[Illustration: Example 44. Fragment of Mendelssohn.]
The co-operation, or interaction, of the principles of Unity and Variety, is nowhere more strikingly shown than in the formulation of the musical period. Either element has the right to predominate, to a reasonable degree, though never to the exclusion or injury of the other. In the above example, the principle of Unity predominates to a somewhat unusual extent:—not only the figures (marked 1-2-3-4), and the motives (a-b), are uniform, in the Antecedent phrase itself, but the melody of the Consequent phrase corresponds very closely throughout to that of the Antecedent, only excepting a trifling change in the course (marked N. B.), and the last few tones, which are necessarily so altered as to transform the semicadence into a perfect cadence. It is this significant change, at the cadence, which prevents the second phrase from being merely a "repetition" of the first one,—which makes it a "Consequent," a response to the one that precedes.
Further (Mendelssohn, No. 23):—
