Fossilized allomorphy

Unlike allomorphy resulting from phonetically motivated replacement, deletion, or epenthesis, the allomorphy discussed in this section cannot be attributed to the operation of an active and transparent phonetic rule. “Fossilized” allomorphs can be deceptively unlike each other in form, yet they are historically related – they are cognates. The morpheme variants in this group arose as a consequence of systematic changes in pre-Old English times, going all the way back to Proto-Germanic and even to Indo-European. Within English, i.e., after the fifth century, the conditions for these early changes became obscured, and their results can no longer be seen as regular or highly predictable. If we set up a scale of predictability for allomorphic variation, allomorphs due to assimilation or vowel weakening will rank very high; mostly, the conditions for these changes are transparent and recoverable. At the low end of the scale would be the non-productive allomorphic variation described in historical terms, for which the conditions have ceased to exist. At the bottom of the scale are completely unpredictable alternate forms of cognates, the subject of Section 5. Historical, or fossilized, allomorphy tends to follow several general patterns; familiarity with those can help us discern etymological cognates in spite of their overt differences.

Two such fossilized processes are gradation and rhotacism. Originally they affected root morphemes, and occurred regularly in conjunction with specific grammatical changes within a paradigm, e.g., the present vs. the past-tense form of one and the same verb, the nominative vs. the genitive case of the same noun.