Now it is possible to regard such spaces or panels as more or less unrelated, and simply as the boundaries of an individual composition or picture of some kind. Yet even so considered a certain sense of geometric control would come in in the selection of our lines and masses, both in regard to each other and in regard to the shape of the inclosing boundary. We seem to feel the need of some answering line or re-echo in the character of the composition to the shape of its boundary, to give it its distinctive reason for existence in that particular form—just as we should expect a shell-fish to conform to the shape of its shell. Such a re-echo or acknowledgment might be ever so slight, or might be quite emphatic and dominate as the leading motive, but for perfectly harmonious effect it must be there.
A strictly simple and logical linear filling of such spaces might be expressed in the most primitive way, as in the illustration on p. [109].
By these means certain primitive types of ornament are evolved, such as the Greek volute and the Greek key or fret, the logical ornament of a logical people.
Such arrangements of line form simple linear patterns, and a decorative effect of surface is produced simply by their repetition, especially if the principle of alternation be observed. This principle may be expressed by taking, say, a series of squares or circles, and placing them either in a line as for a border arrangement, or for extension vertically and laterally over a surface, and filling only the alternate square or circle, leaving the alternate ones, or dropping them out altogether (see illustration, p. [111]).
When we desire to go beyond such primitive linear ornaments, however, and introduce natural form, we should still be guided by the same principles, if we desire to produce a strictly decorative effect, while varying them in application to any extent.
It matters not what forms we deal with, floral, animal, human; directly we come to combine them in a design, to control them by a boundary, to inclose them in a space, we shall feel this necessity of controlling line, which, however concealed, is yet essential to bring them into that harmonious relation which is the essence of all design (see illustration, p. [112]).