According to Ross (see below), the main problem with Wurm's classification is that he did not take contact-induced change into account. For example, several of the main branches of his Trans–New Guinea phylum have no vocabulary in common with other Trans–New Guinea languages, and were classified as Trans–New Guinea because they are similar grammatically. However, there are also many Austronesian languages that are grammatically similar to Trans–New Guinea languages due to the influence of contact and bilingualism. Similarly, several groups that do have substantial basic vocabulary in common with Trans–New Guinea languages are excluded from the phylum because they do not resemble it grammatically. Wurm believed the Papuan languages arrived in several waves of migration with some of the earlier languages (perhaps including the Sepik–Ramu languages) being related to the Australian languages, a later migration bringing the West Papuan, Torricelli and the East Papuan languages and a third wave bringing the most recent pre-Austronesian migration, the Trans–New Guinea family. Two of Wurm's isolates have since been linked as the and since Wurm's time another isolate and two languages belonging to a new family have been discovered, Foley summarized the state of the literature. Besides Trans–New Guinea and families possibly belonging in TNG (see), he accepted the proposals for,  Malcolm Ross re-evaluated Wurm's proposal on purely lexical grounds. That is, he looked at shared vocabulary, and especially shared idiosyncrasies analogous to English "I" and "me" vs. German "ich" and "mich". The poor state of documentation of Papuan languages restricts this approach largely to pronouns. Nonetheless, Ross believes that he has been able to validate much of Wurm's classification, albeit with revisions to correct for Wurm's partially typological approach. (See Trans–New Guinea languages.) "Ethnologue" (2009) largely follows Ross. It has been suggested that the families that appear when comparing pronouns may be due to pronoun borrowing rather than to genealogical relatedness. However, Ross argues that Papuan languages have closed-class pronoun systems, which are resistant to borrowing, and in any case that the massive number of languages with similar pronouns in a family like Trans–New Guinea preclude borrowing as an explanation. Also, he shows that the two cases of alleged pronoun borrowing in New Guinea are simple coincidence, explainable as regular developments from the protolanguages of the families in question: as earlier forms of the languages are reconstructed, their pronouns become "less" similar, not more.