[…] another department of the action of the abbreviating tendency; its aid is essential to the conversion of what was once an independent word into an affix, an appended element denoting relation.
- Whitney (1875), Pag. 52

[…] so long as the independent word, in its individual shape and meaning, is plainly recognized in the combination, so long does this remain a compound rather than a form […] a disguising alteration is needed to help make an affix- a “formative element,” as it is properly termed, in distinction from the “radical element,” the root or base, or the crude-form, to which it is appended.
- Whitney (1875), Pag. 124

Of all the plural signs, the one which had the most distinctive character was ‘s’. The attention of the language-users became centred upon this as an affix by which the plural modification of sense was made, and they proceeded to apply it in words where it had not before been used; and the movement, once started, gathered force in this progress, until it swept in nearly all the nouns of the language. So with the verb.
- Whitney (1875), Pag. 74-75