Objection 1. It would seem that the angels do not differ in species. For since the "difference" is nobler than the 'genus,' all things which agree in what is noblest in them, agree likewise in their ultimate constitutive difference; and so they are the same according to species. But all angels agree in what is noblest in them--that is to say, in intellectuality. Therefore all the angels are of one species.
Objection 2. Further, more and less do not change a species. But the angels seem to differ only from one another according to more and less--namely, as one is simpler than another, and of keener intellect. Therefore the angels do not differ specifically.
Objection 3. Further, soul and angel are contra-distinguished mutually from each other. But all souls are of the one species. So therefore are the angels.
Objection 4. Further, the more perfect a thing is in nature, the more ought it to be multiplied. But this would not be so if there were but one individual under one species. Therefore there are many angels of one species.
On the contrary, In things of one species there is no such thing as "first" and "second" [prius et posterius], as the Philosopher says (Metaph. iii, text 2). But in the angels even of the one order there are first, middle, and last, as Dionysius says (Hier. Ang. x). Therefore the angels are not of the same species.
I answer that, Some have said that all spiritual substances, even souls, are of the one species. Others, again, that all the angels are of the one species, but not souls; while others allege that all the angels of one hierarchy, or even of one order, are of the one species.
But this is impossible. For such things as agree in species but differ in number, agree in form, but are distinguished materially. If, therefore, the angels be not composed of matter and form, as was said above (Article 2), it follows that it is impossible for two angels to be of one species; just as it would be impossible for there to be several whitenesses apart, or several humanities, since whitenesses are not several, except in so far as they are in several substances. And if the angels had matter, not even then could there be several angels of one species. For it would be necessary for matter to be the principle of distinction of one from the other, not, indeed, according to the division of quantity, since they are incorporeal, but according to the diversity of their powers; and such diversity of matter causes diversity not merely of species, but of genus.
Reply to Objection 1. "Difference" is nobler than "genus," as the determined is more noble than the undetermined, and the proper than the common, but not as one nature is nobler than another; otherwise it would be necessary that all irrational animals be of the same species; or that there should be in them some form which is higher than the sensible soul. Therefore irrational animals differ in species according to the various determined degrees of sensitive nature; and in like manner all the angels differ in species according to the diverse degrees of intellectual nature.
Reply to Objection 2. More and less change the species, not according as they are caused by the intensity or remissness of one form, but according as they are caused by forms of diverse degrees; for instance, if we say that fire is more perfect than air: and in this way the angels are diversified according to more or less.
Reply to Objection 3. The good of the species preponderates over the good of the individual. Hence it is much better for the species to be multiplied in the angels than for individuals to be multiplied in the one species.
Reply to Objection 4. Numerical multiplication, since it can be drawn out infinitely, is not intended by the agent, but only specific multiplication, as was said above (Question 47, Article 3). Hence the perfection of the angelic nature calls for the multiplying of species, but not for the multiplying of individuals in one species.
Source: Summa Theologica (New Advent)