Every practical observer of the world around him knew that life develops spontaneously from nonliving matter by the action of heat, light, moisture, and (after it was discovered) electricity. Maggots come from decaying meat, and lice from sweat-soaked clothing. Beetles develop from rotting wood, and horseflies from transmuted manure.

It is difficult to put forward so thoroughly eroded an idea as spontaneous generation today without arousing smiles from the listeners. If ever a generally accepted idea was revealed by careful experiments to be nothing but old wives' tales, spontaneous generation was.

Francisco Redi demonstrated more than 300 years ago that meat, shielded from egg-laying flies by cloth, never developed maggots. Others following him showed that nutrient broths that are boiled and then kept isolated from airborne contamination never produce microorganisms.

Spontaneous generation died hard; its proponents claimed that the life forces were delicate and were destroyed by boiling. The early experiments were crude, and failed just often enough to keep the controversy alive.

This reluctance to abandon spontaneous generation was not an example of the obstinacy of the superstitious, but was the stubbornness of those who considered themselves defenders of the rational approach, and the only alternative to divine whimsy.