Galton has shown, worked great mischief by sterilising for centuries many of the gentlest and noblest in each generation; but this tendency was adventitious to Christianity, and would never have taken root on Palestinian soil. The cult of virginity has lasted on, with much else that belongs to the later Hellenistic age, in Catholicism. In the Middle Ages the population question slumbered. The miserable chaos into which the old civilisation sank after the barbarian invasions, the orgies of massacre and plunder, the almost total oblivion of medical science, and the pestiferous condition of the medieval walled town, which could be smelt miles away, averted any risk of over-population. Families were very large, but the majority of the children died. Millions were swept away by the Black Death; millions more by the Crusades. Such books as that of Luchaire, on France in the reign of Philip Augustus, bring vividly before us the horrible condition of society in feudal times, and explain amply the sparsity of the population. The early modern period contains another notable example of a sudden and unaccountable decline in population. The scene is Spain, which, after playing an active and very prominent part in the world's history, sank quickly into the lethargy from which it has never recovered. It may be noted that here, as in the case of Rome, the decay of population and energy followed a great influx of plundered wealth. On the other hand, the increase of population in our newly-planted North American colonies must have been extremely rapid for two or three generations. The enormous multiplication of the European races since the middle of the eighteenth century is a phenomenon quite unique in history, and never likely to be repeated.[16] It was rendered possible by the new labour-saving inventions which immensely increased the exports which could be exchanged for food, and by the opening up of vast new food-producing areas. The chief method by which the increase was effected, especially in the later period, has been the lengthening of human life by improved sanitation and medical science.[17] Since 1865 the average duration of life in England and Wales has been raised by a little more than one-third. Other European countries show the same ratio of improvement. This astonishing result, so little known and so seldom referred to, was bound to have a great effect on the birth-rate. So long as the swarming period continued at its height, a net annual increase of 15 or even 20 per thousand could be sustained; but the expansion of the European peoples has now passed its zenith, and a tendency to revert to more normal conditions is almost everywhere observable. One of the most advanced nations, France, has already reached the equilibrium towards which other civilised nations are moving. The old-established families in the United States are believed to be actually dwindling.