[…] one class to be noticed: that of the semivowels, or sounds which stand nearly on the division-line between vowel and consonant.
- Whitney (1875), a pag.65

[…] 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 and 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

The vowels are much more liable to wholesale alteration than are the consonants […].
- Whitney (1875), a pag.57

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 practise. It is the most general kind of assimilating influence exerted by consonant and vowel upon one another: each class draws the other toward itself: the vowels become more consonantal; the consonants become more vocalic.
- Whitney (1875), a pag.69