From year to year the majority of those who discuss the causes of hypertensive cardiovascular disease list heredity as the chief, or at least a very important, factor. This view is doubtless due to two clinical observations: In the first place, families in which a high percentage of the members in several generations have died from cardiovascular disease are frequently seen; but a generation ago the same was true of tuberculosis and led to the widespread idea that consumption was hereditary. In the second place, in the history of a majority of families, cardiovascular disease appears in each succeeding generation with noticeable frequency, owing, of course, to the high incidence of the disease in the general population. If the incidence of any hereditary trait is as high as 40 per cent, more than half the families in the general population will show the trait in both parents and children whether