Those imposing agglomerations of houses and dwellers we call cities (in most cases political or commercial capitals) have shown a notable rate of progress during the last two or three decades. More and more do the centripetal forces at work in almost every nation make for the growth of the capital at the expense of the rural community. A century ago a million human beings dwelling side by side under a single municipal government was almost of itself one of the great wonders of the world. Men spoke of London with bated breath and wondered where it would all end. Reports of monster cities in China with a population double that of London were dismissed as travellers' tales. Travellers' tales, verily, they have proved to be, seeing that Peking even to-day has fewer than a million souls. But what would our forefathers have said of these twentieth-century "wens," these "gloomy or glowing, febrile and throbbing concentrations" of human life, numbering not merely two, but three, four, and even five millions of souls? [Illustration: LONDON: THE ADMINISTRATIVE COUNTY OF LONDON, WITH WHICH THE OTHER CITIES ARE COMPARED, IS SHOWN BY THE SHADED PORTION.] Let us take London as the basis of our diagrams. London is an indeterminate quantity. It may mean the City of London, which comprises only 673 acres, or it may mean the Administrative County of London, which boasts nearly 117 square miles, or 74,839 acres, or Greater London, which embraces the Metropolitan Police district, and has an area of no less than 692 square miles, or 443,420 acres. If we take the second of these Londons we shall find it to consist of twenty-nine large and small cities, ranging in population from 334,991 to 51,247 inhabitants. These are called the Metropolitan boroughs; but as it is rather geographical size than population which here concerns us, we may state that the largest of these boroughs is Wandsworth, with an area of 9,130 acres, and the smallest is Holborn, with 409 acres. The average area of these boroughs, if we exclude the City, is about four square miles. Within these borders of London--which must not be confounded with Greater London--there were in 1901 4,536,541 souls, living in 616,461 houses. Within this area, besides buildings, must be counted 12,054 acres of grass, including the public parks and gardens.