2. The earliest trace of a difference of species appears to be found in the passage on the subject of the Mixo-lydian mode quoted above (p. 24) from Plutarch's _Dialogue on Music_. In that mode, according to Plutarch, it was discovered by a certain Lamprocles of Athens that the Disjunctive Tone was the highest interval, that is to say, that the octave in reality consisted of two conjunct tetrachords and a tone: [Music: Mesê Disj. Tone] As the note which is the meeting-point of the two tetrachords is doubtless the key-note, we shall not be wrong in making it the Mesê, and thus finding the octave in question in the Perfect System and in the oldest part of it, viz. the tetrachords Mesôn and Synêmmenôn, with the Nêtê Diezeugmenôn. How then did this octave come to be recognised by Lamprocles as distinctively Mixo-lydian? We cannot tell with certainty, because we do not know what the Mixo-lydian scale was before his treatment of it. Probably, however, the answer is to be sought in the relation in respect of pitch between the Dorian and Mixo-lydian keys. These, as we have seen (p. 23), were the keys chiefly employed in tragedy, and the Mixo-lydian was a Fourth higher than the other. Now when a scale consisting of white notes is transposed to a key a Fourth higher, it becomes a scale with one [Symbol: Flat]. In ancient language, the tetrachord Synêmmenôn (_a-b[Symbol: Flat]-c-d_) takes the place of the tetrachord Diezeugmenôn. In some such way as this the octave of this form may have come to be associated in a special way with the use of the Mixo-lydian key. However this may be, the change from the tetrachord Diezeugmenôn to the tetrachord Synêmmenôn, or the reverse, is a change of mode in the modern sense, for it is what the ancients classified as a change of System ([Greek: metabolê kata systêma])[1]. Nor is it hard to determine the two 'modes' concerned, if we may trust to the authority of the Aristotelian _Problems_ (_l. c._) and regard the Mesê as always the key-note. For if _a_ is kept as the key-note, the octave _a-a_ with one [Symbol: b] is the so-called Dorian (_e - e_ on the white notes). In this way we arrive at the somewhat confusing result that the ancient Dorian species (_e - e_ but with _a_ as key-note) yields the Hypo-dorian or modern Minor mode: while the Dorian mode of modern scientific theory[2] has its ancient prototype in the Mixo-lydian species, viz. the octave first brought to light by Lamprocles. The difficulty of course arises from the species of the Octave being classified according to their compass, without reference to the tonic character of the Mesê.