Darwin first raised the question of sex ratio evolution, and saw it as both important and enigmatic. He was, however, unable to make much headway with the problem and declared it a puzzle for the future. This particular future arrived about 60 years later, when R. A. Fisher (1930) pointed out that under autosomal inheritance half of the genes passed to zygotes in any generation come from males and half come from females. Fisher noted that this one-mother/one-father symmetry generates frequency­ dependent natural selection on sex ratio, resulting in an evolutionary equi­ librium in which half of the reproductive resources are devoted to daughters and half to sons. Although widespread interest in sex ratio as a phenotypic trait did not occur for another 30 years, it is difficult for us to overestimate the impor­ tance of Fisher's brief and characteristically cryptic remarks. Almost all of the innovations in thinking about sex ratio can be viewed as alterations of one or more implicit assumptions in Fisher's scheme. The present book on insect sex ratios is testimony to the fruitfulness of his original ideas and of their descendants.