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THE FIRST RONDO-FORM.
EVOLUTION .-- It cannot have escaped the observant student of the foregoing pages, that the successive enlargement of the structural designs of musical composition is achieved by a process of natural growth and progressive evolution.

No single form intrudes itself in an arbitrary or haphazard manner; each design emerges naturally and inevitably out of the preceding, in response to the necessity of expansion, and conformably with the same constant laws of unity and variety,--the active agents, along the entire unbroken line of continuous evolution, being _reproduction_ (Unity) and legitimate _modification_ (Variety); or, in other words, _modified repetition_.
It is upon the indisputable evidence of such normal evolution in the system of musical structure, that our conviction of the legitimacy and permanence of this system rests.
The diagrams which appear on pages 78 and 98 partly illustrate the line of evolution, which, in its fullest significance, may be traced as follows: the _tone_, by the simplest process of reproduction, became a _figure_; the figure, by multiplication or repetition, gave rise to the _motive_; the latter, in the same manner, to the _phrase_.

The repetition of the phrase, upon the infusion of a certain quality and degree of modification (chiefly affecting the cadences) became the _period_; the latter, by the same process, became the double-period.
The limit of coherent phrase-succession (without a determined interruption) being therewith reached, the larger Part-forms became necessary.

The _Two-Part_ form emerged out of the double-period, the two "connected" periods of which separated into two "independent" Parts, by the determined interruption in the center.