Gould maintains that, 'traits thatĮnhance survival during an extinction do so in ways that are incidental and unrelated to the causes of their evolution in the first place.' Gould earlier coined the term exaptation to describe fortuitously beneficial traits, which are adaptive but arise for reasons other than incremental natural selection. Because fitness for existing conditions does not guarantee long-term survival - particularly when conditions change catastrophically - the survival of many species depends more on luck than conventional features of anatomical superiority. Gould stressed that his argument was not based on randomness but rather contingency a process by which historical outcomes arise from an unpredictable sequence of antecedent states, where any change in the sequence alters the final result. Gould proposed that given a chance to 'rewind the tape of life' and let it play again, we might find ourselves living in a world populated by descendants of Hallucigenia rather than Pikaia (the ancestor of all vertebrates).

All of the Burgess animals, Gould argues, were exquisitely adapted to their environment, and there exists little evidence that the survivors were any better adapted than their extinct contemporaries. However most of these phyla left no modern descendants. Gould argues that during this period just after the Cambrian explosion there was a greater disparity of anatomical body plans ( phyla) than exist today. He based his argument on the extraordinarily well preserved fossils of the Burgess Shale, a rich fossil-bearing deposit in Canada's Rocky Mountains, dating 505 million years ago. Gould's thesis in Wonderful Life was that contingency plays a major role in the evolutionary history of life. Charles Doolittle Walcott (1850-1927), who discovered the Burgess Shale, with his children Sidney Stevens Walcott (1892-1977), and Helen Breese Walcott (1894-1965).

Burgess shale book