The process of evolution in the several Georgian types of the Philadelphia neighbourhood was slow in its working, perhaps, but unmistakable as a comparison of examples will show. Indeed, a glance at the illustrations accompanying this chapter will discover easily distinguished differences of contour and detail corresponding to the evolutionary stages. Fortunately, history comes to aid us, removing all element of conjecture and giving us, instead, a comfortable certainty of the ground we are treading on. It is, of course, impossible to set any exact and unalterable dates for our three Georgian types; our purpose will be best subserved by giving approximate dates between which certain characteristics may be looked for and certain changes expected to take place. We may, roughly speaking, say that the first type flourished between 1720 and 1740, the second type from 1740 to 1770 and the third type from 1770 to 1805. Several parts of these three type divisions were marked by times of great building activity and others again by times of comparative idleness. From 1720 to 1730 there was a great deal going on. Then again, about 1760, we find a regular epidemic of house construction breaking out. Just before, during and after the Revolutionary War, as one would naturally assume, public stress, peril and uncertainty discouraged the prosecution of new plans, although the builders, even then, were not wholly idle. What has just been said applies particularly to country seats, as we have fuller data concerning them than we have about most of the town houses. What were once country seats have been selected, too, because they are, for the most part, intact, while comparatively few of the town houses remain in their original interior state, being, as they chiefly are, in a part of the city now given over to business or to the housing of the foreign population.