There is a degree of assimilation effected in vowels by the consonants with which they come into immediate connection; yet the cases are rather sporadic and often doubtful. The influence of vowels on other vowels, even when separated from them by consonants, is more marked, and leads to some important classes of phenomena […] Though assimilation is the leading principle in the mutual adjustment of sounds, its opposite, dissimilation, is not altogether unknown, as the close recurrence of two acts of the same organs is felt as burdensome, and avoided by the alteration of one of them.
- Whitney (1875), Pag. 71