In the time which followed Byrd and Bull the best energies of composers were chiefly directed to the development of such instrumental forms as the Suite and the Canzona, and the earlier kinds of Sonata ; and Sets of Variations were not so common. There are a few examples among Frescobaldi's compositions ; as the ' Aria detta Balletto ' in the second book of Toccatas, Can- zonas, etc., which is curious on account of the way the variations are put into different times ; but his works of the kind are on the whole neither so interesting nor so satisfactory as Byrd's. It is also common to meet with an occasional variation on one or more of the regu- lar dance-movements in the Suites; and in that position they were commonly called Doubles. There is a curious and unusual experiment in a Suite of Kuhnau's in E minor, in which the Courante in 6-4 time is a complete variation of the Allemande in common time that precedes it. But the art of varying a theme of some sort was cultivated to a greater extent about this time under other guises. In Germany com- posers were fond of harmonising their Chorales in all sorts of ingenious ways, such as are found later in perfection in Bach's Cantatas and Pas- sions ; they also used the Chorales as a kind of Canto fermo upon which they based elaborate movements for the organ, full of ingenious and effective figures and various devices of counter- point; and not a little of the great development of organ-playing, which culminated in J. S. Bach, was carried on by the cultivation of this form of art. Another form which was more obviously allied to the sets of variations, and indeed can in some cases hardly be distinguished from them, was the ground -bass or basso ostinato, which was a very favourite form of art all over Europe during the greater part of the i7th century. The principle of following the bass of the theme is indeed constantly made use of in variations, and in theory the only difference between the two forms is that in a ground- bass the bass passage, which is repeated over and over again, is the whole bond of connec- tion which joins the series together ; while in variations the bass may change entirely so long as the theme is recognisable either by means of the melody or the succession of the harmonies. But in practice, though there are many exam- ples in which a good clear bass figure is made to persist with obstinate regularity in this form, it often gave place to the succession of the har- monies, or was itself so varied as to become scarcely recognisable. For instance, a so-called Ground by Blow in E minor, with twenty- eight divisions, begins with a section that is much more like a theme for variations ; and though the bass moves in good steps, it has no very decided figure whatever. A comparison of the first half of the so-called ground with the corresponding part of the bass of the twentieth division will show that the view musicians then took of the repetitions was at least a liberal one :
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���In this case the outline of the bass as defined by the successive steps downwards is pretty well maintained, but in a few other divisions which are more elaborately constructed, not only is the bass altered, but even harmonies which do not strictly correspond to the originals are intro- duced. Such treatment clearly destroys the in- dividuality of the form of art, and makes the work to all intents a theme with variations, under limitations. The real type of movement constructed on a ground-bass has a decided character of its own, as the obstinate reiteration. of a good figure is necessarily a striking bond of connection throughout the piece ; and if the figures built upon it are well varied it can be made very amusing. In Purcell's use of this form, which he was evidently fond of, the type is kept much purer, and the divisions on the ground are really what they pretend to be. A quotation of the bass of a ground in one of his Suites will illustrate better than any description the differ- ence between the real thing and a hybrid like Blow's :
��But even so genuine a specimen as Purcell's is closely allied to a theme with variations ; and at a time when the form was so popular that it was not only a favourite with composers, but the constant resource of performers with any talent for extemporising to show off their skill in two directions at once, it seems very likely that the more elastic but less pure form adopted by Blow and others should have been easily allowed to pass in the crowd of experiments ; and thus composers were constantly developing the form of ' Theme and Variations ' under another name.
A celebrated example which bears upon this question is the twelfth and last Sonata of Co- relli's Opera Quinta, which is called ' La Follia.' This is sometimes described as a Theme and twenty-two variations, and sometimes as Divi- sions on a ground. The bass of the theme was well known in those days as Farinelli's Ground, from the inventor, and was commonly used by musicians and composers, as for instance by Vivaldi. Hawkins speaks of it as ' the favourite air known in England as Farinelli's Ground,'