For some breeds, such as the Thoroughbred, Arabian influence of specific animals is documented in written stud books. For older breeds, dating the influx of Arabian ancestry is more difficult. For example, while outside cultures, and the horses they brought with them, influenced the predecessor to the Iberian horse in both the time of Ancient Rome and again with the Islamic invasions of the 8th century, it is difficult to trace precise details of the journeys taken by waves of conquerors and their horses as they traveled from the Middle East to North Africa and across Gibraltar to southern Europe. Mitochondrial DNA studies of modern Andalusian horses of the Iberian peninsula and Barb horses of North Africa present convincing evidence that both breeds crossed the Strait of Gibraltar and influenced one another. Though these studies did not compare Andalusian and Barb mtDNA to that of Arabian horses, there is evidence that horses resembling Arabians, whether before or after the breed was called an "Arabian", were part of this genetic mix. Arabians and Barbs, though probably related to one another, are quite different in appearance, and horses of both Arabian and Barb type were present in the Muslim armies that occupied Europe. There is also historical documentation that Islamic invaders raised Arabian horses in Spain prior to the Reconquista; the Spanish also documented imports of Arabian horses in 1847, 1884 and 1885 that were used to improve existing Spanish stock and revive declining equine populations.