109. But there is a still more important reason for it, of a directly contrary nature to that which its formality would seem to require. In all beautiful designs of exterior descent, a certain regularity is necessary; the lines should be graceful, but they must balance each other, slope answering to slope, statue to statue. Now this mathematical regularity would hurt the eye excessively in the midst of scenes of natural grace, were it executed in bare stone; but, if we make part of the design itself foliage, and put in touches of regular shade, alternating with the stone, whose distances and darkness are as mathematically limited as the rest of the grouping, but whose nature is changeful and varied in individual forms, we have obtained a link between nature and art, a step of transition, leading the feelings gradually from the beauty of regularity to that of freedom. And this effect would not be obtained, as might at first appear, by intermingling trees of different kinds, at irregular distances, or wherever they chose to grow; for then the design and the foliage would be instantly separated by the eye, the symmetry of the one would be interrupted, the grace of the other lost; the nobility of the design would not be seen, but its formality would be felt; and the wildness of the trees would be injurious, because it would be felt to be out of place. On principles of composition, therefore, the regular disposition of decorative foliage is right, when such foliage is mixed with architecture; but it requires great taste, and long study, to design this disposition properly. Trees of dark leaf and little color should be invariably used, for they are to be considered, it must be remembered, rather as free touches of shade than as trees.