Since the birth-rate is persistently diminishing, interest in the laws by which nativity is governed is steadily increasing. This interest became still more intense when, on account of the war, a large number of human lives were destroyed, causing a decrease of population in many countries. Those who fell in this war happened to be men in the prime of life, a fact which manifested itself after the war in a decreased number of births, apart from the temporary increase of births in consequence of the return of the mobilised men to their native towns.