IN breeding for improved egg production, as in breeding for improved milk yield, measurement can be made only in one sex. The male is always unknown, and this unknown 50% of the inheritance in each generation on the average drags back the progeny of a good producer towards the mean of the breed. It was found in practice, as was shown by the classic experiments of Pearl and Surface, that starting with an ordinary farm flock of birds laying about 100 eggs per annum and breeding only from the best layers made initial improvement, but that a ceiling of about 160 eggs per annum average was soon reached, above which selection in one sex only made no appreciable improvement.