Genetic analysis of a White Leghorn closed flock apparently plateaued for egg production. Poultry Sci. 37: 565-580. rp = r6 hx hy + rE ex ey where, ex and ey are the square roots of l-h2x and l-h2y, respectively. Although many estimates of genetic, phenot

THE principles of selection for quanti-tative characters under artificial selection have been extensively explored theoretically. These theories are being gradually tested under laboratory condi-tions (Bell et al., 1955; Clayton et al. 1957a, b; Falconer 1953; and others). The concepts and techniques of these theories have been introduced to poultry breeding and many papers dealing with estimation of genetic parameters such as heritabili-ties and genetic correlations have been published during recent years. Most of the selection experiments in chickens have presented evidence that se-lection for several important quantitative characters resulted in steady improve-ment in the traits involved (see Lerner, 1950). Conclusions as to the effectiveness of selections, however, are not clear in most cases. Although the portion of such improvement which may be attributed to the selection of superior breeders cannot be determined precisely, there is little doubt that much of it is genetic. An ideal selection experiment would allow precise estimations of the effectiveness of selec-tion by use of control populations to evaluate the effect of extraneous environ-mental influences on the population means. Studies on the behavior of suc