Each art has its own medium of expression and to this medium it absolutely adheres in its periods of greatness, adapting itself to its limitations, working within them, and even making them tributary to its excellence. Limitations are, after all, the greatest gift of God to man instead of being, as the last century feigned, something that deplorably existed only to be transcended; without them man would revert to the condition of the jellyfish or the primal ooze of the depths of the sea; with them he has an opportunity to demonstrate his divine elements through a development that is both in spite of and because of the firm lines they draw that cannot be overpassed. One of the great things in art is its revelation of the possibility of spiritual achievement by and through the narrowest physical limitations. When, on the other hand, architecture tries to assimilate the peculiar methods of bridge building; when painting intrudes into the province of sculpture, literature, photography, or even music; when music becomes mathematical or takes to itself the habits of the circus performer; when sculpture deliquesces into a sloppy kind of black-and-white illustration; when stained glass and tapestry become pictorial; when, in fact, all the arts forsake their own provinces and deny their own limitations, as they have tended to do during the last century, splashing over, the one into the other, they cease to be arts at all and become unprofitable aberrations. Only three great arts have come into existence during the last two thousand years--music, stained glass, and tapestry--and each developed its own exact and individual mode of expression. Music was as old as architecture, painting, sculpture, poetry, and the drama, but under the influences of Christianity it gradually transformed itself into what was almost a new art and one that has remained the only vital art through all the unfriendly influences of modern civilisation. Stained glass was an absolutely new art, taking its rise in the twelfth century, culminating in the thirteenth, decaying through the two following centuries, and entirely disappearing in the eighteenth century. It is an art of Christianity, of Frank genius, and of the Île de France. Tapestry is also a new art, beginning about 1350, culminating a century later, dying almost in a day (as a great art), about 1520. It also is Christian, but, unlike glass, it is primarily secular, and it is explicitly and almost exclusively Flemish, the great contribution of a distinguished race to the imperishable art of the world.