p. 671~672
The first of these is that there is a huge amount of variation in the way the "same" phonological unit is pronounced, whether this unit is the phone, syllable, or word. The relatively short list of allophones given in conventional phonemic descriptions of languages is just the "tip of the iceberg."
The second fundamental fact that motivates us to look at phonetics for an understanding of sound change is that a great deal of phonetic variation parallels sound change, that is, synchronic variation, including that which we find in present-day speech, resembles diachronic variation.
p. 674-675
But these two facts immediately raise the question: could this synchronic variation actually be sound change observed "on the hoof"? Logically this would be difficult to accept, because if this were the case then we would find sound change progressing at a rate very much faster than we do -- in fact, several orders of magnitude faster than present evidence suggests. All of the sound changes that transformed Proto-Indo-European over five or six millennia into the present-day Indo-European languages would be accomplished in a day or less. Somehow pronunciation remains relatively stable over time in spite of the great variation in everyday speech. But if present-day variation is not sound change, then how do we account for the uncanny similarities between them?
