The great advantage which the classical historians possess over those of modern times is in unity of plan, of course greatly facilitated by the narrower sphere to which their researches were confined.

Except Herodotus, the great historians of Greece--we exclude the more modern compilers, like Diodorus Siculus--limited themselves to a single period, or at least to the contracted sphere of Grecian affairs.

As far as the Barbarians trespassed within the Grecian boundary, or were necessarily mingled up with Grecian politics, they were admitted into the pale of Grecian history; but to Thucydides and to Xenophon, excepting in the Persian inroad of the latter, Greece was the world.

Natural unity confined their narrative almost to chronological order, the episodes were of rare occurrence and extremely brief.

To the Roman historians the course was equally clear and defined.