It is to be observed that while in its various isolated developments, art has proceeded from the immature to the mature, there has been no general evolution, as in natural life, but on the other hand there seems to be a limit to its progress. So far as our imagination can divine, no higher reaches in art are attainable than those already achieved. The mind can conceive of nothing higher than the spiritual, and this cannot be represented in art except by means of form; while within the range of human intelligence, no suggestion of spiritual form can rise above the ideals of Phidias. Of the purely human form, nothing greater than the work of Praxiteles and Raphael can be pictured on our brains. There may be poets who will rival Homer and Shakespeare, but it is exceedingly doubtful. In any case we must discard the law of evolution as applicable to the arts, with the one exception of music, which, on account of the special functioning of its signs, must be put into a division by itself.[a] [a] See Chap. III. But although there has been no general progression in art parallel with the growth of the sciences and civilization, there have been, as already indicated, many separate epochs of art cultivation in various countries, sometimes accompanied by the production of immortal works, which epochs in themselves seem to provide examples of restricted evolution.[4] It is desirable to refer to these art periods, as they are commonly called, for the purpose of removing, if possible, a not uncommon apprehension that they are the result of special conditions operating an æsthetic stimulus, and that similar or related conditions must be present in any country if the flame of art there is to burn high and brightly.[5] The well-defined periods vary largely both in character and duration, the most important of them--the Grecian development and the Italian Renaissance--covering two or three centuries each, and the others, as the French thirteenth century sculpture expansion, the English literary revival in the sixteenth century,[6] and the Dutch development in painting in the seventeenth, lasting only a few decades. These latter periods can be dispensed with at once because they were each concerned with one art only, and therefore can scarcely have resulted from a general æsthetic stimulus. But the Grecian and Italian movements applied to all the arts. They represented natural developments from the crude to the advanced, of which all nations produce examples, and were only exceptional in that they reached higher levels in art than were attained by other movements. But there is no evidence to show that they were brought about by special circumstances outside of the arts themselves. While there were national crises preceding the one development, there was no trouble of consequence to herald the other, nor was there any parallel between the conditions of the two peoples during the progress of the movements. A short reference to each development will show that