Why did Linnaeus classify whales as fish?

The premise that Carolus Linnaeus classified whales as fish is a common historical observation regarding early taxonomy. In the first edition of his landmark work, Systema Naturae (1735), Linnaeus grouped whales with fish primarily based on their aquatic habitat and streamlined, fish-like morphology, which are examples of convergent evolution rather than shared ancestry.12

At the time, biological classification relied heavily on observable anatomical characteristics and environmental niches. Because whales possess flippers and live entirely in the water, they were traditionally associated with fish in early systems of biological classification. It was only through later scientific advancements and a deeper understanding of mammalian physiology—such as the fact that whales breathe air with lungs, are warm-blooded, and nourish their young with milk—that they were correctly reclassified as mammals.2