e
_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. And, be it well
understood, each new design having once been thus established, its
enlargement within its own peculiar boundaries followed as a matter of
course; I mean, simply, that the two Parts did not need to remain the
_periods_ that were their original type; the process of growth cannot
be stopped. The _Three-Part_ form resulted from adding to the Two-Part
the perfecting reversion to the starting-point, and confirmation of the
principal statement. The _Five-part_ form, and the _Song with Trio_
are enlargements of the Three-Part forms by repetition or
multiplication; and with the latter the limit of this particular
process appears to be achieved. Any further growth must take place
from within, rather than by addition from without.

But the process of evolution continues steadily, as the student will
witness. To one vital fact his attention is here called,--a fact which
he is enjoined to hold in readiness for constant application,--namely,
_that perfection of structural design is attained in the Three-Part
form, and that every larger (or higher) form will have its type in this
design, and its basis upon it_. The coming designs will prove to be
expansions of the Three-Part form.

THE RONDO-FORMS.--The structural basis of the Rondo, and other larger
or (as they are sometimes called) higher forms, is the Subject or