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.

(3) The birth rate of civilized countries has also fallen markedly
during the nineteenth century, especially during the latter half. On the
whole, this is a good thing. The birth rate should decrease with the
death rate. This leaves more energy to be used in other things; but when