The task has been by no means easy. So great was the extent of the material which had to be discussed that the difficulties of concise presentation might well seem almost insuperable. The author has had to renounce the attempt to deal with the problem in all its extension and all its complexity, and has confined himself to the consideration of salient features. In the execution of this design he has received the unwearied and invaluable help of his wife, Gisela Michels.