The chief thing which the imitative arts add to nature is permanence, the lack of which is the saddest defect of many natural beauties. The forces which determine natural forms, therefore, determine also the forms of the imitative arts. But the non-imitative arts supply organisms different in kind from those which nature affords. If we seek the principle by which these objects are organized, we shall generally find that it is likewise utility. Architecture, for instance, has all its forms suggested by practical demands. Use requires our buildings to assume certain determinate forms; the mechanical properties of our materials, the exigency of shelter, light, accessibility, economy, and convenience, dictate the arrangements of our buildings. Houses and temples have an evolution like that of animals and plants. Various forms arise by mechanical necessity, like the cave, or the shelter of overhanging boughs. These are perpetuated by a selection in which the needs and pleasures of man are the environment to which the structure must be adapted. Determinate forms thus establish themselves, and the eye becomes accustomed to them. The line of use, by habit of apperception, becomes the line of beauty. A striking example may be found in the pediment of the Greek temple and the gable of the northern house. The exigencies of climate determine these forms differently, but the eye in each case accepts what utility imposes. We admire height in one and breadth in the other, and we soon find the steep pediment heavy and the low gable awkward and mean. It would be an error, however, to conclude that habit alone establishes the right proportion in these various types of building. We have the same intrinsic elements to consider as in natural forms. That is, besides the unity of type and correspondence of parts which custom establishes, there are certain appeals to more fundamental susceptibilities of the human eye and imagination. There is, for instance, the value of abstract form, determined by the pleasantness and harmony of implicated retinal or muscular tensions. Different structures contain or suggest more or less of this kind of beauty, and in that proportion may be called intrinsically better or worse. Thus artificial forms may be arranged in a hierarchy like natural ones, by reference to the absolute values of their contours and masses. Herein lies the superiority of a Greek to a Chinese vase, or of Gothic to Saracenic construction. Thus although every useful form is capable of proportion and beauty, when once its type is established, we cannot say that this beauty is always potentially equal; and an iron bridge, for instance, although it certainly possesses and daily acquires aesthetic interest, will probably never, on the average, equal a bridge of stone. _Form and adventitious ornament._