According to the division of the subject in the beginning of this work, the period from the Christian era to that of Palestrina, A.D. 1600, is one of apprentice work, in which the details of art were being mastered, but in which no music, according to our acceptation of the term, was produced. The history of this period is somewhat obscure, the writers who throw light on it averaging scarcely more than one to a century, scattered about in different parts of Europe. Nevertheless, the most important changes in the history of music took place during this period. The monody and empyrical tonality of the ancients gave place to polyphony and harmonized melodies resting upon the relations of tones in key. New instruments came in, and the entire practice of the art of music was deepened, ennobled and immeasurably enlarged in every direction. There were four causes co-operating in this transformation of the art, and it is not easy to say of any one of them that this one was the chief. First of these, in the Roman empire, or in the south of Europe more particularly, for about 800 years the Greek principles remained more or less in force. The Church is here the foremost influence, and its part in the transformation already noted will be considered presently. In the north of Europe the Goths, Celts and Scandinavians built mighty empires and impressed their enthusiastic and idealistic natures upon the whole form of modern art. The Saracens conquered a foot-hold in the south of France about 819, and remained there for twenty years. Their influence was very important in the development of music, and became still more active after the crusades, where the armies of the west came again in contact with this peculiar civilization. Besides these three sources measurably unprofessional and outside of music, or amateur, as we say now, there was the work of the professional musicians strictly so-called, who, from about 1100 in the old French school, commenced the development of what is now known as polyphony, which culminated in the hands of the Netherlanders, about 1580, Palestrina himself being one of the latest products of this school. These influences reacted upon each other, and all have entered into modern art, and have imparted to it their most essential elements.