The true domesticated cattle are the descendants of a group of races of Bos primigenius, the Urus or Aurochs, as this species is commonly called in Europe. Their economic importance has increased in the course of time as civilization advanced and to-day they are perhaps the most important of all domesticated animals, as producers of meat, milk and hides. It is certain that domestication was undertaken long before 3000 b.c., but nothing is known about its actual beginnings. Many theories have been developed to explain the differences of present-day breeds by deriving them from various types of wild cattle. It is possible also that interspecific crossing has played a part in the development of some of the eastern breeds.