazine in March 1926, while Reader in Biochemistry at the University of Cambridge. The essay argued that gravity, surface tension, and diffusion impose strict scaling constraints that determine each organism's optimal size, and that what works at one scale fails at another. It was collected in Possible Worlds and Other Essays (1927) and has since been cited in biology, urban planning, and architectural theory.