Darwin built his theory of descent with modifications from many quarters. He took uniformitarianism from the geologist Charles Lyell, the struggle for existence from the economist Thomas Malthus, and homology from a number of continental biologists. Perhaps most surprising is his debt to a theologian, William Paley. At university, Darwin had Paley’s Natural Theology (Darwin, 1887b) almost by heart.

Paley pointed to the complexity of organisms and claimed that such complexity required a supernatural intelligence. Darwin’s chief achievement was to provide a scientific explanation for adaptive complexity. Paley had famously built his argument from a rock and a clock (Paley, 1802). A stone, he argued, did not beg for any special explanation. It was simple, predictable, unchanging, devoid of obvious purpose. It might have been put there by some intelligence, but nothing about it begged for that explanation. A watch told a different story. The gears, levers, and springs work together in intricate harmony, causing the hands to move across the labeled face and measure time. Such complexity of design or purpose could not arise by chance. The watch must have had a designer, a watchmaker. Paley then applied the argument to organisms and their parts. The eye has a complex arrangement of parts that have a clear purpose, endowing its bearer with sight, and such complexity of purpose seemed to imply a designer and a maker. Throughout the rest of the book, Paley polishes the argument and applies it to other cases, including the sting of the worker honey bee, which he called a neutral bee.

Darwin won the argument with Paley long ago. Both had candidate explanations for complexity, but only Darwin also described a natural mechanism for adaptation and a natural explanation for the changes observed in fossils. Only Darwin explained aspects of biology that were nonadaptive consequences of history, from vestigial organs and other homologies to biogeographical patterns. Our understanding that organisms are a mix of historical constraint and adaptation by natural selection has led to many successful predictions about the natural world, whereas Paley’s theory stands mute about the details. In other words, Darwin’s theory is much richer than a simple explanation for design; it makes many further extensions and predictions. Some of these extensions and predictions were not fully appreciated in Darwin’s time. The last several decades have seen increased attention to a further important question