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IN the absence of disturbing factors, a phenotypic trait whose genetic variance is determined by alleles at a single locus will respond to selection pressure until fixation of the most desirable allele is attained. Presumably similar exhaustion of genetic variation may account for deceleration and eventual cessation of gains from selection of a character under polygenic control. It is, however, also possible that other factors may interfere with the realisation of genetic gains long before the limitations imposed by the decrease in genetic variability enter the picture to a significant degree. Attention to a number of factors possibly responsible for this phenomenon has been drawn by Lush (I94), Dempster and Lerner (ig4), Lerner (1950) and others. In for a character not intrinsically connected with viability, rate of maturation, and fecundity is practiced, the individuals excelling in the desired trait and thus selected to be the parents of the next generation are not always those possessing the greatest fitness under natural selection. If they are actually less fit in the latter sense than individuals nearer the mean of the population with respect to the trait under artificial selection, it is clear that some of the artificial 75

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