Claus Peter Zoller also investigated shared innovations in Outer IA, but with a different historical explanation for the differences. He limited examination to "d"~"ḍ" alternation; "c, j" > "ċ, (d)z"; and the -"l(l)"- past, finding the rest of Southworth's evidence unconvincing. He argued for a more wave model-type conception of Inner IA, imposing itself on the older OIA layer that is preserved in Outer IA, with greater retention radiating outwards from Inner IA to Outer IA, Dardic, and Nuristani. "n individual language is either more Outer and less Inner Language or vice versa, depending on the amount of typical Outer Language features characterizing that individual language."