In this volume the author surveys the many experiments using new molecular techniques that have revealed the enormous wealth of hereditary variation among individuals and have quantified the genetic changes that take place in the origin of new species. Dr. Lewontin proposes new theories to attack the problems which still confront the scientist. While a tremendous amount of variation has been revealed, a satisfactory explanation of the origin and maintenance of such variation is still lacking. It is not at all clear whether adaptive evolution makes use of the kind of genetic diversity that is now known to be so common. Populatin genetic theory, Dr. Lewontin observes, leads to conflicting conclusions about the forces operating on the variation, and it appears that current theory is inadequate to cope with the data. Noting that the interaction among genes in evolution is of primary importance in the interpretation of genetic change, he urges that theory needs to be developed which takes into account the evolution of the genome as a whole rather than the independent evolution of each gene. A book which summarizes in an unusually felicitous way the findings of the rapidly growing science of molecular evolution and points out new directions for its future.