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The present study grew out of an earlier inquiry 2 into the effects which selection of dams might have on sire indexes. That study showed the importance of knowing the degree to which the observed variations in a character are hereditary, if one is to predict the consequences of various breeding plans or to discount the effects of selection or other practices on sire indexes and on estimates of a cow's breeding worth. It also indicated that the heritability of differences in annual butter fat production was similar in two different kinds of data (Iowa D.ILI.A. prior to 1937 and lKolstein-Friesian H.I.R. prior to October, 1938) and was not very different from that indicated by several other studies. This suggested that heritability of butterfat production might be nearly enough constant in all cattle populations which are of much interest to dairy breeders, that there would be little error in using a single figure for it when, estimating the probable results of alternative breeding procedures.
Lush et al. (Sun,) studied this question.