‘NATURE PROCEEDS . . . by such small steps.’ Or, to express the same thought obversely and in Latin, Natura non facit saltum – nature does not jump. The tag is familiar. It was one of Darwin’s favourite slogans; in the Origin alone it appears seven times. Huxley, famously, thought it a needless weakness of the theory.* It is a recurring motif in Aristotle too: explicitly when he speaks of plant-like sponges or how, in some animals, bone seems to blend into fishbone; implicitly when he says that snakes are elongate lizards (are, indeed, their ‘kindred’), that seals are ‘deformed’ tetrapods and
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