Since this work was accepted for publication Professor Shipp has been prevented by illness from undertaking the
final stages of its preparation. The typescript, however, was already completed, and editing of it has been confined to minor typographical corrections, the incorporation of the author's own amendments, and a few matters of arrangement or presentation which would clearly have received his attention in a final checking. No change or addition has been made to the substance of the work. In carrying out such editing as was necessary I have been grateful for the collaboration of Professor A.P. Treweek, who read carefully through the whole of the typescript, and or. A,W, James, and also for the help of Mrs, Olive Newman with any typing that was required.