A review treating statistically previously published data. An attempt has been made to portray the interaction of recognized biological factors leading to milk production. Heredity in its broad sense is found to be the primal cause of variation. It was further shown that the relative amount of milk secreted and the butter-fat content of the same were independent in their inheritance for Holstein-Friesian cattle. Finally the cow and her relatives were shown to be correlated in milk production, etc., as would be expected for Men-delian inheritance in its most generalized case, multiple factors and random mating.