How then can the subject be best treated with the object of presenting the main facts of the sanitary evolution of London, and deducing from them the lessons of experience and guidance for the future? Probably by a sort of compromise between these two methods--taking groups of districts instead of separate districts--and groups of matters pertaining to the public health, instead of separate subjects--and, furthermore, dealing with the whole subject in certain definite periods. Groups of parishes have already, for certain health purposes, been classified into central, eastern, northern, western, and southern. That classification can be adhered to here. And inasmuch as almost the only reliable statistics as to many matters relating to the public health are those afforded every decade by the census, the narrative can best be treated by taking decennial periods, and utilising the reliable information of the census for the deduction of conclusions which on any other basis might be unsound. This method, then, though in many respects imperfect, is adopted as probably the best for tracing the sanitary evolution of the great metropolis. Foremost among the central group, but standing by itself, and in the main outside the scope of the legislation, was the "City." To the description of its condition already given nothing need be added beyond the statement of the fact that the great economic forces at work therein were displaying their results in the "City" itself in very striking manner. Under their potent influence the population there had begun to rapidly decline. In 1851 it had been 127,533. In 1861 it had come down to 111,784. The number of inhabited houses was likewise rapidly declining. In 1851 there had been 14,483; in 1861 there were 13,218. Under the irresistible demands for greater business and trading accommodation, the inhabited houses there were being rapidly converted to the more profitable purpose of business offices, or warehouses. As the number of business premises and shops increased in a locality, so did the better-to-do residents leave it, and migrate to pleasanter or more healthy localities. Some of the houses thus vacated became promptly tenanted by numerous families of a lower, or even the lowest classes; until they too were converted to business purposes, and their inhabitants once more turned adrift to seek other habitation. Some of these people secured in the neighbouring parishes residence in one or part of one of those jerry-built and insanitary constructions which land-owners and builders were erecting as rapidly as possible upon any unbuilt ground which they owned, or which they could lay hands upon--the majority contented themselves with squeezing somehow into tenement houses already overcrowded.