[…] the relations of vowel and consonant: which, though their distinction is of the highest importance in phonetics, are by no means separate and independent systems, but only poles, as it were, in one continuous unitary series, and with a doubtful or neutral territory between them: they are simply the opener and closer sounds of the alphabetic system. Upon their alternation and antithesis depends the syllabic or “articulate” character of human speech: the stream of utterance is broken into ‘articuli’, ‘joints,’ by the intervention of the closer sounds between the opener, connecting the latter at the same time that they separate them, giving distinctness and flexibility, and the power of endlessly variable combination. A mere succession of vowels passing into one another would be wanting in definite character; it would be rather sing-song than speech; and, on the other hand, a mere succession of consonants, though pronounceable by sufficient effort, would be an indistinct and disagreeable sputter.
- Whitney (1875), a pag.68

That our written vowels have from three to nine values each, is owing to the fact that we have altered their original unitary sounds in so many different ways during the historic period; and there lies yet further back another like history of change.
- Whitney (1875), a pag.56

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.
- Whitney (1875), a pag.71

To reduce the length of swing of these transitions [between vowel and consonant, between opener and closer positions], by reducing the openness of the open sounds and the closeness of the close ones, is an economy which the articulating organs- of course, unconsciously- find out for themselves by experience and learn to practice. It is the most assimilating influence exerted by consonant and vowel upon one another: each class draws the other toward itself; the vowels become more consonantal; the consonant become more vocalic.
- Whitney (1875), a pag.69

Upon this openest tone [‘a’] various modifications are produced by narrowing the oral cavity, at different points and to different degrees. The less marked modifications, which, though they alter decidedly the quality of the tone; yet leave predominant the element of tone, of material, give rise to the sounds which we call vowels.
- Whitney (1875), a pag.61