What do I mean by a slope?  That I hope to make clear in the course of this chapter and the next.  But, as readers may expect something to go on with, I will explain immediately that, though I recognise the continuity of the stream of art, I believe that it is possible and proper to divide that stream into slopes and movements.  About the exact line of division there can be no certainty.  It is easy to say that in the passage of a great river from the hills to the sea, the depth, the width, the colour, the temperature, and the velocity of the waters are bound to change; to fix precisely the point of change is another matter.  If I try to picture for myself the whole history of art from earliest times in all parts of the world I am unable, of course, to see it as a single thread.  The stuff of which it is made is unchangeable, it is always water that flows down the river, but there is more than one channel:  for instance, there is European art and Oriental.  To me the universal history of art has the look of a map in which several streams descend from the same range of mountains to the same sea.  They start from different altitudes but all descend at last to one level.  Thus, I should say that the slope at the head of which stand the Buddhist masterpieces of the Wei, Liang, and T’ang dynasties begins a great deal higher than the slope at the head of which are the Greek primitives of the seventh century, and higher than that of which early Sumerian sculpture is the head; but when we have to consider contemporary Japanese art, Graeco-Roman and Roman sculpture, and late Assyrian, we see that all have found the same sea-level of nasty naturalism.

By a slope, then, I mean that which lies between a great primitive morning, when men create art because they must, and that darkest hour when men confound imitation with art.  These slopes can be subdivided into movements.  The downward course of a slope is not smooth and even, but broken and full of accidents.  Indeed the procession of art does not so much resemble a river as a road from the mountains to the plain.  That road is a sequence of ups and downs.  An up and a down together form a movement.  Sometimes the apex of one movement seems to reach as high as the apex of the movement that preceded it, but always its base carries us farther down the slope.  Also, in the history of art the summit of one movement seems always to spring erect from the trough of its predecessor.  The upward stroke is vertical, the downward an inclined plane.  For instance, from Duccio to Giotto is a step up, sharp and shallow.  From Giotto to Lionardo is a long and, at times, almost imperceptible fall.  Duccio is a fine decadent of that Basilian movement which half survived the Latin conquest and came to an exquisite end under the earlier Palaeologi.  The peak of that movement rises high above Giotto, though Duccio near its base is below him.  Giotto’s art is definitely inferior to the very finest Byzantine of the eleventh and twelfth centuries, and Giotto is the crest of a new movement destined and doomed inevitably to sink to depths undreamed of by Duccio.