The two great divisions of Mediæval architecture are, _firstly_, that which preceded, and, _secondly_, that which followed the great transition of the latter half of the twelfth century. The whole may be viewed as the one great development of arcuated construction into a style of art, and its two great divisions are the round-arched and the pointed-arched styles. It is my purpose during the present session to limit myself very much to the former; but viewing it, not only in its own bearings, but also as the precursor of the latter. Though I intend to choose my illustrations almost wholly from buildings in our own country, it would be taking a very narrow view of our subject if we were to consider the great round-arched style otherwise than as a whole, and our own portion of it other than as a branch of that mighty bifurcated tree whose boughs, whether growing from its eastern or its western stem, spread themselves over the whole civilised world. It has been well remarked, by Mr. Freeman, in his _History of Architecture_, that the ancient Roman manner of building was essentially an _arcuated_ style, though its true character was artificially overlaid by the features belonging to the purely _trabeated_ style of Greece; and that the whole course of change through which it, in after ages, passed, may be described as the gradual throwing off the trabeated overlayings and the perfecting into an architectural style its vital germ,--the _arcuated system_. This process was carried on equally in the East as in the West, though under circumstances accidentally differing. The two great metropolises of the Christian Roman empire, commencing with the same architecture, gradually changed it into two distinct branches, though clearly belonging to the same great trunk. In _both_ the changes or developments took for their starting-point the architecture, not of Greece, but of Rome. In the West, they continued to follow the natural suggestions of that style, influenced deeply by the changed religion, and subsequently curbed and held down, first by the removal of the seat of government to Constantinople, and then by the continuous waves of the northern invaders who gradually brought down to a very low ebb the civilisation and arts of the Western empire. In the East, the influence of the Christian worship was at least equally deep; while the presence of the imperial court and government offered greater advantages to development, and the accidental preference for _domed_ construction gradually gave a wholly new tone to the general character of the architecture, while the proximity of ancient Greek remains had a very strong influence on the ornamentation.