The problem that occupied all the Renaissance architects was how to restore the manner of ancient Rome as far as possible, adapting it to the modern requirements of ecclesiastical, civic, and domestic buildings. Of Greek art they knew comparatively nothing: nor indeed could Greek architecture have offered for their purpose the same plastic elements as Roman--itself a derived style, admitting of easier adjustment to modern uses than the inflexibly pure art of Greece. At the same time they possessed but imperfect fragments of Roman work. The ruins of baths, theatres, tombs, temple-fronts, and triumphal arches, were of little immediate assistance in the labour of designing churches and palaces. All that the architects could do, after familiarising themselves with the remains of ancient Rome, and assimilating the spirit of Roman art, was to clothe their own inventions with classic details. The form and structure of their edifices were modern; the parts were copied from antique models. A want of organic unity and structural sincerity is always the result of those necessities under which a secondary and adapted style must labour; and thus the pseudo-Roman buildings even of the best Renaissance period display faults similar to those of the Italian Gothic. While they are remarkable for grandeur of effect in all that concerns the distribution of light and shade, the covering and enclosing of space, and the disposition of masses, they show at best but a superficial correspondence between the borrowed forms and the construction these are used to mask.[28] The edifices of this period abound in more or less successful shams, in surface decoration more or less pleasing to the eye; their real greatness, meanwhile, consists in the feeling for spatial proportions and for linear harmonies possessed by their architects. Three periods in the development of Renaissance architecture may be roughly marked.[29] The first, extending from 1420 to 1500, is the age of experiment and of luxuriant inventiveness. The second embraces the first forty years of the sixteenth century. The most perfect buildings of the Italian Renaissance were produced within this short space of time. The third, again comprising about forty years, from 1540 to 1580, leads onward to the reign of mannerism and exaggeration, called by the Italians _barocco_. In itself the third period is distinguished by a scrupulous purism bordering upon pedantry, strict adherence to theoretical rules, and sacrifice of inventive qualities to established canons. To do more than briefly indicate the masterpieces of these three periods, would be impossible in a work that does not pretend to treat of architecture exhaustively: and yet to omit all notice of the builders of this age and of their styles, would be to neglect the most important art-phase of the time I have undertaken to illustrate.