s of expression music became a
living language of communication, ready for that development which,
through the genius of the Classic and Romantic masters, it was
destined to show. The essential feature of all the above forms is the
emphasis laid on _one theme_. This is strictly true of the polyphonic
forms, the Canon, Fugue[87] and Invention and of the Two-part form;
and although in the Three-part form we have a second theme, this is
merely for contrast and is often of rather slight import. The same
comment holds true of the Rondo where, notwithstanding the new
contrasting themes of the episodes, the centre of attraction is the
_single main theme_, to which constant recurrence is made. Obviously
the Varied Air is the expansion of a single theme. But the principal
characteristic of the Sonata-Form, now to be studied, is that we find
therein _two themes_ of coequal importance, which may well be compared
to the hero and heroine of a novel or the two leading characters in a
drama. It is true that a composer will often in the creations of his
imagination show a marked preference for one theme over the other;
just as, in the family group to which the child owes its life, either
the man or the woman is likely to be the stronger character. But as
there can be no child without two parents, so the organism of the
Sonata-Form derives its vitality from the presence and interaction of
two living musical personalities, the first and second themes. The
first theme is so called because it is the one first presented and
because it generally furnishes the prevailing rhythmic pulse of the
movement. Yet the second theme,--exactly as important in its own way,
is often of a greater beauty; its title of "second theme" implying
nothing of a secondary nature, but merely its position in order of
appearance. No greater step was ever taken in the growth of musical
structure than this introduction of a second coequal theme; for the
principle of duality, of action and reaction between two forces, runs
throughout nature both human and physical, as is seen from the import
of the terms: man and woman, active and passive, positive and
negative, heat and cold, light and darkness. The first theme, in fact,
often resembles, in its vigor and directness, a masculine personality;
while the second theme, in grace and tenderness, resembles the
feminine. As long as music confined itself to the presentation of but
one main theme it was hampered by the same limitat