The issue in or, rather, the excuse for this somewhat undisciplined documentary is the fight for control of Constantinople and Mediterranean trade routes to the east. The limitations of the topic lead Mr. Robinson into some supplementary discussions, repetitions, and informal stylistic meanderings. In presenting the ups and downs of Venice and Genoa--both individually and interdependently--he compares their geographic, economic, and cultural-characteristics, profiles major figures from Popes to Doges, and introduces the indirect causality of political and religious events beyond the two cities' borders. The instability of this rendering of history comes from its peculiarly darting chronology (from early centuries A.D. to Renaissance times and back again) and its inconsistent selection of explanatory background material; many of the accompanying line drawings are blurry, carelessly placed, and ill-captioned. The author's enthusiasm usually redeems the disentanglement from the petty sibling-rival tedium it approaches. . . but the fact remains that this series is not the most direct route through history.