Its effect is not restricted to a given historical event or epoch, but, except when temporarily met by some strong counteracting force, tends to make itself felt under varying guise in all succeeding history.

It is the permanent element in the shifting fate of races.

Islands show certain fundamental points of agreement which can be distinguished in the economic, ethnic and historical development of England, Japan, Melanesian Fiji, Polynesian New Zealand, and pre-historic Crete.

The great belt of deserts and steppes extending across the Old World gives us a vast territory of rare historical uniformity.

From time immemorial they have borne and bred tribes of wandering herdsmen; they have sent out the invading hordes who, in successive waves of conquest, have overwhelmed the neighboring river lowlands of Eurasia and Africa.