During my undergrad in biology, we were assigned to read biologist J. B. S. Haldane’s essay “On Being the Right Size”, collected in his 1927 book Possible Worlds and Other Essays.
The essay centers on the inextricable link between an organism’s size and its form; for example, the mechanics of a rabbit would not work at the scale of a hippopotamus, nor would the mechanics of a whale work at the scale of a herring. Many examples are based on the square-cube law, which describes the relationship between surface area and volume. As an organism’s size grows, its volume grows much more rapidly than its surface area. This has all kinds of biological consequences for the types of organisms that make sense at each size, which the essay explores.
Two of the examples Haldane gives are surface tension and gravity.
Small animals have much to fear from water: a wet mouse must carry its own body weight in water; a wet fly must carry many times its own body weight. But a person coming out of the bath will not perceive the weight of the thin film of water he or she carries.
Gravity is the opposite. Here Haldane gives a particularly vivid illustration, which is probably the main reason why the essay has stuck with me:
Gravity, a mere nuisance to Christian, was a terror to Pope, Pagan, and Despair. To the mouse and any smaller animal it presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away. A rat would probably be killed, though it can fall safely from the eleventh story of a building; a man is killed, a horse splashes.
(The first sentence is a reference to the giants in The Pilgrim’s Progress, forced to support one thousand times the weight of a man on bones with only one hundred times the cross-sectional area.)
The end of the essay turns, as essays are wont to do, toward the wider world. In this case, politics:
And just as there is a best size for every animal, so the same is true for every human institution. In the Greek type of democracy all the citizens could listen to a series of orators and vote directly on questions of legislation. Hence their philosophers held that a small city was the largest possible democratic state. The English invention of representative government made a democratic nation possible, and the possibility was first realized in the United States, and later elsewhere. […] To the biologist the problem of socialism appears largely as a problem of size. The extreme socialists desire to run every nation as a single business concern. […] But while nationalization of certain industries is an obvious possibility in the largest of states, I find it no easier to picture a completely socialized British Empire or United States than an elephant turning somersaults or a hippopotamus jumping a hedge.
Haldane was in fact a prominent socialist, later an official member of the Communist Party of Great Britain, which he left in 1950. Tensions over Lysenkoism and other anti-scientific policies pursued by the Soviet Union contributed to his departure.
This idea—that size constrains the forms available to an organism—came to be known as Haldane’s principle. As Haldane himself did at the end of the essay, later thinkers such as Jane Jacobs applied the same idea beyond biology to human institutions.