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_.