The theories about the nonhomogeneous origin of the Slovak language assume its late integration in the 13th to 14th centuries or even after the 16th century. They claim that the Proto-Slavic basis of Slovak emerged on the border of early Western, Southern and Eastern Slavic macro dialects or that Slovak emerged from early or late mixing of neighboring languages. A prominent Slovak linguist, Samuel Czambel (1856-1909), believed that Western Slovak dialects are derived from early Western Slavic, that Central Slovak dialects are remains of the South Slavic language area (Czechized over centuries) and that Eastern Slovak dialects come from Old Polish and Old Ukrainian. Samuil Bernstein supported a similar theory. István Kniezsa suggested a mixing of languages in today's Central Slovakia after the Mongol invasion of Europe and Ottoman wars, thus forming modern Central Slovak dialects. The opinion about the late integration is not compliant with the current state of knowledge about the development of Slovak dialects from Proto-Slavic.