This discussion of the philosophy of evolutionary biology is based on the author's idea that species are not kinds of organisms but wholes composed of organisms - individuals in the broadest ontological sense. Although the book's primary focus is on species and speciation, it deals with a wide variety of other fundamental units and basic processes and provides a reexamination of the role of classification in biology and other sciences.

In explaining his individuality thesis, Michael T. Ghiselin provides extended discussions of such philosophical topics as definition, the reality of various kinds of groups, and how we classify traits and processes.

He develops and applies the implications for general biology and other sciences and makes the case that a better understanding of species and of classification in general puts biologists and paleontologists in a much better position to understand nature in general, and such processes as extinction in particular.