This episode looks at three pre-Darwinian accounts of the development of organic nature: the Great Chain of Being, Design arguments (Paley) and Jean-Baptiste Lamarck's 'transformatism'. All of these accounts assume, in one way or another, that the explanation of organic life requires the postulation of purpose and design in nature. This assumption is in striking contrast with mechanical explanations in astronomy and physics. The idea of 'evolution' was well accepted before Darwin. But Darwin's achievement consists in the articulation of a testable mechanism - natural selection - which explained evolution, without requiring design and purpose. The episode ends with a succinct description of Darwin's 'revolution'.
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