They cease when the first enthusiasm spends itself, or when outside competition is intensified, or the material rewards decrease.
[Sidenote: The case of Spain.] An illustration is found in the mediaeval history of Spain.

The intercontinental location of the Iberian Peninsula exposed it to the Saracen conquest and to the constant reinforcements to Islam power furnished by the Mohammedanized Berbers of North Africa.

For seven centuries this location was the dominant geographic factor in Spain's history.

It made the expulsion of the Moors the sole object of all the Iberian states, converted the country into an armed camp, made the gentleman adventurer and Christian knight the national ideal.

It placed the center of political control high up on the barren plateau of Castile, far from the centers of population and culture in the river lowlands or along the coast.