[13] It is true that many scholars estimate a high-water mark for the Roman population in excess of two millions; and one daring authority, by throwing out suburbs _ad libitum_ into the Campagna, suburbs of which no trace remains, has raised the two to ten. The Colosseum could, no doubt, seat over 80,000 spectators; the circuit of the bench frontage of the Circus Maximus was very nearly a mile in length, and the Romans of Imperial times certainly used ten times as much water as the modern Romans. But, on the other hand, habits change, and Rome as it is defined by lines drawn at the times of its greatest ascendancy--the city, that is, enclosed by the walls of Aurelian and including all the regiones of Augustus, an enclosure from which there could have been no reason for excluding half or more of its population--could have scarcely contained a million. It would have packed very comfortably within the circle of the Grands Boulevards of Paris--the Paris, that is, of Louis XIV., with a population of 560,000; and the Rome of to-day, were the houses that spread so densely over the once vacant Campus Martius distributed in the now deserted spaces in the south and east, and the Vatican suburb replaced within the ancient walls, would quite fill the ancient limits, in spite of the fact that the population is under 500,000. But these are incidental doubts on a very authoritative opinion, and, whatever their value, they do not greatly affect the significance of these new great cities, which have arisen all over the world, as if by the operation of a natural law, as the railways have developed. [14] It will be plain that such towns must have clearly defined limits of population, _dependant finally on the minimum yearly produce of the district they control_. If ever they rise above that limit the natural checks of famine, and of pestilence following enfeeblement, will come into operation, and they will always be kept near this limit by the natural tendency of humanity to increase. The limit would rise with increasing public intelligence, and the organization of the towns would become more definite. [15] I owe the fertilizing suggestion of this general principle to a paper by Grant Allen that I read long ago in _Longman's Magazine_. [16] It is worth remarking that in 1801 the density of population in the City of London was half as dense again as that of any district, even of the densest "slum" districts, to-day. [17] Be it noted that the phrase "available area" is used, and various other modifying considerations altogether waived for the present. [18] Their temporary suppression of the specialist is indeed carried to such an extent that one may see even such things as bronze ornaments and personal jewellery listed in Messrs. Omnium's list, and stored in list designs and pattern; and their assistants will inform you that their brooch, No. 175, is now "very much worn," without either blush or smile.