This brings us very close to the more general problem of conservation in the mind, which has so far hardly been discussed, but is so interesting and important that we may take the opportunity to pay it some attention, even though its relevance is not immediate. Since the time when we recognized the error of supposing that ordinary forgetting signified destruction or annihilation of the memory-trace, we have been inclined to the opposite view that nothing once formed in the mind could ever perish, that everything survives in some way or other, and is capable under certain conditions of being brought to light again, as, for instance, when regression extends back far enough. One might try to picture to oneself what this assumption signifies by a comparison taken from another field. Let us choose the history of the Eternal City as an example.[3] Historians tell us that the oldest Rome of all was the _Roma quadrata_, a fenced settlement on the Palatine. Then followed the phase of the Septimontium, when the colonies on the different hills united together; then the town which was bounded by the Servian wall; and later still, after all the transformations in the periods of the republic and the early Caesars, the city which the Emperor Aurelian enclosed by his walls. We will not follow the changes the city went through any further, but will ask ourselves what traces of these early stages in its history a visitor to Rome may still find to-day, if he goes equipped with the most complete historical and topographical knowledge. Except for a few gaps, he will see the wall of Aurelian almost unchanged. He can find sections of the Servian rampart at certain points where it has been excavated and brought to light. If he knows enough—more than present-day archaeology—he may perhaps trace out in the structure of the town the whole course of this wall and the outline of _Roma quadrata_. Of the buildings which once occupied this ancient ground-plan he will find nothing, or but meagre fragments, for they exist no longer. With the best information about Rome of the republican era, the utmost he could achieve would be to indicate the sites where the temples and public buildings of that period stood. These places are now occupied by ruins, but the ruins are not those of the early buildings themselves but of restorations of them in later times after fires and demolitions. It is hardly necessary to mention that all these remains of ancient Rome are found woven into the fabric of a great metropolis which has arisen in the last few centuries since the Renaissance. There is assuredly much that is ancient still buried in the soil or under the modern buildings of the town. This is the way in which we find antiquities surviving in historic cities like Rome.