Every building when reduced to its elements, as will be done in these papers, may be considered as made up of its (1) floor or plan, (2) walls, (3) roof, (4) openings, (5) columns, and (6) ornaments, and as marked by its distinctive (7) character, and the student must be prepared to find that the openings are by no means the least important of these elements. In fact, the moment the method of covering openings was changed, it would be easy to show, did space permit, that all the other elements, except the ornaments, were directly affected by the change, and the ornaments indirectly; and we thus find such a correspondence between this index feature and the entire structure as renders this primary division a scientific though a very broad one. A division of buildings into such great series as these can not, however, supersede the more obvious historical and geographical divisions. The architecture of every ancient country was partly the growth of the soil, _i. e._, adapted to the climate of the country, and the materials found there, and partly the outcome of the national character of its inhabitants, and of such influences as race, colonization, commerce, or conquest brought to bear upon them. These influences produced strong distinctions between the work of different peoples, especially before the era of the Roman Empire. Since that period of universal dominion all buildings and styles have been influenced more or less by Roman art. We accordingly find the buildings of the most ancient nations separated from each other by strongly marked lines of demarcation, but those since the era of the empire showing a considerable resemblance to one another. The circumstance that the remains of those buildings only which received the greatest possible attention from their builders have come down to us from any remote antiquity, has perhaps served to accentuate the differences between different styles, for these foremost buildings were not intended to serve the same purpose in all countries. Nothing but tombs and temples have survived in Egypt. Palaces only have been rescued from the decay of Assyrian and Persian cities; and temples, theaters, and places of public assembly are the chief, almost the only remains of architecture in Greece.