The Arabs introduced the Indian buffalo which has spread considerably, and is now found in South Italy and the Balkan peninsula. The pig has been banished from parts of the region on religious grounds, but elsewhere it chiefly thrives where oak forests grow, the acorn being an important part of its food. The really important ungulates, however, are sheep and goats, which are often very numerous, and which, apart from birds and fish, furnish the most important part of the animal food of the inhabitants. The milk furnishes cheese, which is an important element of diet, while leather, wool and hair are also important products. The goats chiefly feed upon the young shoots of shrubs, and frequent the denser thickets, while the sheep browse upon the grasses and herbs to be found in the more open forms of maquis. The climate permits the animals to remain in the open during the whole year, and this prevents the collection of the manure for the arable lands. Further, the summer drought makes it difficult for even these hardy animals to obtain food, and necessitates in many regions a curious form of nomadism, to which the name of transhumance is given. Transhumance, still well developed in Spain, is the periodic and alternating displacement of flocks and herds between two regions of different climate. As we have had frequent reason to remark, the rainlessness of the Mediterranean summer is locally modified by many causes, notably by elevation. Mountains may receive frequent showers, while the plains are parched and brown, and therefore there may be pasture on the mountains while there is none in the plains. On lofty mountains also the winter snow lingers long enough to promote the growth of summer pasture. While there are considerable herds of sheep and goats, then, it may be necessary for the flocks and their keepers to travel to the mountains in summer and back to the plains in winter. In Spain these periodic migrations, now largely made by means of the railway, formerly took place by well-defined routes, along which the immense army of sheep, accompanied by a smaller army of attendants, passed twice a year, causing enormous destruction to the cultivated lands through which they passed. Everywhere the conflict between shepherd and husbandman is more or less acute, but it seems to have been especially acute in Spain, which is in some respects a link between Africa and Europe. Its constant liability to Arab invasion made agriculture especially difficult, while frequent wars favoured the pastoral industry; for flocks may be removed to a place of safety on an alarm, but agriculture must have some security before it can develop. In the semi-desert regions of North Africa some form of pastoral nomadism, with the social polity which comes from pastoral nomadism, was the natural result of the physical and climatic conditions, and Spain, like the lands of the eastern part of Europe, has been constantly liable to have its nascent agriculture