Charles Darwin's theory of natural selection, laid out in the 19th century, spreads far past biology into politics, religion, and how people think about being human. Andrew Marr traces that spread across three episodes, showing how the idea got used to justify imperial expansion and the treatment of indigenous peoples abroad, and eugenics programs built on selective breeding in the early 20th-century United States at home. Marr also follows a more hopeful thread, arguing that Darwin is effectively the father of ecology and that the modern environmental movement grew out of his insight that all life is bound together in a single web of dependence. The series closes on scientists now assembling what Marr calls a flotilla of Darwinian Noah's Arks, conservation projects built on evolutionary logic to keep species from disappearing. Marr moves between archival material and expert interviews to track one scientific idea as it hardens into ideology, then softens into environmental policy.