 The mathematical models of quantitative genetics could be readily extended from selective breeding to evolutionary change by having theoretical genotypes from a large number of loci each contribute to parameters for surviving and leaving offspring—so-called selection coefficients. Data on the variation for a trait in a specific group or population could be analyzed so as to estimate the parameters in the model that would generate the observed changes in the average value of the trait over time. Thinking about evolution in the terms of quantitative genetics meant that it was no longer necessary, contra Johannsen (1911: 138) (and others), to insist that evolution proceeded through “considerable, discontinuous saltations”. Notice, again the separate theoretical genotypes and their contributions, this time to selection coefficients, remain unobserved; the focus of the data analysis could be on differences in the trait, not the mechanisms of trait development. The complexity of developmental mechanisms, which involve interactions with the environment, was collapsed in the models into the selection coefficients modulated by parameters for dominance between alleles (i.e., variants of a gene) within, and epistasis between, theoretical genotypes.