The window became one of the most important and characteristic features of Gothic buildings. These large open spaces gave opportunity for elegant shapes and splendid colors, both the form of the opening and the dividing ribs, or tracery, as it was called, being made with the utmost beauty and grace. The round windows, called rose windows and wheel windows, were often exquisitely designed, as the following example shows (Fig. 84). [Illustration: FIG. 84.--WHEEL WINDOW, FROM CATHEDRAL. _Toscanella._] The window is illustrative of the influence which climate may have on the development of architectural style. In warm countries where spaces were left open, window forms and painted glass were, of course, never employed; but in more northern lands they became one of the most marked features in important edifices. A whole book might be written about these windows and be very interesting also, but we can give no more space to them here. Gothic architecture gradually extended from the centre of Italy to the most northern bounds of civilization, and though practised by so many nations, was as much the architectural expression of a religion as the architecture of a single ancient nation had been the outgrowth of its peculiar religious belief. During the Middle Ages the priests and monks preserved learning in the midst of general darkness and ignorance, and were the chief patrons of all art which survived the decline of the time. They built up the Christian faith by every means in their power. The monks were missionaries. They went to various countries, and selecting favorable spots they founded abbeys; around these abbeys a poor population settled; gradually churches were built, and it frequently happened that the monks not only planned the work to be done, but also executed it with their own hands. Many of them were masons and builders, and several bishops were architects. St. Germain, Bishop of Paris, designed the church in that city now called by his name, and was also sent to Angers to build another church, and to Mans to erect a monastery. The finest buildings being thus made for religious purposes and under the direction of the clergy, they must have been as full an expression of Christianity as were the temple-palaces of Egypt an expression of the religion of Osiris and Isis, when the kings were both priests and sovereigns, and dwelt in these palaces. And this was true as long as Gothic art was in the hands of the clergy and used almost entirely for religious purposes.