Other barbarian tribes invaded Europe at about the same time as the Goths, and during the succeeding centuries. One of the most powerful was the Huns, a people of rude culture but great virility, belonging probably to the Mongolic or Tatar stock, who appeared about the fourth century A.D. They were followed by other races from the same general region and belonging to the same great stock, the Avars who arrived about 555, and the Magyars who put in an appearance at the close of the ninth century. The most recent explanation of the migrations of these Asiatic tribes is that their habitat suffered a change of climate from one of those great cycles about which we are beginning to have some information, which resulted in drying up the region, and furnishing a much smaller amount of subsistence than the people had been accustomed to. This is overpopulation, and furnishes another case of that great economic cause.[8] Another powerful Asiatic invader was Timur or Tamerlane, who with his Tatar hordes devastated Asia Minor during the latter part of the fourteenth century. A conquest is almost the reverse of an invasion. In this case the people of higher culture take the aggressive. It is an overflow of civilization, of manners, of organization, of government,—not to any great extent, of population. Conquest occurs when a well-developed state, full of vigor, sends its armies over the territory of less advanced peoples, imposing its political system upon them, and laying them under tribute, but not slaying the people or destroying their wealth any more than is necessary to secure subjection. It is an enterprise of the state, seeking its own glory and aggrandizement. The movement of population to the conquered territory may be insignificant, and in this, conquest differs from all the other forms of migratory movements. Consequently the effects on the racial stock of the conquered people may be very slight, and in most cases are. The effect on the mores, on the other hand, including the language, may be profound and lasting. Conquest differs from the other forms of migration also in the fact that the motives belong more nearly to the positive, or attractive, group than in any of the others. It is energy, ambition, etc., which lead to conquest rather than fear, cowardice, etc. Many of the individuals who change their residence under conquest are state officials, sent out in the pursuit of their duties to the sovereign, not because of any particular choice of their own.