As the two sides of meaning and application in the predicative or verbal roots are verb and noun, so in the demonstrative […] the two sides may be said to be pronoun and adverb. From the latter class come those earliest words of place and direction, readily convertible also into words of time, which are of adverbial quality. Yet even these are claimed by some to be properly case-forms of pronouns; and the rule is laid down that everything in language is by origin an inflected form either of verb or of noun.
- Whitney (1875), a pag.208

Our adverbial ending ‘ly’ […] by which most of our adverbs are made, and which to us is only a suffix, is really the product of an alteration of a case-form of a compounded adjective, a word originally independent.
- Whitney (1875), a pag.41

The oldest of them [prepositions] were originally- as many of them still continue also to be- adverbs, modifiers of verbal action, only aiding to determine the noun-case which that action should take as its further adjunct.
- Whitney (1875), a pag.94