[…] many combination of surd consonant with sonant have that degree of difficulty which we call impossibility (this is only a matter of degree); and nothing is more frequent in all language than the interchange of surd and sonant utterance. There is also a more general movement here: since the sonant elements in connected speech are (including the vowels) much more numerous than the surd, the general weight of the assimilative force is in the direction of sonancy, and surds are converted into sonants more often than the reverse.
- Whitney (1875), a pag.70-71

‘Ofer’ […] has become ‘over’ with us, by the conversion of a surd into its corresponding sonant sound, a phenomenon of very wide range and great frequency in language […].
- Whitney (1875), a pag.57

Along with ‘k’, ‘t’, ‘p’, in the first place, go their nearest kindred ‘g’, ‘d’, ‘b’. These are their sonant (or vocal, phthongal, intonated) counterparts. In the former, namely, there is no audible utterance, but complete silence, during the continuance of the closure; the antithesis to ‘a’ is absolute; the explosion is their whole sensible substance. In the latter there is, even while the closure lasts, a tone produced by the vibration of the vocal chords, a stream of air sufficient to support vibration for a very brief time being forced up from the lungs into the closed cavity or receiving-box of the pharynx and mouth. This is the fundamental distinction of “surd” and “sonant”; anything else is merely a consequence of this and subordinate to it; the names strong and weak, hard and soft, sharp and flat, and so on, founded […] upon these subordinate characteristic, are to be rejected.
- Whitney (1875), a pag.62-63