In this book, Gould presents his thesis that contingency (luck) was one of the decisive factors in the evolution of life on earth. His device is the fauna of the Burgess Shale, animals from just after the Cambrian explosion, half a billion years ago.
Gould's point is that though the Burgess animals were exquisitely adapted to their environment, most of them left no descendant and, more importantly, the surviving creatures did not seem better adapted than their now dead contemporaneous neighbors. This seems to indicate that fitness does not ensure survival, and that less fit organisms are as likely to survive as organisms better adapted to their immediate environment.
Most of the book's conclusions were deemed controversial at publication and some of Gould's examples have already been shown incorrect. However, the ultimate theme of the book is still being debated among evolutionary thinkers today.