It does not, however, follow that the Greeks knew of no musical forms analogous to our Major and Minor modes, or to the mediaeval Tones. On the contrary, the course of the discussion has led us to recognise distinctions of this kind in more than one instance. The doctrine against which the argument has been mainly directed is not that ancient scales were of more than one species or 'mode' (as it is now called), but that difference of species was the basis of the ancient Greek Modes. This will become clear if we bring together all the indications which we have observed of scales differing from each other in species, that is, in the _order_ of the intervals in the octave. In doing so it will be especially important to be guided by the principle which we laid down at the outset, of arranging our materials according to chronology, and judging of each piece of evidence strictly with reference to the period to which it belongs. It is only thus that we can hope to gain a conception of Greek music as the living and changing thing that we know it must have been. 1. The principal scale of Greek music is undoubtedly of the Hypo-dorian or common species. This is sufficiently proved by the facts (1) that two octaves of this species (_a-a_) constitute the scale known as the Greater Perfect System, and (2) that the central _a_ of this system, called the Mesê, is said to have been the key-note, or at least to have had the kind of importance in the scale which we connect with the key-note (Arist. _Probl._ xix. 20). This mode, it is obvious, is based on the scale which is the descending scale of the modern Minor mode. It may therefore be identified with the Minor, except that it does not admit the leading note. It should be observed that this mode is to be recognised not merely in the Perfect System but equally in the primitive octave, of the form _e - e_, out of which the Perfect System grew. The important point is the tonic character of the Mesê (_a_), and this, as it happens, rests upon the testimony of an author who knows the primitive octave only. The fact that that octave is of the so-called Dorian species does not alter the _mode_ (as we are now using that term), but only the compass of the notes employed. The Hypo-dorian octave is seen in two of the scales of the cithara given by Ptolemy (p. 85), viz. those called [Greek: tritai] and [Greek: tropoi], and the Dorian octave (_e - e_) in two scales, [Greek: parupatai] and [Greek: ludia]. It is very possible (as was observed in commenting on them) that the two latter scales were in the key of _a_, and therefore Hypo-dorian in respect of mode. The Hypo-dorian mode is also exemplified by three at least of the instrumental passages given by the _Anonymus_ (_supra_, p. 89).