And it must also be accepted that at any speaking point along this backward journey the monosyllable may change: all the vowels may replace one another in the history of a root, for the vowels are the voice itself, which knows no discontinuity or rupture; the consonants, on the other hand, are modified according to certain privileged channels: gutturals, linguals, palatals, dentals, labials, and nasals all make up families of homophonous consonants within which changes of pronunciation are made for preference, though without any obligation.