[…] the inflectional system of the noun, and less distinctly that of the verb, reached a fullness which has [...] undergone a gradual reduction. Not that there has been generally a diminution of ability to express distinctions, but means of another kind have been more and more resorted to: auxiliaries, form-words, instead of suffixes, formative elements in words […].
- Whitney (1875), a pag.211

An early (perhaps the first) and most important act in the history of linguistic development out of […] rather scanty beginnings was that whereby a separation was made between noun (substantive and adjective) and verb.
- Whitney (1875), a pag.202

Most of the later tongues of our family still retain that adapted ness of the qualifying adjective, in gender and number and case, to the noun qualified, which, inherited from the time when adjective and substantive were not separated, was characteristic of their ancestors; to this we preserve nothing whatever that is correspondent; that an adjective should change its form on account of the character of the noun it belongs to is as strange to us as to many languages it is that the verb should change its form on account of the character of the subject of which it predicates something.
- Whitney (1875), a pag.218

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. And everything in Indo-European speech from predicative roots is by origin either a verb or noun, a form either of conjugation or of declension.
- Whitney (1875), a pag.205