The different types of unity are by no means exclusive of each other and are usually found together in any complex work of art. Symmetry usually involves a combination of harmony and balance. The symmetrical halves of a Greek vase, for example, are harmonious in so far as their size and shape are the same, yet balanced as being disposed in opposite directions, right and left. Rhythm is temporal symmetry, and so also represents a combination of harmony and balance. Static rhythm is only apparent; for in every seeming case, the rhythm really pervades the succession of acts of attention to the elements rather than the elements themselves; a colonnade, for example, is rhythmical only when the attention moves from one column to another. There is harmony in rhythm, for there is always some law--metrical scheme in poetry, time in music, similarity of column and equality of interval between them in a colonnade--pervading the elements. But there is also balance; for as the elements enter the mind one after the other, there is rivalry between the element now occupying the focus of the attention and the one that is about to present an equal claim to this position. Because of its intrinsic value, we tend to hold on to each element as we hear or see it, but are forced to relinquish it for the sake of the one that follows; only for a moment can we keep both in the conscious span; the recurrence and overcoming of the resulting tension, as we follow the succession through, creates the pulsation so characteristic of rhythm. The opposition of the elements as in turn they crowd each other out does not, however, interfere with the harmony, for they have an existence all together in memory, where the law binding them can be felt,--a law which each element as it comes into consciousness is recognized as fulfilling. Since we usually look forward to the end of the rhythmical movement as a goal, rhythm often exists in combination with evolution, and is therefore the most inclusive of all artistic structural forms. In a poem, for example, the metrical rhythm is a framework overlying the development of the thought. Dramatic unity is found combined with balance even in the static arts, as, for example, in the combination of blue and gold, where the balance is not quite equal, because of a slight movement from the blue to the more brilliant and striking gold. I have already shown how harmony, opposition, and evolution may be combined in a melody. In the drama, also, all three are present. There is a balance of opposing and conflicting wills or forces; this is unstable; whence movement follows, leading on to the catastrophe, where the problem is solved; and throughout there is a single mood or atmosphere in which all participate, creating an enveloping harmony despite the tension and action. And other illustrations of combinations of types will come to the mind of every reader.