(1) The population of all civilized countries, with one or two exceptions, has been increasing rapidly since the beginning of the nineteenth century. Previous to that time we have no statistics that are reliable, but it seems probable that the population of Europe stood practically stationary during the Middle Ages and increased only slowly down to the nineteenth century; but during the nineteenth century the population of the leading industrial nations has increased very rapidly. This is due primarily, without doubt, to improved economic conditions, which has made it possible for a larger population to subsist within a given area. Back of these improved economic conditions, however, has been increased scientific knowledge in ways of mastering physical nature, and accompanying them has been a very greatly decreased death rate, due in part at least to the advance of medical science. (2) This increase in population has been due, not to an increase in birth rate, but to a decreased death rate. During the nineteenth century the death rate decreased markedly in practically all civilized countries. As we have already noted, this is due primarily to improved living conditions, particularly in the food, clothing, and shelter for the masses, but it has also been due in no small part to the advance in medical science, and especially that branch of it which we know as "public sanitation." Because the death rate decreases with improved material, and probably also with improved moral conditions, it is a relatively good measure, at least of the material civilization or progress of a people. We may note that the death rate is measured by the number of deaths that occur annually per thousand in a given population. The death rate of the countries most advanced in sanitary science and in industrial improvement apparently tends to go down to about fifteen or sixteen per thousand annually.