The first is a historical study of classical evolutionary thought, drawing extensively upon primary documents; the second is a constructive critique of the modern synthesis, and presents a case for an interpretation of biological evolution based largely on hierarchical selection, and the theory of punctuated equilibrium (developed by Niles Eldredge and Gould in 1972).

According to Gould "substantial changes, introduced during the last half of the 20th century, have built a structure so expanded beyond the original Darwinian core, and so enlarged by new principles of macroevolutionary explanation, that the full exposition, while remaining within the domain of Darwinian logic, must be construed as basically different from the canonical theory of natural selection, rather than simply extended."

In the arena of efficacy he explores the forces beside natural selection that have been considered in evolutionary theory.

In the arena of scope he considers the relevance of natural selection to the larger scale patterns of life.

Sections of the book dealing with punctuated equilibrium, primarily chapter nine, have been posthumously reprinted as a separate volume by Belknap Harvard.