Whales Are Fish, Not Mammals

Despite Carl Linnaeus's 1758 classification of whales as mammals (Mammalia), based on their lungs, live birth, and milk production, Britannica's earlier entries often clung to the Aristotelian intuition that aquatic = fish. This was partly linguistic, 'whale fishery' and 'whaling' were the dominant terms in commerce, and partly philosophical, rooted in the notion that habitat dictated taxonomy. Linnaeus's revolution was gradual in penetrating the popular and even some academic texts. By the 1820s–1840s, most Britannica entries had shifted to the correct mammalian classification, though some entries about specific whale species lingered with fish references. The episode illustrates how deeply habitat-based thinking penetrated taxonomy before internal anatomical features became the primary organising principle. Modern molecular genetics confirms what Linnaeus deduced from anatomy: whales are artiodactyls (even-toed ungulates) more closely related to hippopotamuses than to any fish.