[…] the general historical development of the alphabet. The primitive language of our family had not half the sounds given in the scheme; and those which it had were the extreme members of the system: among the vowels, only ‘a’, ‘i’, and ‘u’, the corners of the vowel triangle; among the consonants, mainly the mutes, along with the nasals ‘m’ and ‘n’, which are also mutes as concerns their mouth-position; of the whole double class of fricatives, only the ‘s’. The ‘l’ was not yet distinctly separated from the ‘r’, nor the ‘w’ and ‘y’ from ‘u’ and ‘I’. There has been a filling-up of the scheme by the production of such new sounds as are intermediate in character, made by less strongly differentiated positions of the organs.
- Whitney (1875), a pag.68

[…] we are able to gain some idea of our spoken alphabet as of an orderly system of sounds, and of the lines and degrees of relationship which bind its members together, and help to determine their transition.
- Whitney (1875), a pag.59

Above the vibrating reed-apparatus is set, after the fashion of a sounding-box, the cavity of the pharynx, with that of the mouth, and the nasal passage; and movements of the throat and mouth-organs under voluntary control so alter the shape and size of this box as to give to the tone produced a variety of tones- which are the sounds of our spoken alphabet.
- Whitney (1875), a pag.59-60

It [human alphabet] must lie between the completely open ‘a (far)’ and the completely close mutes; these are its natural and necessary limits; and it may be expected to fall into classes according to the intermediate degrees of closure. But there are also other lines of relationship in it. Theoretically an indefinite number of mute-closures are possible, all along the mouth, from the lips to as far back in the throat as the organs can be brought together; in practice, however, they are found to be prevailingly three: […] the labial closure, giving ‘p’ […] the palatal (or guttural) closure, giving ‘k’ […] the lingual (or dental) closure, giving ‘t’ […] And the same tendency toward a triple classification, of front, back and intermediate, appears also in the other classes of sounds, so that these arrange themselves, in the main, nearly upon the lines of gradual closure proceeding from the neutrally open a ‘(far)’ to the shut ‘p’, ‘t’, ‘k’. This adds, then, the other element which is needed in order to convert the mass of articulate utterances into an orderly system [their sonant counterparts].
- Whitney (1875), a pag.61-62