History of Classification

Classification schemes have changed dramatically across more than two thousand years of biological thought. The earliest schemes used only crude criteria — whether an animal was beneficial or harmful to humans — and one ancient system recognised just five broad groups: domestic, wild, creeping, flying and sea animals, based purely on habitat and outward appearance.

Aristotle (384–322 BC) produced the first genuinely systematic classification, in his History of Animals. He split all living things into Plants and Animals, and then classified animals by locomotion — walking (terrestrial), flying (aerial) and swimming (aquatic) — as well as by the presence or absence of red blood (Enaima versus Anaima). Because it relied on single observable traits, this system misgrouped unrelated organisms: birds, bats and flying insects were lumped together simply because they fly, even though flightless birds like ostriches and emus do not fit that logic, and frogs — gilled, aquatic tadpoles that metamorphose into air-breathing, land-capable adults — resisted any single-category placement. Despite these flaws, Aristotle's system persisted for roughly two thousand years, until about 1700. …