On Being the Right Size
J.B.S. Haldane’s 1926 essay, collected in Possible Worlds and Other Essays. His central point: sheer size determines what bodily equipment an animal must have. A tiny insect can get oxygen by simple diffusion; a larger animal needs a circulatory system. The square-cube law means as you get bigger, your weight grows faster than your strength, so proportions have to shift. Haldane famously notes that a mouse could be dropped down a thousand-yard mineshaft and walk away, but a rat would be killed, a man broken, and a horse splashed — because terminal velocity scales with size, and surface area (for impact) doesn’t keep up.
The principle has been adopted outside biology — Jane Jacobs and Christopher Alexander both cited it to argue that human institutions, like animals, have a best size.
Haldane’s essay, collected in Possible Worlds and Other Essays.