The importance of this time element for a solution of anthropo-geographic problems becomes plainer, where a certain locality has received an entirely new population, or where a given people by migration change their habitat. The result in either case is the same, a new combination, new modifications superimposed on old modifications. And it is with this sort of case that anthropo-geography most often has to deal. So restless has mankind been, that the testimony of history and ethnology is all against the assumption that a social group has ever been subjected to but one type of environment during its long period of development from a primitive to a civilized society. Therefore, if we assert that a people is the product of the country which it inhabits at a given time, we forget that many different countries which its forbears occupied have left their mark on the present race in the form of inherited aptitudes and traditional customs acquired in those remote ancestral habitats. The Moors of Granada had passed through a wide range of ancestral experiences; they bore the impress of Asia, Africa and Europe, and on their expulsion from Spain carried back with them to Morocco traces of their peninsula life. A race or tribe develops certain characteristics in a certain region, then moves on, leaving the old abode but not all the accretions of custom, social organization and economic method there acquired. These travel on with the migrant people; some are dropped, others are preserved because of utility, sentiment or mere habit. For centuries after the settlement of the Jews in Palestine, traces of their pastoral life in the grasslands of Mesopotamia could be discerned in their social and political organization, in their ritual and literature. Survivals of their nomadic life in Asiatic steppes still persist among the Turks of Europe, after six centuries of sedentary life in the best agricultural land of the Balkan Peninsula. One of these appears in their choice of meat. They eat chiefly sheep and goats, beef very rarely, and swine not at all.[29] The first two thrive on poor pastures and travel well, so that they are admirably adapted to nomadic life in arid lands; the last two, far less so, but on the other hand are the regular concomitant of agricultural life. The Turk's taste to-day, therefore, is determined by the flocks and herds which he once pastured on the Trans-Caspian plains. The finished terrace agriculture and methods of irrigation, which the Saracens had learned on the mountain sides of Yemen through a schooling of a thousand years or more, facilitated their economic conquest of Spain. Their intelligent exploitation of the country's resources for the support of their growing numbers in the favorable climatic conditions which Spain offered was a light-hearted task, because of the severe training which they had had in their Arabian home.