Most of the open questions discussed by the author were settled during the twentieth century. The author’s footnotes are labelled using printer’s marks*; footnotes showing where corrections to the text have been made are labelled numerically1. Minor typographical corrections are documented in the LATEX source. This document is designed for two-sided printing. Consequently, the many hyperlinked cross-references are not visually distinguished. The document can be recompiled for more comfortable on-screen viewing: see comments in source LATEX code
