Push chains are not as well accepted as drag chains; they suggest functional explanations for a class of sound change. In historical phonetic change, when the phonetic realization of one phoneme becomes very similar to that of another phoneme, three things could logically happen. First, they could merge. This is not necessarily -- or perhaps at all -- deterred by the ``functional load'' of the distinction (that is, the amount of homonymy that would result from merger). Mergers can occur even when a sound distinction carries a huge functional load, as shown by the merger in Middle-Indo-Iranian of all the strident fricatives, s, s, , resulting in large numbers of homonyms, or of the merger in Early Modern English of EME /e:/ and /:/ (as in meet and meat)(Prins 1972:122). The second possible consequence of phonetic approximation is near-merger, discussed in the next section, where sounds remain approximated to one another but do not merge. Finally, a push chain could occur, where one of the phonemes becomes distinguished by some new phonetic difference. Given the tolerance for both merger and near-merger, it is something of a mystery why push chains should occur. In fact, some theoreticians (King 1969) claim for independent theoretical reasons that they are impossible, though others (e.g., Labov, Yaeger, & Steiner 1972:212) accept their existence. But this mystery is no more unusual than the existence of both mergers and near-mergers: Why should approximating vowels merge or instead remain distinct, though approximated?B.2 The historical conditioning of merger versus unmerger is not known; it cannot therefore be surprising that the conditioning of the third alternative is not known either.
The explanation given here for the raising of the nuclei of /e:, o:/ to [ie, uo] involves a push chain, whereby /ou/ encroaches on /o:/ and then /o:/ becomes differentiated by a raised nucleus and a different glide direction. This explanation thus a small contribution to the discussion of push chains. If it is true, then all three results: merger, near-merger, and push-chain shifts can occur. Opponents of push-chains must provide a better explanation for this set of vowel shifts.