to the linguistic center of gravity principle, the most likely point of origin of a language family is in the area of its greatest diversity. By this criterion, Northern India, home to only a single branch of the Indo-European language family (i.e., Indo-Aryan), is an exceedingly unlikely candidate for the Indo-European homeland, compared to Central-Eastern Europe, for example, which is home to the Italic, Venetic, Illyrian, Albanian, Germanic, Baltic, Slavic, Thracian and Greek branches of Indo-European. Both mainstream Urheimat solutions locate the Proto-Indo-European homeland in the vicinity of the Black Sea. Dialectal variation It has been recognized since the mid-19th century, beginning with Schmidt and Schuchardt, that a binary tree model cannot capture all linguistic alignments; certain areal features cut across language groups and are better explained through a model treating linguistic change like waves rippling out through a pond. This is true of the Indo-European languages as well. Various features originated and spread while Proto-Indo-European was still a dialect continuum. These features sometimes cut across sub-families: for instance, the instrumental, dative and ablative plurals in Germanic and Balto-Slavic feature endings beginning with -m-, rather than the usual -*bh-, e.g. Gothic dative plural sunum 'to the sons' and Old Church Slavonic instrumental plural synъ-mi 'with sons', despite the fact that the Germanic languages are centum, while Balto-Slavic languages are satem. The strong correspondence between the dialectal relationships of the Indo-European languages and their actual geographical arrangement in their earliest attested forms makes an Indian origin, as suggested by the Out of India Theory, unlikely. Substrate influence Already in the 1870s the Neogrammarians realised that the Greek/Latin vocalism couldn't be explained on the basis of the Sanskrit one, and therefore must be more original. The Indo-Iranian and Uralic languages influenced each other, with the Finno-Ugric languages containing Indo-European loan words. A telling example is the Finnish word vasara, "hammer", which is related to vajra, the weapon of Indra. Since the Finno-Ugric homeland was located in the northern forest zone in northern Europe, the contacts must have taken place – in line with the placement of the proto-Indo-European homeland at the Pontic-Caspian steppes – between the Black Sea and the Caspian Sea. Dravidian and other South Asian languages share with Indo-Aryan a number of syntactical and morphological features that are alien to other Indo-European languages, including even its closest relative, Old Iranian. Phonologically, there is the introduction of retroflexes, which alternate with