By extending and developing such imitation as this, composers came to write in "double counterpoint," which means the construction of two parts in such a way that their different portions can be transferred from one to the other just as they are in the selection from Perotin. The reader will see at once that composing in this manner required a great deal of calculation, and was a constant tax on the ingenuity of the musician. In its early stages it prevented any attempt at making music expressive, and reduced composition to a mere exercise of scientific skill. But it forced the composers to a close study of the materials of their art, and they acquired a great mastery over them and constantly learned more and more about the possibilities of music. Double counterpoint, which is at the foundation of the most rigid forms of polyphonic writing, was generally known in the French school at least as early as the thirteenth century, but the example from Perotin seems to show that at least some of the composers knew it much earlier. This might account for the existence of the celebrated example of early English polyphony, a "canon" called "Sumer is icumen in." This was discovered by Sir John Hawkins, who wrote a history of music in 1776. Its manuscript was copied by a monk of Reading, John of Forneste, in 1228, and it must have been composed shortly before that time. The fact that Walter Odington, an Englishman, wrote a treatise on music in 1230, when the only famous school was that of Paris, leads me to believe that these early English composers were disciples of the French. But it is certain that the composer of "Sumer is icumen in" was a greater master of counterpoint than his teachers, because this "canon" is the finest specimen of polyphonic writing that has come down to us from those early times. Here is its beginning:-- [Music: "Sumer is icumen in" for Six Voices.] Chorus: Su-mer is i-cu-men in . . . Lhu-de sing Cuc-cu, Grow-eth sed, and blow-eth med, And springth the w-de nu . . . Sing Cuc-cu . . .