Professor J.S. McKenzie says "The growth of large cities constitutes perhaps the greatest of all the problems of modern civilization." While the city is a problem in itself, creating certain biological and psychological conditions which are new to the race, the city is perhaps even more an intensification of all our other social problems, such as crime, vice, poverty, and degeneracy. The city is in a certain sense a relatively modern problem, due to modern industrial development. While great cities were known in ancient times, the number was so few that the total population affected by city living conditions was comparatively small. Moreover, the populations of ancient cities have often been exaggerated. Probably at the height of its power, the population of Athens did not exceed 100,000; Carthage, 700,000; Rome, 500,000; Alexandria, 500,000; Nineveh and Babylon, 1,000,000. All the great cities of the ancient world practically disappeared with the fall of Rome. After Rome's fall, Constantinople was the only large city with over 100,000 population in all Europe for centuries. Down to 1600 A.D., indeed, there were only fourteen cities in all Europe with a population of over 100,000; and even in 1800, at the beginning of the nineteenth century, there were only twenty-two such cities. But at the end of the nineteenth century, in 1900, there were one hundred and thirty-six such cities in Europe, representing twelve per cent of the entire population. Moreover, while in 1800 less than three per cent of the total population of Europe lived in cities, in 1900 the total urban population was twenty-five per cent. Again, all of the great European capitals developed their present enormous population almost wholly within the nineteenth century. Thus, the population of London in 1800 was 864,000, while in 1901 it had reached 4,536,000, or in the total area policed, 6,581,000; the population of Paris in 1800 was 547,000, in 1901 it was 2,714,000; the population of Berlin in 1800 was only 172,000, in 1901 it was 1,888,000; the population of Vienna in 1800 was 232,000, in 1901 it was 1,674,000. These figures are cited to show that from four fifths to nine tenths of the growth of the greatest cities of the world has taken place within the nineteenth century.