This, the last period of Classic Art, comprehends a space of about 350 years, terminating with the transference of the seat of empire by Constantine, A. D. 306. Of this interval, however, only the smaller portion must be given to a taste even comparatively pure; for, great as were its resources, symptoms of the decay of art, continually increasing, are detected even from the first years of the imperial government. Without entering minutely into these gradations, the death of Hadrian, A. D. 138, may be assumed as including both the noblest erections and the better taste of the empire. That to this date, the essential characteristics of elegance and purity continued in a degree untainted, there is evidence in the works of Hadrian at Athens. Thus, during an interval of not less than 574 years, from Pericles to the last mentioned emperor, architecture, in this respect more fortunate than either sculpture or painting, flourished in splendor and excellence not greatly impaired. Of all the fine arts--poetry not excepted--architecture is the only one into which the Roman mind entered with the real enthusiasm of natural and national feeling. Success corresponded with the exalted sentiment whence it arose; here have been left for the admiration of future ages, the most magnificent proofs of original genius. This originality, however, depends not upon _invention_ so much as upon _application_ of modes. To the architectonic system, indeed, the Romans claim to have added two novel elements in their own Doric, or Tuscan, and Composite orders. But in the restless spirit of innovation which these betray, the alleged invention discovers a total want of the true feeling and understanding of the science of Grecian design. In this very desire of novelty, and in the principles upon which it was pursued, are to be traced the immediate causes of ruin to the art, while yet its resources were unimpaired. The Romans unfortunately viewed the constituents of the Greek orders, and even the orders themselves, as so many conventional ornaments, which might be changed or superseded on the laws of association, in the same manner as they were supposed to have been framed. This it is of importance to mark, for the very same have been the sources, and are still the operating causes, of inferiority in modern architecture. But the very opposite of all this is the case. Of this system, the Greeks, in the course of centuries, had founded what was conventional upon what is necessary; they had united beauty with science, by combinations the most pleasing to taste--because of this very union of effect and principle. Architecture, with them, was thus not more conventional than is every part of knowledge not immediately derived from sense--not more, for instance, than geometry; and its modes, therefore, as constituting one whole, became immutable, being only conventional, as expressions or representatives of truth.