[…] the further we go back, the less are substantive and adjective distinguished from one another; they are made by the same suffixes, they share the same inflection: things, in fact are named from their qualities, and whether the quality denoting word shall be used attributively or appellatively is at the outset a matter of comparative indifference; though the two come finally to be distinct enough.
- Whitney (1875), Pag. 205

The genesis of the noun as a part of speech, in its two forms, substantive and adjective, was implied in that of the verb: when one set of forms became distinctly verb, the rest were left as noun.
- Whitney (1875), Pag. 205