"The non-Indo-Aryan nature of the terms and names noted earlier also has to be juxtaposed with the fact that the place-names and river names in northern India are almost all Indo-Aryan. These names are, to my mind, the single most important element in considering the existence of a non-Indo-Aryan substratum position. Unlike people, tribes, material items, flora, and fauna, they cannot relocate or be introduced by trade (although their names can be transferred by immigrants). In other words, it is difficult to exclude the possibility that the foreign personal and material names in the Rgveda were intrusive into a preexisting Indo-Aryan area as opposed to vice versa. This argument of lexical transiency can much less readily be used in the matter of foreign place-names. Place-names tend to be among the most conservative elements in a language. Moreover, it is a widely attested fact that intruders into a geographic region often adopt the names of rivers and places that are current among the peoples that preceded them. Even if some such names are changed by the immigrants, some of the previous names are invariably retained (e.g., the Mississippi river compared with the Hudson, Missouri state compared with New England)."