Roman domes of earlier date are to be found, and that this style should, as it were, be born in perfect manhood without having passed through the stages of infancy and growth. These no doubt existed, although we have as yet been unable to trace them. Possibly, as Professor Baldwin Brown suggests, the dome is of eastern origin, and its enlarged construction may have been worked out in some of the Hellenistic cities, such as Alexandria, where the earlier examples have now perished. Along with the introduction of the above new and splendid development of vaulting in their interiors, the Romans still adhered in the decoration of their exteriors to the Italo-Grecian portico, with its entablature and pediment. It was not till the time of the Lower Empire that these elements came to be modified and slowly abandoned. The stages by which the trabeated forms were by degrees stripped off can, however, be distinctly traced. The arches and vaults employed in the baths, tombs, &c., no doubt conduced to that result. In these the arch became the important feature internally, and naturally in course of time it assumed a more prominent position externally also. Archivolts, or curved architraves running round the arches, such as were in common use in buildings like the Colosseum, had gradually intruded themselves amongst the Greek pilasters and entablatures of the exterior elevations; while in later edifices, such as the palace of Diocletian at Spalatro, the straight architrave was omitted, and only the arched one retained. The early Christian sarcophagi shew the same important step. In these a common design consists of an arcade containing the figure of an apostle in each arch, and these arches or archivolts spring directly from the caps of the columns, without any straight architrave being employed. Of this a good example has been given above, page 63. In all transitional styles it is difficult, and indeed scarcely possible, to draw the line where one style terminates and another begins. This is especially difficult in connection with the passage from Roman to mediæval architecture. The latter was in fact for centuries not a different style but simply a continuation of that of the Empire.