_Mortality._--In the civilised countries of Europe the death-rate in the twentieth century varies between 15 and 32 per mille. Among the chief causes of transient variations in the death-rate are: war, civil disorders, and rise in prices. A rise in the price of the necessaries of life affects the lower classes of the population more especially, but its influence upon the general death-rate is trifling. Death-rate varies in accordance with occupation. The lower classes have a higher death-rate than the upper; the weekly wage-earners have a higher death-rate than the rest of the population; and mortality is greater in the towns than it is in the country. Of late years, there has been a gradual decline in the death-rate, the decline in the towns being proportionately greater than the decline in the country districts; in this case also the decline must be attributed mainly to the advance in medical science. _The Productive Age and the Unproductive Age._--The distinction of the productive age from the unproductive age is a matter of great importance. The former extends from the age of 15 to the age of 65; the latter, the unproductive age, comprises the years before the age of 15 and those after the age of 65. Certain other classifications of the population, such as the distinction of those of an age for school-attendance from those at other ages (those of the former age comprising about one-sixth of the population), and the distinction between youthful and adult criminals, are of no interest in relation to our special inquiry. _Classification of the Population according to Age._--In the civilised countries of modern Europe, persons at ages of 0 to 10 comprise about 24 per cent. of the population; those at ages 10 to 20 comprise about 20 per cent.; and those at ages 20 to 30 comprise about 16 per cent. of the population. Those under the age of 15 years comprise about 30 per cent. of the population, each year of life corresponding to from 2 to 3 per cent.; infants (those under 1 year) making up 3 per cent., and each year of life after that only a little more than 2 per cent. At all ages under 15 the boys are more numerous than the girls. The age-pyramid of the population has a form which depends upon the birth-rate. When the birth-rate is higher, or the excess of births over deaths greater, the base of the pyramid is comparatively wide. Thus, in the majority of the civilised states of Europe, about 30 per cent. of the population consists of those under 15 years of age; but in France, where the birth-rate is exceptionally low, those under 15 years of age comprise a much smaller proportion of the total population.