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._

Sec. 41. Beauty of form is the last to be found or admired in artificial
as in natural objects. Time is needed to establish it, and training
and nicety of perception to enjoy it. Motion or colour is what first
interests a child in toys, as in animals; and the barbarian artist
decorates long before he designs. The cave and wigwam are
daubed with paint, or hung with trophies, before any pleasure is
taken in their shape; and the appeal to the detached senses,