You could say that this book consists of variations on the theme of variation. The first section of the book is a crash course in basic statistics.
Scientists use statistics to summarise variation as best they can.
Each measure has its strengths and weaknesses. Throughout, he stresses that statistics are an abstraction and that variation is the reality. He uses as examples of the median and the mean his diagnosis with cancer in the early 1980s, the evolutionary bush of horses, and trends in baseball, adding some innovative statistics of his own to show that the decline in batting averages in the past fifty years is more apparent than real. In the last chapter Gould speculates on human limits, most interestingly in the creative arts, where, he feels, we may be reaching a point where further novelty or originality is not possible. One of the reasons I like reading Stephen Jay Gould is that he leaves loopholes, something to think about or to disagree with.