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.